Content:

OS – Ocean Sciences

OS1.1 – Improved Understanding of Ocean Variability and Climate

EGU21-3825 | vPICO presentations | OS1.1

What is the heat uptake potential of Antarctic Bottom Water?

Jan Zika, Abhishek Savita, Ryan Holmes, and Taimoor Sohail

Antarctic Bottom Water (AABW) is a cold dense water mass which sinks around Antarctica keeping the abyssal ocean relatively cool. Recent observations have suggested a component of recent deep ocean warming is linked to AABW. Here we explore how much changes in AABW could affect changes in vertical ocean heat transport in a warming climate. If the AABW circulation were to be completely extinguished, for example due to increases in upper ocean thermal stratification, AABW would cease to cool the deep ocean and hence lead to an effective warming of the abyss. Therefore, we propose that long term mean vertical heat transport of the AABW circulation is an effective upper bound on the change in heat transport that can be affected by changes in AABW. We call this upper bound the ‘heat uptake potential’. We analyse AABW circulations in an ensemble of numerical climate models. We find that the AABW circulation contributes between 0.05Wm-2 and 0.15Wm-2 to the global vertical heat balance in the model’s pre-industrial states. Indeed, under abrupt CO2 forcing changes, AABW heat transport systematically reduces (in some cases completely), with the largest reductions occurring in models with the largest pre-industrial mean heat transports. The AABW circulation vertical heat transport is found to be highly correlated with the minimum of the Meridional Overturning Circulation at 50oS in the models, suggesting there may be observable constraints on the heat uptake potential of AABW.

How to cite: Zika, J., Savita, A., Holmes, R., and Sohail, T.: What is the heat uptake potential of Antarctic Bottom Water?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3825, https://doi.org/10.5194/egusphere-egu21-3825, 2021.

EGU21-8096 | vPICO presentations | OS1.1

Recent trajectory of ocean heat uptake estimated from novel 1972-2017 ocean sea-ice model hindcast simulations

Maurice Huguenin, Ryan Holmes, and Matthew England

Uptake and storage of heat by the ocean plays a critical role in modulating the Earth's climate system. In the last 50 years, the ocean has absorbed over 90% of the additional energy accumulating in the Earth system due to radiative imbalance. However, our knowledge about ocean heat uptake (OHU), transport and storage is strongly constrained by the sparse observational record with large uncertainties. In this study, we conduct a suite of historical 1972–2017 hindcast simulations using a global ocean-sea ice model that are specifically designed to account for a cold start climate and model drift. The hindcast simulations are initialised from an equilibrated control simulation that uses repeat decade forcing over the period 1962-1971. This repeat decade forcing approach is a compromise between an early unobserved period (where our confidence in the forcing is low) and later periods (which would result in a shorter experiment period and a smaller fraction of the total OHU). The simulations are aimed at giving a good estimate of the trajectory of OHU in the tropics, the extratropics and individual ocean basins in recent decades. Many modelling studies that look at recent OHU rates so far use a simpler approach for the forcing. For example, they use repeating cycles of 1950-2010 Coordinated Ocean Reference Experiment (CORE) forcing that is consistent with the Ocean Model Intercomparison Project 2 (OMIP-2). However, this approach cannot account for model drift. The new simulations here highlight the dominant role of the extratropics, and in particular the Southern Ocean in OHU. In contrast, little heat is absorbed in the tropics and simulations forced with only tropical trends in atmospheric forcing show only weak global ocean heat content trends. Almost 50% of the heat taken up from the atmosphere in the Southern Ocean is transported into the Atlantic Ocean. Two-thirds of this Southern Ocean-sourced heat is then subsequently lost to the atmosphere in the North Atlantic but nevertheless this basin gains heat overall. Our results help to estimate the large-scale cycling of anthropogenic heat within the ocean today and have implications for heat content trends under a changing climate.

How to cite: Huguenin, M., Holmes, R., and England, M.: Recent trajectory of ocean heat uptake estimated from novel 1972-2017 ocean sea-ice model hindcast simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8096, https://doi.org/10.5194/egusphere-egu21-8096, 2021.

EGU21-12769 | vPICO presentations | OS1.1

Absorption of Ocean Heat Along and Across Isopycnals in HadCM3

Louis Clement, Elaine McDonagh, Jonathan Gregory, Quran Wu, Alice Marzocchi, and George Nurser

Anthropogenic warming added to the climate system accumulates mostly in the ocean interior and discrepancies in how this is modelled contribute to uncertainties in predicting sea level rise. Temperature changes are partitioned between excess, due to perturbed surface heat fluxes, and redistribution, that arises from the changing circulation and perturbations to mixing. In a model (HadCM3) with realistic historical forcing (anthropogenic and natural) from 1960 to 2011, we firstly compare this excess-redistribution partitioning with the spice and heave decomposition, in which ocean interior temperature anomalies occur along or across isopycnals, respectively. This comparison reveals that in subtropical gyres (except in the North Atlantic) heave mostly captures excess warming in the top 2000 m, as expected from Ekman pumping, whereas spice captures redistributive cooling. At high-latitudes and in the subtropical Atlantic, however, spice predicts excess warming at the winter mixed layer whereas below this layer, spice represents redistributive warming in southern high latitudes.

 

Secondly, we use Eulerian heat budgets of the ocean interior to identify the process responsible for excess and redistributive warming. In southern high latitudes, spice warming results from reduced convective cooling and increased warming by isopycnal diffusion, which account for the deep redistributive and shallow excess warming, respectively. In the North Atlantic, excess warming due to advection contains both cross-isopycnal warming (heave found in subtropical gyres) and along-isopycnal warming (spice). Finally, projections of heat budgets —coupled with salinity budgets— into thermohaline and spiciness-density coordinates inform us about how water mass formation occurs with varying T-S slopes. Such formation happens preferentially along isopycnal surfaces at high-latitudes and along isospiciness surfaces at mid-latitudes, and along both coordinates in the subtropical Atlantic. Because spice and heave depend only on temperature and salinity, our study suggests a method to detect excess warming in observations.

How to cite: Clement, L., McDonagh, E., Gregory, J., Wu, Q., Marzocchi, A., and Nurser, G.: Absorption of Ocean Heat Along and Across Isopycnals in HadCM3, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12769, https://doi.org/10.5194/egusphere-egu21-12769, 2021.

EGU21-4791 | vPICO presentations | OS1.1

A new process-based vertical advection/diffusion theoretical model of ocean heat uptake

Gabriel Wolf, Rémi Tailleux, Antoine Hochet, Till Kuhlbrodt, David Ferreira, and Jonathan Gregory

Ocean heat uptake is a key process for climate change owing to its control of global mean temperature trends. To understand the underlying internal ocean processes and vertical heat transfer controlling it, ocean heat uptake has been often analysed in terms of the simple one-dimensional vertical advection diffusion model. The standard version of this model, formulated in terms of the horizontally-averaged potential temperature is known to poorly capture important effects such as isopycnal mixing, density-compensated temperature anomalies, meso-scale eddy-induced advection and the depth-varying ocean area.

To overcome this problem a new theoretical model of vertical heat transfer for the ocean heat uptake has been developed in an isopycnal framework that exploits advances achieved in the theory of water masses over the past 30 years or so. The new theoretical model describes the temporal evolution of the isopycnally-averaged thickness-weighted potential temperature in terms of an effective velocity that depends uniquely on the surface heating conditionally integrated in density classes, an effective diapycnal diffusivity controlled by isoneutral and dianeutral mixing, and an additional term linked to the meridional transport of density-compensated temperature anomalies by the diabatic residual overturning circulation. The advantage of the isopycnally-averaged construction over the horizontally-averaged construction is that all the terms that enters it have explicit analytical expressions that are more easily evaluated from observations or model outputs, as well as having clearer physical interpretations.

As a first step, the terms of this new model of ocean heat uptake are evaluated by using a range of different datasets, net surface heat flux products and temporal averages to evaluate their sensitivity to input fields. One key feature of the new model is that its effective velocity and diffusivity are positive over most of the ocean column depth. This is in contrast to the horizontally-averaged construction, in which downwelling and ant-diffusive behavior were occasionally observed in previous studies. The hope is that this insight can then be used to develop an improved representation of ocean heat uptake in simple climate models.

How to cite: Wolf, G., Tailleux, R., Hochet, A., Kuhlbrodt, T., Ferreira, D., and Gregory, J.: A new process-based vertical advection/diffusion theoretical model of ocean heat uptake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4791, https://doi.org/10.5194/egusphere-egu21-4791, 2021.

EGU21-10319 | vPICO presentations | OS1.1

The influence of geostrophic coupling between Added and Redistributed heat on ocean warming patterns

Emily Newsom, Laure Zanna, and Samar Khatiwala

Nearly all of the excess heat in the climate system resides in the global ocean, though the distribution of this heat varies widely in space and is concentrated above the pycnocline. The geographic pattern of ocean warming is a primary control on regional sea level rise and strongly modulates the global radiative feedback strength. The drivers of this pattern are not fully understood, however, complicated by their dual dependence on how preindustrial ocean dynamics passively transport surface temperature anomalies into the interior (or "Added" heat), and on how changes in ocean dynamics redistribute pre-existing ocean heat (or "Redistributed" heat). Most previous studies attribute heat redistribution to changes in high-latitude processes, namey deep overturning, convection, and mixing in the North Atlantic and Southern Oceans. Here we instead propose that a substantial component of global heat redistribution is explained by the local geostrophic adjustment of the velocity field to warming within the pycnocline. We explore this hypothesis by comparing patterns of Added and Redistributed heat in a coupled climate model (the University of Victoria Earth System Climate Model) forced with an 8.5 emission scenario, where Added heat is estimated using a Green's Function of the model's preindustrial ocean transport. Throughout most of the model's subtropical and tropical pycnocline, where the majority of ocean warming occurs, patterns of Added and Redistributed heat are strongly anti-correlated (R2 >≈0.85). This anti-correlation arises because changes in the ocean's velocity field, acting across pre-existing temperature gradients, redistribute heat away from regions of strong passive heat convergence. Over broad scales, this advective response can be estimated from changes in upper ocean density alone, using the Thermal Wind relation. These advective changes smooth spatial gradients in Added heat and alter the distribution of subtropical pycnocline depth.  Together, these results highlight the strong geostrophic coupling between Added and Redistributed heat, emphasizing the importance of subtropical and mid-latitude ocean dynamics on the evolution of the future climate response.

How to cite: Newsom, E., Zanna, L., and Khatiwala, S.: The influence of geostrophic coupling between Added and Redistributed heat on ocean warming patterns, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10319, https://doi.org/10.5194/egusphere-egu21-10319, 2021.

EGU21-8358 | vPICO presentations | OS1.1 | Highlight

Ocean Heat Transport’s Response to Future Climate Projections

Jennifer Mecking and Sybren Drijfhout

This study investigates the response of the meridional Ocean Heat Transports (OHT) to future climate projections in both CMIP5 and CMIP6 models.  Globally the OHT transport is declining/becoming more southward across all latitudes in the Northern Hemisphere, while at latitudes south of 10°S the OHT is icreasing/becoming more northward.  These changes in OHT are much stronger in CMIP6 models relative to CMIP5, especially for the rcp2.6/ssp126 scenario relative to the rcp85/ssp585 scenario.   Throughout the entire Atlantic basin the northward heat transport is reduced and can be tied to the velocity driven overturning (Atlantic Meridional Overturning Circulation (AMOC)) contribution to the OHT.  While the temperature driven changes in the Atlantic basin dampen the changes in the OHT.  In the Indo-Pacific basin the OHT transport north of the equator does not change much since the temperature and velocity driven changes balance each other.   However, south of the equator the increase in northward heat transport is caused by the overturning velocity driven changes and again dampened by temperature driven changes.  These changes in the Indo-Pacific basin can be tied to changes in wind driven subtropical overturning cells.

How to cite: Mecking, J. and Drijfhout, S.: Ocean Heat Transport’s Response to Future Climate Projections, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8358, https://doi.org/10.5194/egusphere-egu21-8358, 2021.

EGU21-7514 | vPICO presentations | OS1.1

Comparing inferred surface energy fluxes with observation-based flux estimates over the ocean

Johannes Mayer, Michael Mayer, and Leopold Haimberger

We combine atmospheric energy transports from ECMWF's latest reanalysis dataset ERA5 with observation-based TOA fluxes from CERES-EBAF to infer net surface energy fluxes (FSinf) for the period 1985-2018. We present an extensive comparison at scales ranging from global to local using 15 in-situ buoy measurements, parameterized surface fluxes from ERA5, and previous evaluations of FSinf using ERA-Interim. We also combine FSinf with various estimates of the ocean heat content tendency (OHCT) and observation-based oceanic heat transports from RAPID and moorings in Fram Strait and Barents Sea Opening to evaluate the oceanic energy budget in the North Atlantic Ocean basin.

Our results show that the indirectly estimated FSinf has a 1985-2018 ocean mean of 1.7 W m-2 (see J.Mayer et al. (2021); under review), which is in good agreement with the long-term mean OHCT derived from ocean reanalyses as well as independent surface flux estimates presented in recent literature (e.g., von Schuckmann et al. (2020); https://doi.org/10.5194/essd-12-2013-2020), suggesting an only small global ocean mean bias of FSinf. Moreover, our FSinf product is temporally more stable than parameterized surface fluxes from ERA5 and previous FSinf estimates using ERA-Interim, at least from 2000 onwards. The evaluation of the oceanic energy budget in the North Atlantic shows good agreement between FSinf and observation-based divergence of oceanic heat transports and OHCT such that its residual is on the order of <0.2 PW (~7 W m-2). Even on station-scale, FSinf agrees reasonably well with buoy-based surface flux measurements with a bias of 19.7 W m-2 over all 15 buoys  (compared to 21.7 W m-2 for parameterized surface fluxes), with largest biases in the Indian Ocean. This assessment demonstrates that our inferred surface flux estimate using ERA5 data outperforms parameterized fluxes from the model on all considered spatial scales (global-regional-local) in terms of bias and temporal stability and thus is well-suited for climate studies and model evaluations.

 

How to cite: Mayer, J., Mayer, M., and Haimberger, L.: Comparing inferred surface energy fluxes with observation-based flux estimates over the ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7514, https://doi.org/10.5194/egusphere-egu21-7514, 2021.

EGU21-14331 | vPICO presentations | OS1.1

What determines the position of the transition zone between alpha and beta regions in the ocean? A model study.

Romain Caneill, Fabien Roquet, Gurvan Madec, and Jonas Nycander

The stratification is primarily controlled by the temperature in subtropical regions (alpha ocean), and by salinity in subpolar regions (beta ocean). Between these two regions lies a transition zone where intense frontal systems are usually found, either in the Southern Ocean or in the North Atlantic and North Pacific basins. Transition zones are often characterized by deep mixed layers in winter responsible for the ventilation of intermediate layers. Here we want to investigate what determines the latitudinal position of the transition zone. It is generally assumed that this position is set by the wind stress pattern forcing Ekman downwelling, however the position of the transition zone does not match so well the wind stress convergence zone in the observations. Another possibility would be that it is controlled by the distribution of air-sea fluxes. The equation of state (EOS) for seawater determines the relative impact of heat and freshwater forcing on the buoyancy forcing. A key property of seawater is that the density becomes less sensitive to temperature at low temperatures (caused by an important nonlinearity of the EOS), increasing the effect of salinity on the stratification in polar region. We hypothesize that the decreasing of the relative influence of temperature on density is a major component in setting the position of the transition zone. To test this hypothesis, we developed an idealized triple-gyre configuration with the ocean global circulation model NEMO (Nucleus for European Modelling of the Ocean). A range of simplified EOS have been ran to test the effect of the buoyancy forcing on the position of the transition zone and the convective area. Under restoring conditions for the temperature and the salinity, augmenting or reducing the sensitivity of the density to the temperature is used as a way to modify the relative contribution of temperature and salinity to the buoyancy forcing. We show that the position of the convective area corresponds to a surface density maximum and is not directly related to the Ekman pumping zone. Moreover, alpha - beta ocean distinction becomes possible because the EOS is nonlinear. The first order influence of the forcing evolution on setting the localization of the transition zone and the associated deep water formation challenges the classical theories of thermocline ventilation by Ekman pumping.

How to cite: Caneill, R., Roquet, F., Madec, G., and Nycander, J.: What determines the position of the transition zone between alpha and beta regions in the ocean? A model study., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14331, https://doi.org/10.5194/egusphere-egu21-14331, 2021.

EGU21-9942 | vPICO presentations | OS1.1

Ocean model formulation influences climate sensitivity

Tido Semmler, Johann Jungclaus, Christopher Danek, Helge F Goessling, Nikolay Koldunov, Thomas Rackow, and Dmitry Sidorenko

The climate sensitivity is known to be mainly determined by the atmosphere model but here we discover that the ocean model can change a given transient climate response (TCR) by as much as 20% while the equilibrium climate sensitivity (ECS) change is limited to 10%. In our study, two different coupled CMIP6 models (MPI-ESM and AWI-CM) in two different resolutions each are compared. The coupled models share the same atmosphere-land component ECHAM6.3, which has been developed at the Max-Planck-Institute for Meteorology (MPI-M). However, as part of MPI-ESM and AWI-CM, ECHAM6.3 is coupled to two different ocean models, namely the MPIOM sea ice-ocean model developed at MPI-M and the FESOM sea ice-ocean model developed at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). A reason for the different TCR is different ocean heat uptake through greenhouse gas forcing in AWI simulations compared to MPI-M simulations. Specifically, AWI-CM simulations show stronger surface heating than MPI-ESM simulations while the MPI-M model accumulates more heat in the deeper ocean. The vertically integrated ocean heat content is increasing stronger in MPI-M model configurations compared to AWI model configurations in the high latitudes. Strong vertical mixing in MPI-M model configurations compared to AWI model configurations seems to be key for these differences. The strongest difference in vertical ocean mixing occurs inside the Weddell Gyre, but there are also important differences in another key region, the northern North Atlantic. Over the North Atlantic, these differences materialize in a lack of a warming hole in AWI model configurations and the presence of a warming hole in MPI-M model configurations. All these differences occur largely independent of the considered model resolutions.

How to cite: Semmler, T., Jungclaus, J., Danek, C., Goessling, H. F., Koldunov, N., Rackow, T., and Sidorenko, D.: Ocean model formulation influences climate sensitivity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9942, https://doi.org/10.5194/egusphere-egu21-9942, 2021.

EGU21-12801 | vPICO presentations | OS1.1

Using the depth of the centre of gravity as an indicator on the state of the general ocean circulation

Benjamin Schmiedel and Fabien Roquet

An approach is here investigated that uses the depth of the centre of gravity as a central ocean property, thought to give a clear and practical indicator on the state of the general ocean circulation. The depth of the gravity centre can be directly linked to the volume-integral of potential energy, or of dynamic enthalpy when making the Boussinesq approximation, and therefore to the strength of the global mean stratification. Because the stratification is directly linked to the global overturning circulation, it is hypothesized that the depth of the centre of gravity can be used to assess the state of global circulation. In order to test this hypothesis, the depth of the centre of gravity is diagnosed in an ocean model simulation for an idealized square basin configuration with the NEMO model. The centre of gravity is compared to the value it would have if the ocean was perfectly well mixed, giving a state of maximum potential energy. We find in our idealized simulation that the centre of gravity is lowered by only 22 cm compared to the reference well-mixed state, reflecting the potential energy that would be required to destroy the ocean stratification. The smallness of that number highlights the inefficiency of the ocean engine. Furthermore, the dynamic balance setting the depth of the gravity centre is investigated, diagnosing separately the tendency terms on the equation of conservation of potential energy. A positive change (sinking) of the centre of gravity indicates an input of high density water into lower levels or low density water in upper levels, essentially enhancing the global mean stratification, while for a negative change (lifting) it is reversed. The goal is to compare the relative role of the wind stress, surface buoyancy forcing and internal mixing in setting the general circulation.

How to cite: Schmiedel, B. and Roquet, F.: Using the depth of the centre of gravity as an indicator on the state of the general ocean circulation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12801, https://doi.org/10.5194/egusphere-egu21-12801, 2021.

EGU21-10037 * | vPICO presentations | OS1.1 | Highlight

Global-scale surface salinity change since the 1870s.Implications for the global hydrological cycle

W John Gould and Stuart Cunningham

Based on the first ever combined analysis of observations from the round-the-world voyages of HMS Challenger and SMS Gazelle in the 1870s, early in the industrial era, this paper shows that the amplification of the global surface salinity signal (saline areas becoming saltier and fresh areas fresher) has increased by 63±5% since the 1950s compared to the period 1870s to 1950s. Other analyses of regional salinity change between the mid-20th century and present day have linked this amplification to anthropogenically-driven strengthening of the global hydrological cycle in line with increasing global temperatures. Our results show that the rate of change has indeed accelerated but more closely in line with changes in sea surface temperature than with surface air temperature over almost 150 years. This is the first global-scale analysis of salinities from these two expeditions in the 1870s and the first observational evidence of changes in the global hydrological cycle since the late 19th century.

How to cite: Gould, W. J. and Cunningham, S.: Global-scale surface salinity change since the 1870s.Implications for the global hydrological cycle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10037, https://doi.org/10.5194/egusphere-egu21-10037, 2021.

EGU21-9630 | vPICO presentations | OS1.1

Historical changes in fresh water transport from sub-tropical to sub-polar oceans

Taimoor Sohail, Jan Zika, Damien Irving, and John Church

Warming-induced global water cycle changes pose a significant threat to biodiversity and humanity.  The atmosphere transports freshwater from the sub-tropical ocean to the tropics and poles in two distinct branches. The resulting air-sea fluxes of fresh water and river run-off imprint on ocean salinity (S) at different temperatures (T), creating a characteristic `T-S curve' of mean salinity as a function of temperature. Using a novel tracer-percentile framework, we quantify changes in the observed T-S curve from 1970 to 2014.  The warming ocean has been characterised by freshening tropical and sub-polar oceans and salinifying sub-tropical oceans. Over the 44 year period investigated, a net poleward freshwater transport out of the sub-tropical ocean is quantified, implying an amplification of the net poleward atmospheric freshwater transport. Historical reconstructions from the 6th Climate Model Intercomparison Project (CMIP6) exhibit a different response, underestimating the peak salinification of the ocean by a factor of 4, and showing a weak freshwater transport into the sub-polar ocean. Results indicate this discrepancy between the observations and models may be attributed to consistently biased representations of evaporation and precipitation patterns, which lead to the the weaker amplification seen in CMIP6 models.

How to cite: Sohail, T., Zika, J., Irving, D., and Church, J.: Historical changes in fresh water transport from sub-tropical to sub-polar oceans, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9630, https://doi.org/10.5194/egusphere-egu21-9630, 2021.

EGU21-11063 | vPICO presentations | OS1.1

Decadal changes in the storage of anthropogenic carbon in the Atlantic Ocean

Verónica Caínzos, Fiz F. Pérez, Antón Velo, Cristina Arumí-Planas, Melania Cubas Armas, Daniel Santana-Toscano, M. Dolores Pérez-Hernández, and Alonso Hernández-Guerra

The Atlantic Meridional Circulation (AMOC) plays a major role in the life cycle of nutrients and chemical species in the ocean, as they are introduced into the ocean by deep water formation and resurface as part of the upwelling. We aim to obtain decadal changes in the latitudinal and vertical distribution of nutrients and carbon species in the Atlantic Ocean, using data from three inverse models carried out for the 1990-99, 2000-09 and 2010-19.  We have used in situ quality-controlled data from GLODAPv2, the neural network CANYON-B for nutrients, and total alkalinity and dissolved inorganic carbon. We then compute the transport of each property, taking into account the results of mass transport balance from the inverse model for each decade. The inverse model has been applied to the whole Atlantic basin with 11 neutral density layers. With these results, we will be able to find out if the CO2 variability arises from changes in circulation or from other processes. On top of that, the availability of several zonal sections for the Atlantic enables the latitudinal division in boxes in which we may find differences in the regional anthropogenic carbon uptake. Our results will allow us to estimate how much anthropogenic carbon is being released or captured within each box, as well as the balance for other variables related to the carbon cycle.

How to cite: Caínzos, V., Pérez, F. F., Velo, A., Arumí-Planas, C., Cubas Armas, M., Santana-Toscano, D., Pérez-Hernández, M. D., and Hernández-Guerra, A.: Decadal changes in the storage of anthropogenic carbon in the Atlantic Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11063, https://doi.org/10.5194/egusphere-egu21-11063, 2021.

EGU21-13682 | vPICO presentations | OS1.1

Towards inferring the variability in oceanic CO2 fluxes at high latitudes using atmospheric O2 observations

Nicolas Mayot, Corinne Le Quéré, Andrew Manning, Ralph Keeling, and Christian Rödenbeck

The oceanic CO2 sink displays year-to-year to decadal variabilities which are not fully reproduced by global ocean biogeochemistry models, especially in the high-latitude oceans. Oceanic CO2 is influenced by the same climate variability and the same ecosystem processes as oceanic oxygen (O2), although in different proportions. Unlike for CO2, oceanic O2 flux is not influenced directly by the rise in atmospheric CO2, and therefore its variability reflects purely climatic and biogeochemical variability and trends. Therefore, natural climate variability and changes in oceanic processes controlling air-sea exchanges of CO2 can be studied by focusing on oxygen (O2), where the signal is unencumbered by direct anthropogenic influence. A global time series of oceanic O2 flux was obtained by building a global O2 budget, with an approach similar to the one used for the global carbon budget. The global O2 budget is based on atmospheric O2 observations and fossil fuel statistics, and infers the partitioning of the land and ocean fluxes using constant C:O2 ratios for land processes. One key result of this analysis is that air-sea O2 exchange induced significant year-to-year variability in observed atmospheric O2. Estimates of regional oceanic O2 fluxes were obtained from an atmospheric transport inversion analysis that inferred air-sea O2 exchange based on global atmospheric O2 observations and a global atmospheric transport model. For the Southern Ocean, a comparison was made between time series of winter oceanic O2 fluxes from this inversion method and winter mixed layer depths from Argo floats. Results from this comparison confirmed the previously suggested relationship between the winter ocean mixing and air-sea O2 exchange, which might be controlled by the climate variability induced by the Southern Annular Mode. Finally, these global and regional air-sea O2 fluxes were compared with outputs from six global ocean biogeochemistry models to examine their current skills in simulating O2 variability. Preliminary results suggested that all models underestimated the interannual variability in oceanic O2 fluxes, however they were able to simulate some of the observed multi-annual variability in O2 fluxes at high latitudes. We discuss the implications for the model’s representation of the variability in CO2 fluxes.

How to cite: Mayot, N., Le Quéré, C., Manning, A., Keeling, R., and Rödenbeck, C.: Towards inferring the variability in oceanic CO2 fluxes at high latitudes using atmospheric O2 observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13682, https://doi.org/10.5194/egusphere-egu21-13682, 2021.

EGU21-10367 | vPICO presentations | OS1.1

The impact of climate change on swell events significant wave heights from multiple origins

Gil Lemos, Alvaro Semedo, Mark Hemer, Melisa Menendez, and Pedro Miranda

Swell waves dominate the ocean surface, propagating across ocean basins, with minor attenuation. Here, a state-of-the-art swell tracking algorithm is applied to a global dynamic ensemble of CMIP5 wave climate simulations, isolating swell events from the remaining local sea state conditions based on the behavior of the peak wave period (Tp) and peak mean wave direction (MWDp). The swell events related significant wave height (Hs) projected changes for the late 21st century, as well as the overall contribution of swells from different origins to the total Hs projections, are then characterized. The propagation of the projected changes, from the overlaying winds (U10) at the wave generation areas, to the swell arrival locations, through swell waves, is also analyzed and quantified. Results indicate that the arriving swells’ Hs projected changes, along the tropical and subtropical latitudes, are highly dependent on the direction of the incoming waves, being mostly compatible with the Hs and U10 projections at the respective wave generation areas, especially when statistical significance is accounted for. Clear implications on sediment transport, coastal accretion and erosion, and offshore infrastructures and navigation arise from the disproportionate flux of energy carried by swell waves in each direction, increasing the need for adequate measures to mitigate its effects, towards the end of the 21st century.

How to cite: Lemos, G., Semedo, A., Hemer, M., Menendez, M., and Miranda, P.: The impact of climate change on swell events significant wave heights from multiple origins, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10367, https://doi.org/10.5194/egusphere-egu21-10367, 2021.

EGU21-5460 | vPICO presentations | OS1.1

Increased ocean heat transport into the Nordic Seas and Arctic Ocean over the period 1993-2016

Takamasa Tsubouchi, Kjetil Våge, Bogi Hansen, Karin Larsen, Svein Østerhus, Clare Johnson, Steingrímur Jónsson, and Héðinn Valdimarsson

Warm water of subtropical-origin flows northward in the Atlantic Ocean and transports heat to high latitudes. This poleward heat transport has been implicated as one possible cause of the declining sea ice extent and increasing ocean temperatures across the Nordic Seas and Arctic Ocean, but robust estimates are still lacking. Here we use a box inverse model and over 20 years of volume transport measurements to show that the mean ocean heat transport was 305±26 TW for 1993-2016. A significant increase of 21 TW occurred after 2001, which is sufficient to account for the recent accumulation of heat in the northern seas. Therefore, ocean heat transport may have been a major contributor to climate change since the late 1990s. This increased heat transport contrasts with the Atlantic Meridional Overturning Circulation (AMOC) slowdown at mid-latitudes and indicates a discontinuity of the overturning circulation measured at different latitudes in the Atlantic Ocean.

How to cite: Tsubouchi, T., Våge, K., Hansen, B., Larsen, K., Østerhus, S., Johnson, C., Jónsson, S., and Valdimarsson, H.: Increased ocean heat transport into the Nordic Seas and Arctic Ocean over the period 1993-2016, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5460, https://doi.org/10.5194/egusphere-egu21-5460, 2021.

EGU21-5888 | vPICO presentations | OS1.1

A centennial-scale Arctic - North Atlantic recharge oscillator in a coupled climate model

Robin Waldman, Christophe Cassou, and Aurore Voldoire

In global climate models, low-frequency natural variability related to the Atlantic Ocean overturning circulation is a common behaviour. Such intrinsic climate variability is a potential source of decadal climate predictability. However, over longer term scenario simulations, this natural variability becomes a major source of uncertainty. In this study, we document a large and sustained centennial variability in the 3500-year pre-industrial control run of the CNRM-CM6 coupled climate model which is driven by the North Atlantic ocean, and more specifically its meridional overturning circulation (AMOC). We propose a new AMOC dynamical decomposition highlighting the dominant role of mid-depth density anomalies at the western boundary as the driver of this centennial variability. We relate such density variability to deep convection and overflows in the western subpolar gyre, themselves controlled by and intense salinity variability of the upper layers. Finally, we show that such salinity variability is the result of periodic freshwater recharge and descharge events from the Arctic Ocean, themselves triggered by stochastic atmospheric forcing.

How to cite: Waldman, R., Cassou, C., and Voldoire, A.: A centennial-scale Arctic - North Atlantic recharge oscillator in a coupled climate model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5888, https://doi.org/10.5194/egusphere-egu21-5888, 2021.

The Nordic and Barents Seas play a critical role in the climate system resulting from water mass transformation, triggered by intense air-sea heat fluxes, that is an integral component of the Atlantic Meridional Overturning Circulation (AMOC). These seas are undergoing rapid warming, associated with a retreat in ice cover. Here we present a novel analysis, covering the period 1950-2020, of the spatiotemporal variability of the air-sea heat fluxes along the region’s boundary currents, where the impacts on the water mass transformation are large.  We find that the variability is a function of the relative orientation of the current and the axis of sea-ice change that can result in up to a doubling of the heat fluxes over the period of interest. This implies enhanced water mass transformation is occurring along these currents. In contrast, previous work has shown a reduction in fluxes in the interior sites of the Nordic Seas, where ocean convection is also observed, suggesting that a reorganization may be underway in the nature of the water mass transformation, that needs to be considered when ascertaining how the AMOC will respond to a warming climate.

How to cite: Moore, K., Våge, K., Renfrew, I., and Pickart, B.: Evolving air-sea interaction due to sea-ice retreat points to a re-organisation of water mass transformation in the Nordic and Barents Seas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10382, https://doi.org/10.5194/egusphere-egu21-10382, 2021.

Greenland Ice Sheet melt and freshening of the Arctic Ocean lead to increased discharge of freshwater into the East Greenland Current. If this additional freshwater reaches the convective regions of the Subpolar North Atlantic it could weaken deep mixing and affect the strength of the Atlantic Meridional Overturning Circulation. In particular, freshwater exported away from the South-East Greenland shelf could affect deep convection in the Irminger Sea, which has recently been shown to have a key role in the Atlantic overturning circulation. Though export of fresh shelf surface water is well observed west of Greenland, there is still little insight into surface water export from the East Greenland shelf to the Irminger Sea.

The East Greenland Current Drifter Investigation of Freshwater Transport drifter deployment conducted in August 2019 at 65°N on the eastern side of Greenland, resulted in five out of 30 drifters being exported away from the east Greenland shelf, four of which were exported at Cape Farewell. The specific wind regime at Cape Farewell is a potential driving factor for enhanced freshwater export in the area. While persistent south-eastward barrier winds push surface waters to the coast over most of the eastern shelf, Cape Farewell experiences strong eastward wind events such as tip-jets that could cause off-shelf export. Using wind data from the ERA-5 atmospheric reanalysis, we compute Ekman transport along the east Greenland shelf. We find greater probability for off-shelf Ekman transport at Cape Farewell than along the rest of the shelf, confirming that the area is the most likely to contribute to wind-driven freshwater export to the Irminger Sea. Wind and surface velocity data from a high-resolution model (2 km) are used to further investigate and quantify freshwater export at Cape Farewell and how it relates to local wind events.

How to cite: Duyck, E. and De Jong, F.: Freshwater export from the south-east Greenland shelf into the Irminger Sea and relation to wind events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12036, https://doi.org/10.5194/egusphere-egu21-12036, 2021.

EGU21-9128 | vPICO presentations | OS1.1

Characteristics and variability of ocean ventilation in the high-latitude North Atlantic in an eddy-permitting ocean model

Helen L. Johnson, Graeme MacGilchrist, David P. Marshall, Camille Lique, Matthew Thomas, Laura Jackson, and Richard Wood

A substantial fraction of the deep ocean is ventilated in the high latitude North Atlantic. As a result, the region plays a crucial role in transient climate change through the uptake of carbon dioxide and heat. We investigate the nature of ventilation in the high latitude North Atlantic in an eddy-permitting numerical ocean circulation model using a set of comprehensive Lagrangian trajectory experiments. Backwards-in-time trajectories from a model-defined ‘North Atlantic Deep Water’ (NADW) reveal the times and locations of subduction from the surface mixed layer at high temporal and spatial resolution. The major fraction (∼60%) of NADW ventilation results from subduction directly into the Labrador Sea boundary current, with a smaller fraction (∼25%) arising from open ocean deep convection in the Labrador Sea. There is a notable absence of ventilation arising from subduction in the Greenland–Iceland–Norwegian Seas, due to the re-entrainment of those waters as they move southward. Temporal variability in ventilation arises both from changes in subduction — driven by large-scale atmospheric forcing — and from year-to-year changes in the subsurface retention of newly subducted water, the result of an inter-annual equivalent of Stommel’s mixed layer demon. This interannual demon operates most effectively in the open ocean where newly subducted water is slow to escape its region of subduction. Thus, while subduction in the boundary current dominates NADW ventilation, processes in the open ocean set the variability, mediating the translation of inter-annual variations in atmospheric forcing to the ocean interior.

How to cite: Johnson, H. L., MacGilchrist, G., Marshall, D. P., Lique, C., Thomas, M., Jackson, L., and Wood, R.: Characteristics and variability of ocean ventilation in the high-latitude North Atlantic in an eddy-permitting ocean model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9128, https://doi.org/10.5194/egusphere-egu21-9128, 2021.

EGU21-2503 | vPICO presentations | OS1.1

Mechanisms of decadal North Atlantic climate variability and implications for the recent cold anomaly

Marius Årthun, Robert C. J. Wills, Helen L. Johnson, Léon Chafik, and Helene R. Langehaug

There has recently been a large focus on identifying the mechanisms responsible for Atlantic multidecadal variability (AMV). However, decadal-scale variability embedded within the AMV has received less attention, despite being a prominent feature of observed North Atlantic sea surface temperature (SST) and important for the climate of adjacent continents. These decadal fluctuations in the North Atlantic Ocean are also a key source of skill in decadal climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations and pre-industrial control simulations, we identify a decadal mode of atmosphere-ocean variability in the North Atlantic with a dominant time scale of 13-18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air-sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This strongly suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability, and that we might expect it to become less pronounced over the next few years.

How to cite: Årthun, M., Wills, R. C. J., Johnson, H. L., Chafik, L., and Langehaug, H. R.: Mechanisms of decadal North Atlantic climate variability and implications for the recent cold anomaly, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2503, https://doi.org/10.5194/egusphere-egu21-2503, 2021.

EGU21-10159 | vPICO presentations | OS1.1

Mechanisms linking the Labrador Sea with subtropical Atlantic overturning

Yavor Kostov, Marie-José Messias, Helen Johnson, Herlé Mercier, and David Marshall

We analyze the causal chain linking sea surface buoyancy anomalies in the Labrador Sea and variability in the subtropical Atlantic meridional overturning circulation (AMOC) in the ECCO ocean state estimate on inter-annual timescales. Our study highlights the importance of Lower North Atlantic Deep Water (LNADW) for the north-south connectivity in the Atlantic Ocean. We identify important mechanisms that allow the Labrador Sea to impact the southward transport of LNADW. We show that NAC plays an essential role in the export of buoyancy anomalies from the Labrador Sea – and it furthermore exerts a positive feedback that amplifies these upper ocean anomalies in the eastern subpolar gyre – before they reach the denser water masses along the lower limb of the AMOC. Our results also highlight the contribution of the western Labrador Sea for the surface uptake of tracers that penetrate the LNADW near Denmark Strait, which has implications for the redistribution of ocean heat anomalies.

How to cite: Kostov, Y., Messias, M.-J., Johnson, H., Mercier, H., and Marshall, D.: Mechanisms linking the Labrador Sea with subtropical Atlantic overturning, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10159, https://doi.org/10.5194/egusphere-egu21-10159, 2021.

EGU21-11693 | vPICO presentations | OS1.1

On the Seasonal variability eastern boundary of the North Atlantic Subtropical Gyre

Maria Dolores Pérez-Hernández, Pedro Vélez-Belchí, Verónica Caínzos, Daniel Santana-Toscano, Cristina Arumí-Planas, Melania Cubas Armas, Carmen Presas Navarro, and Alonso Hernández-Guerra

On the eastern region of the North Atlantic Subtropical Gyre, the Canary Current connects the Azores Current with the North Equatorial Current. Several studies link the seasonality of the AMOC (as measured by the RAPID program) to the seasonality of the main flows existing on the Canary basin. Since 2003, the RaProCan project which is the Canary Islands component of the Spanish Institute of Oceanography ocean observing system, monitors the Canary basin. In 2015, the RaProCan project joined efforts with the Seasonal Variability of the AMOC: Canary Current (SeVaCan) project of the Instituto de Oceanografía y Cambio Global (IOCAG) to increase the temporal resolution of the observations. Hence, during 2015 a hydrographic cruise took place in each season (February, April, July, and November) to complete the seasonal cycle of the basin. Here we present results from these cruises to describe the seasonal cycle of the area. A sensitive analysis is carried out to understand the representativeness of the cycle to be able to compare it with the AMOC seasonal cycle.

How to cite: Pérez-Hernández, M. D., Vélez-Belchí, P., Caínzos, V., Santana-Toscano, D., Arumí-Planas, C., Cubas Armas, M., Presas Navarro, C., and Hernández-Guerra, A.: On the Seasonal variability eastern boundary of the North Atlantic Subtropical Gyre, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11693, https://doi.org/10.5194/egusphere-egu21-11693, 2021.

EGU21-12120 | vPICO presentations | OS1.1

Western Boundary of the North Atlantic Subtropical Gyre: Decadal Change

Daniel Santana-Toscano, M. Dolores Pérez-Hernández, Verónica Caínzos, Melania Cubas Armas, Cristina Arumí-Planas, María Casanova-Masjoan, and Alonso Hernández-Guerra

The A20 is a meridional hydrographic section located at 52ºW on the western North Atlantic Subtropical Gyre that encloses the path of the water masses of the Atlantic Meridional Overturning Circulation (AMOC). Using data from three A20 hydrographic cruises carried out in 1997, 2003 and 2012 together with LADCP-SADCP data and the velocities from an inverse box model, the circulation of the western North Atlantic Subtropical Gyre is estimated. The main poleward current of the AMOC is the Gulf Stream (GS) which carries 129.0±10.5 Sv in 2003 and 110.4±12.2 Sv in 2012. Due to the seasonality, the GS position is shifted southward in 2012 - relative to that of 2003 - as both cruises took place in different seasons. In opposite direction, the Deep Western Boundary Current (DWBC) crosses the section twice, first at 39.3-43.2ºN (-34.9±7.5 Sv in 2003 and -25.3±9.4 Sv in 2012) and then at 7.0-11.7ºN (42.0±8.0 Sv in 2003 and 48.0±8.1 Sv in 2012). Additionally, two zonal currents contribute with westward transport below 20ºN: the North Equatorial Current and the North Brazil Current; with a net value of -28.0±4.1 Sv in 2003 and -36.7±3.6 Sv in 2012.

How to cite: Santana-Toscano, D., Pérez-Hernández, M. D., Caínzos, V., Cubas Armas, M., Arumí-Planas, C., Casanova-Masjoan, M., and Hernández-Guerra, A.: Western Boundary of the North Atlantic Subtropical Gyre: Decadal Change, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12120, https://doi.org/10.5194/egusphere-egu21-12120, 2021.

EGU21-15743 | vPICO presentations | OS1.1

Western boundary circulation and sea level patterns in northern hemisphere oceans

Samuel Diabaté, Didier Swingedouw, Joël Hirschi, Aurélie Duchez, Philip Leadbitter, Ivan Haigh, and Gerard McCarthy

The sea level changes along the Atlantic coast of the US have received a lot of attention recently because of an increased rate of rise north of the Gulf Stream separation point since the late 1980s (Sallenger et al., 2012 ; Boon, 2012). While sea-level rise is a major issue for coastal community, sea-level measurements in the region are key to understand the past of the nearby Gulf Stream and the large-scale ocean dynamics. Tide gauges on the coastline have measured the inshore sea-level for many decades and provide a unique window on past oceanic circulation. So far, numerous studies have linked the interannual to multi-decadal coastal sea-level changes to ocean dynamics, including the Gulf Stream strength, the divergence of the Sverdrup transport in the basin interior and the Atlantic meridional overturning circulation. However, other studies argue that local and regional processes, such as the alongshore winds or the river discharges, are processes of greater importance to the coastal sea level.

The general picture in the Atlantic is hence unclear. Yet, the northwest Atlantic is not the only western boundary region where sea-level has been well sampled. In this study we extend the analysis to the northwest Pacific, where links between the state of the Kuroshio and sea-level are evident (Kawabe, 2005; Sasaki et al., 2014). We discuss similarities and dissimilarities between the western boundary regions. We show for each basin, that the inshore sea level upstream the separation points is in sustained agreement with the meridional shifts of the western boundary current extension. This indicates that long duration tide gauges, such as Fernandina Beach (US) and Hosojima (Japan) could be used as proxies for the Gulf Stream North Wall and the Kuroshio Extension state, respectively.

References:

Boon, J. D. (2012). Evidence of sea level acceleration at US and Canadian tide stations, Atlantic Coast, North America. Journal of Coastal Research, 28(6), 1437-1445. 

Kawabe, M. (2005). Variations of the Kuroshio in the southern region of Japan: Conditions for large meander of the Kuroshio. Journal of oceanography, 61(3), 529-537.

Sallenger, A. H., Doran, K. S., & Howd, P. A. (2012). Hotspot of accelerated sea-level rise on the Atlantic coast of North America. Nature Climate Change, 2(12), 884-888.

Sasaki, Y. N., Minobe, S., & Miura, Y. (2014). Decadal sea‐level variability along the coast of Japan in response to ocean circulation changes. Journal of Geophysical Research: Oceans, 119(1), 266-275.

How to cite: Diabaté, S., Swingedouw, D., Hirschi, J., Duchez, A., Leadbitter, P., Haigh, I., and McCarthy, G.: Western boundary circulation and sea level patterns in northern hemisphere oceans, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15743, https://doi.org/10.5194/egusphere-egu21-15743, 2021.

EGU21-1164 | vPICO presentations | OS1.1

Decomposing the time-mean Atlantic Meridional Overturning Circulation and its variability with latitude.

Tomas Jonathan, Mike Bell, Helen Johnson, and David Marshall

The Atlantic Meridional Overturning Circulations (AMOC) is crucial to our global climate, transporting heat and nutrients around the globe. Detecting  potential climate change signals first requires a careful characterisation of inherent natural AMOC variability. Using a hierarchy of global coupled model  control runs (HadGEM-GC3.1, HighResMIP) we decompose the overturning circulation as the sum of (near surface) Ekman, (depth-dependent) bottom velocity, eastern and western boundary density components, as a function of latitude. This decomposition proves a useful low-dimensional characterisation of the full 3-D overturning circulation. In particular, the decomposition provides a means to investigate and quantify the constraints which boundary information imposes on the overturning, and the relative role of eastern versus western contributions on different timescales. 

The basin-wide time-mean contribution of each boundary component to the expected streamfunction is investigated as a function of depth, latitude and spatial resolution. Regression modelling supplemented by Correlation Adjusted coRrelation (CAR) score diagnostics provide a natural ranking of the contributions of the various components in explaining the variability of the total streamfunction. Results reveal the dominant role of the bottom component, western boundary and Ekman components at short time-scales, and of boundary density components at decadal and longer timescales.

How to cite: Jonathan, T., Bell, M., Johnson, H., and Marshall, D.: Decomposing the time-mean Atlantic Meridional Overturning Circulation and its variability with latitude., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1164, https://doi.org/10.5194/egusphere-egu21-1164, 2021.

EGU21-2111 | vPICO presentations | OS1.1

Net surface heat fluxes and Meridional Overturning Circulations

Mike Bell, Pat Hyder, Twm Jonathan, Helen Johnson, David Marshall, David Storkey, and Richard Wood

The geographical patterns of the annual mean net surface heat fluxes (NSHF) simulated by the HadGEM3 GC3.1 coupled atmosphere-ocean models are shown to agree well with the CDEEP analyses. The patterns for the coarse resolution (N96O1) and high resolution (N512O12) simulations are shown to be similar (except near the “cold pool of death”). We argue that they can be interpreted relatively simply in terms of (a) regions of net surface heating where Ekman pumping provides a supply of cold water at the sea surface and (b) regions of net cooling where boundary currents have taken warm water poleward. We extend the simple models of Gnanadesikan (1999), Nikurashin & Vallis (2011) and Bell (2015) for the mid-depth Meridional Overturning Circulation (MOC) to a simple model describing the upper and mid-depth MOC cells. As a first step in investigating whether these ideas simulate the model circulations “realistically”, we show that in the HadGEM3 Pacific Ocean, time-variations in the annual and zonal mean NSHF within 5o of the equator are well correlated (r2=0.6) with those in the annual and zonal mean wind stress along the equator. Finally we explore a warm, salty wedge of water next to the eastern boundary in the north Atlantic N96O1 pre-industrial simulations and interpret its northward heat transport in terms suggested by Bell (2015).     

This work is distributed under the Creative Commons Attribution 4.0 License. This licence does not affect the Crown copyright work, which is re-usable under the Open Government Licence (OGL). The Creative Commons Attribution 4.0 License and the OGL are interoperable and do not conflict with, reduce or limit each other.

How to cite: Bell, M., Hyder, P., Jonathan, T., Johnson, H., Marshall, D., Storkey, D., and Wood, R.: Net surface heat fluxes and Meridional Overturning Circulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2111, https://doi.org/10.5194/egusphere-egu21-2111, 2021.

EGU21-5333 | vPICO presentations | OS1.1

Interaction of Atlantic Meridional Overturning Circulation and Sub-Polar Gyre on decadal timescale

Stephen Ogungbenro, Leonard Borchert, Sebastian Brune, Vimal Koul, Levke Caesar, and André Düsterhus

North Atlantic climate variability is dominated by two important subsystems, the Atlantic Meridional Overturning Circulation (AMOC) and the Sub-Polar Gyre (SPG). While the AMOC is responsible for the transport of mass and heat into higher latitudes, SPG has been linked with large-scale changes in the subpolar marine environment. The changes in strength, intensity and positions of the constituent currents of the SPG impose variabilities in the distribution of heat and salt in the North Atlantic Ocean. Consequently, the predictability on decadal scales of the two subsystems is of huge importance for the understanding of variability in the North Atlantic.

Our contribution investigates the decadal and multi-decadal predictability of these subsystems within the Max Planck Institute for Meteorology Earth System Model (MPI-ESM). We analyse the model’s capability to predict these subsystems as well as the dependence of the two subsystems on each other. These investigations open new opportunities for a better understanding of the impact of the North Atlantic onto important marine ecosystems and its changes in the upcoming decade.

How to cite: Ogungbenro, S., Borchert, L., Brune, S., Koul, V., Caesar, L., and Düsterhus, A.: Interaction of Atlantic Meridional Overturning Circulation and Sub-Polar Gyre on decadal timescale, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5333, https://doi.org/10.5194/egusphere-egu21-5333, 2021.

EGU21-2664 | vPICO presentations | OS1.1

Wind-driven Oscillations in the Meridional Overturning Circulation near the equator

Adam Blaker, Michael Bell, Joel Hirschi, and Amy Bokota

Numerical model studies have shown the meridional overturning circulation (MOC) to exhibit variability on near-inertial timescales, and also indicate a region of enhanced variability on the equator. We present an analysis of a set of integrations of a global configuration of a numerical ocean model, which show very large amplitude oscillations in the MOCs in the Atlantic, Indian and Pacific oceans confined to the equatorial region. The amplitude of these oscillations is proportional to the width of the ocean basin, typically about 100 (200) Sv in the Atlantic (Pacific). We show that these oscillations are driven by surface winds within 10°N/S of the equator, and their periods (typically 4-10 days) correspond to a small number of low mode equatorially trapped planetary waves. Furthermore, the oscillations can be well reproduced by idealised wind-driven simulations linearised about a state of rest. Zonally integrated linearised equations of motion are solved using vertical normal modes and equatorial meridional modes representing Yanai and inertia-gravity waves. Idealised simulations capture between 85% and 95% of the variance of matching time-series segments diagnosed from the NEMO integrations. Similar results are obtained for the corresponding modes in the Atlantic and Indian Oceans. Our results raise questions about the roles of inertia-gravity waves near the equator in the vertical transfer of heat and momentum and whether these transfers will be explicitly resolved by ocean models or need to be parametrised.

How to cite: Blaker, A., Bell, M., Hirschi, J., and Bokota, A.: Wind-driven Oscillations in the Meridional Overturning Circulation near the equator, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2664, https://doi.org/10.5194/egusphere-egu21-2664, 2021.

EGU21-5315 | vPICO presentations | OS1.1

Meridional overturning circulation at 30ºS in the Pacific Ocean: 1992, 2003, 2009 and 2017.

Cristina Arumí-Planas, Maria Casanova-Masjoan, Verónica Caínzos, Daniel Santana-Toscano, Melania Cubas Armas, M. Dolores Pérez-Hernández, and Alonso Hernández-Guerra

The meridional circulation and transports at 32oS in the Pacific Ocean in 1992 and 2017 are compared with analogous data from 2003 and 2009. The hydrographic data comes from the GO-SHIP database and an inverse box model has been applied with several constraints. In 1992, 2003 and 2017 the pattern of the overturning streamfunction is similar, but in 2009 the pattern of the circulation changes in the whole water column. The horizontal distribution of mass transports at all depths in 1992 and in 2017 changes notably from the “bowed gyre” found in 2009 and resembles that regular shape of 2003. The hydrographic data have also been compared with analogous data obtained from the numerical modelling output of GFDL, ECCO, and SOSE. Results show that the numerical modelling output in the upper layers (γn<27.58 kg/m3) have a roughly similar pattern as hydrographic data. This is not the case, however, for deep and bottom layers (γn>27.58 kg/m3), where noticeable differences are found. Furthermore, the temperature transport in 2009 (0.16 ± 0.12 PW) is significantly lower than in 1992 (0.42 ± 0.12 PW), 2003 (0.38 ± 0.12 PW) and 2017 (0.42 ± 0.12 PW). In addition, the freshwater transport result in 2009 (0.50 ± 0.03 Sv) is significantly higher than in 1992 (0.26 ± 0.08 Sv), 2003 (0.25 ± 0.02 Sv) and 2017 (0.34 ± 0.08 Sv). Westward Rossby waves are presumably the dynamical forcing that changes the circulation pattern in 2009.

How to cite: Arumí-Planas, C., Casanova-Masjoan, M., Caínzos, V., Santana-Toscano, D., Cubas Armas, M., Pérez-Hernández, M. D., and Hernández-Guerra, A.: Meridional overturning circulation at 30ºS in the Pacific Ocean: 1992, 2003, 2009 and 2017., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5315, https://doi.org/10.5194/egusphere-egu21-5315, 2021.

EGU21-15083 | vPICO presentations | OS1.1

Heat distribution in the Tropical Indian Ocean during the prolonged La-Nina events during 1958–2017

Soumya Mukhopadhyay, C. Gnanaseelan, J.S. Chowdary, and Sandeep Mohapatra

In the present study, heat distribution in the Tropical Indian Ocean (TIO) associated with the prolonged La-Nina events during 1958–2017 is examined using reanalysis/observations. A detailed analysis revealed that in response to prolonged La-Nina forcing, prominent east-west thermocline gradient in the equatorial Indian Ocean and the eastward extension of thermocline ridge in the southwestern TIO (TRIO) are noted. Anomalous subsurface warming, thermocline deepening, and sea-level increase are also evident in the eastern and southeastern TIO and Bay of Bengal (BoB) during the prolonged La-Nina events. Cross equatorial volume transport near the eastern boundary during the prolonged La-Nina years especially at 50m-150m depth levels indicates the pathways of Pacific water entering the north Indian Ocean (NIO), a feature that has a strong impact on the BoB dynamics and thermodynamics. Intense cooling of TRIO and the Arabian Sea and the eastward extension of TRIO are some of the characteristic features of the prolonged La-Nina years. These may have strong implications on the air-sea interaction associated with inter-annual and intra-seasonal variability over this region. Further, the subsurface heat content (50m–150m) in the eastern and southeastern TIO in general dominated by interannual variability whereas the TRIO region experienced the decadal variability. Subsurface heat content variations associated with prolonged La Niña years are discussed. This study shows that the warming and cooling events of TIO are very closely tied to the internal dynamics of the IO driven remotely by the Pacific through modulation of surface winds.

How to cite: Mukhopadhyay, S., Gnanaseelan, C., Chowdary, J. S., and Mohapatra, S.: Heat distribution in the Tropical Indian Ocean during the prolonged La-Nina events during 1958–2017, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15083, https://doi.org/10.5194/egusphere-egu21-15083, 2021.

EGU21-3535 | vPICO presentations | OS1.1

Intraseasonal variations of Ocean Heat Content in the tropical Indian and Pacific Ocean

Ashneel Chandra, Noel Keenlyside, Lea Svendsen, and Awnesh Singh

The ocean heat content (OHC) is an important thermodynamical parameter in the Earth’s climate system as about 90% of the Earth’s Energy Imbalance (EEI) is stored in the ocean. It is therefore important to understand how this quantity varies on different timescales and how different thermodynamical and dynamical processes affect it. On intraseasonal timescales, there is a two-way interaction between the atmosphere and ocean whereby atmospheric forcing leads to ocean dynamics causing changes in OHC and OHC, in turn, possibly playing a role in affecting the intensity of the Madden-Julian Oscillation (MJO) through air-sea interactions. In this study, we focus on the variations of OHC in the equatorial Indian and Pacific Ocean on intraseasonal timescales. A heat budget analysis for the upper 100 m was performed using HYCOM Reanalysis for the period 2005 – 2015. The simple three-term heat budget comprised of a surface heat flux term (Q), an advection and adiabatic redistribution term (ADV) and finally a residual term (RES) to account for processes not resolved using the reanalysis product. When averaged over the equatorial Pacific Ocean, the heat budget analysis shows that the ADV and RES terms contributed the most to the ocean heat content tendency (OHCT). Zonal wind anomalies are observed to excite intraseasonal Kelvin waves in the equatorial Pacific Ocean. These Kelvin waves are associated with the eastward advection of intraseasonal OHC anomalies from the western Pacific warm pool to the central Pacific. This eastward propagation of intraseasonal OHC anomalies associated with Kelvin waves is seen to contribute to the warming leading to El Niño events such as the 2009 El Niño. In the Indian Ocean, intraseasonal OHC anomalies along the equator were seen to be in phase with the MJO as revealed by the negative intraseasonal outgoing longwave radiation (OLR) anomalies, while the off-equatorial intraseasonal OHC anomalies were seen to be out of phase with the MJO. Off-equatorial intraseasonal OHC anomalies in the Indian Ocean may be a useful parameter to investigate further as it may provide the residual heat energy for air-sea interactions for subsequent MJO events and hence improve subseasonal predictability.

How to cite: Chandra, A., Keenlyside, N., Svendsen, L., and Singh, A.: Intraseasonal variations of Ocean Heat Content in the tropical Indian and Pacific Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3535, https://doi.org/10.5194/egusphere-egu21-3535, 2021.

EGU21-6383 | vPICO presentations | OS1.1 | Highlight

Marine Heat Waves in the Peruvian Upwelling System: from 5-day localized warming to year-long El Niños

Alice Pietri, François Colas, Mogollon Rodrigo, Jorge Tam, and Dimitri Gutierrez

Extreme climatic events, such as marine heatwaves (MHWs), have been shown to globally increase in frequency and magnitude over the last decades, and can disrupt ecosystems significantly. Coastal upwelling systems, because they are biodiversity hot-spots and socioeconomic hubs, are particularly vulnerable to those rapidly developing anomalously warm marine events. The Peruvian coastal system in particular is highly exposed to climate variability because of its proximity to the equator. As such it is regularly impacted by El Niño events whose variability has been related to the longest and most intense MHWs in the region. However the intensively studied El Niño events tend to overshadow the MHWs of shorter duration that also have an important impact on the coastal environment as they can trigger other extreme events such as nearshore hypoxias and harmful algal blooms. 

Using 38 years of satellite sea surface temperature data, we investigate the characteristics (spatial variability, frequency, intensity and duration) and evolution of MHWs in the South Tropical Eastern Pacific, with a focus on the Peru Coastal Upwelling System. The separation of events by duration allows to identify a spectrum, from El Niño events to shorter scale MHWs. Results show that the statistical  distribution of MHWs properties, their spatial organization and preferential season of occurrence varies significantly in function of their duration. Besides, when removing large El Niño events, an increase of occurrences, duration and intensity is observed over the last 38 years, contrary to the reduction that is observed in the region when considering all MHWs. Finally, the possible drivers are discussed to disentangle the role of the local (wind stress) and remote (equatorial variability) forcing in function of the events duration.

How to cite: Pietri, A., Colas, F., Rodrigo, M., Tam, J., and Gutierrez, D.: Marine Heat Waves in the Peruvian Upwelling System: from 5-day localized warming to year-long El Niños, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6383, https://doi.org/10.5194/egusphere-egu21-6383, 2021.

EGU21-13806 | vPICO presentations | OS1.1

Variability and meandering of the East Australian Current jet at 27oS

Bernadette Sloyan, Christopher Chapman, Rebecca Cowley, and Thomas Moore

The East Australian Current (EAC) is the complex and highly energetic western boundary current of the South Pacific Ocean gyre. Low frequency (>2 year) variability of the EAC reflects the changes in the wind and buoyancy forcing over the South Pacific. However, local and regional wind and buoyancy forcing drives higher frequency variability (< 1-2 year) of the EAC. Due to the narrow shelf, the EAC-jet  meandering has an immediate impact on the continental shelf circulation. Here we use the IMOS EAC mooring array between May 2015 to September 2019 and satellite observational data to quantify the quantify the EAC variability and assess the potential drives and impact of the on-shelf meandering of the EAC jet on the properties of the Coral and Tasman Seas.

 

We find that there is considerable variability of Sea Surface Height (SSH) and Sea Surface temperature (SST) that at times co-vary, but at  other times the anomalies are opposed. We compare the surface anomalies with the EAC velocity and transport timeseries. The mean along-slope velocity vectors show poleward velocity dominates from 0-1500 m at the five mooring locations from the 500 m isobath to the deep abyssal basin with the strongest southward flow at the continental shelf. The variance ellipses show that the largest variability in EAC transport is in the along-shore direction. This indicates that the EAC variability is dominated by the movement of the EAC on- and off-shore. The EAC thus maintains its jet structure as it meanders onshore and offshore adjacent to the continental slope. While the mean along-shore velocity vectors provide a picture of the mean EAC, the time-series shows that the EAC has a complex and highly variable structure. Strong southward flow is associated with off-shore flow (positive across-slope velocity). While mostly measuring the EAC core we see times where the flow is northward (positive along-slope velocity). This northward velocity is due to the shelf flow extending from the coast to the shelf, and is generally associated with on-shore flow (negative across-slope velocity). These changes in the direction and strength of the velocity are driven by cyclonic eddies inshore of the jet, and have significant influence on the exchange between the open and shelf ocean.

How to cite: Sloyan, B., Chapman, C., Cowley, R., and Moore, T.: Variability and meandering of the East Australian Current jet at 27oS, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13806, https://doi.org/10.5194/egusphere-egu21-13806, 2021.

EGU21-13852 | vPICO presentations | OS1.1

Extreme ocean weather induced by upstream meandering of the East Australian Current 

Christopher Chapman, Bernadette Sloyan, and Madeleine Cahill

We investigate Marine Heat Waves and Marine Cold Spells (MHWs/MCSs) along the east coast of the Australian continent, a western boundary current region with exceedingly complex dynamics. We provide evidence that episodic MWHs/MCSs along the south-east of the Australian continent are driven by upstream variations in the position, but not necessarily the strength, of the East Australian Current, and that these variations are, in turn, controlled by small-scale (100s of kilometers) eddies that propagate into the region from the east. These eddies are able to alternately 'shut-off' and `turn-on' the poleward transport of warm water by the boundary current in a manner analogous to atmospheric blocking. Precursors to these `blocks' are detectable as much as 60 days prior to the onset of an event. We will discuss the implications of our results for the early prediction of MHW/MCS events.

How to cite: Chapman, C., Sloyan, B., and Cahill, M.: Extreme ocean weather induced by upstream meandering of the East Australian Current , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13852, https://doi.org/10.5194/egusphere-egu21-13852, 2021.

EGU21-1536 | vPICO presentations | OS1.1

Global wave number-4 pattern in the southern subtropics

Balaji Senapati, Mihir Dash, and Swadhin Behera

Presence of a stationary zonal wavenumber-4 (W4) pattern is revealed in the sea surface temperature (SST) anomaly over southern subtropics (20°S-55°S) using empirical orthogonal function analysis. This W4 pattern is found to be seasonally phase-locked to the austral summer (persists up to mid-autumn) and independent of other known tropical and extra-tropical climate phenomena. A thermodynamic coupling of atmosphere and the upper ocean helps in generating the W4 pattern, which later terminates due to the breaking of the ocean-atmosphere positive feedback. Due to anomalous convection over western subtropical pacific near the westerly jet, the signal appears first in the atmosphere during early November. Later, the disturbance gets trapped in the westerly waveguide which circumnavigates the globe and produces an atmospheric W4 pattern in early December (20-30 days later). Then, the signal transported to the ocean through the ocean-atmosphere feedback and sustained in the ocean (after it disappears from the atmosphere) as it has high specific heat capacity. During the positive phase of the W4 event, the cold SST anomaly develops over the southeastern and -western side (SE-NW) of Australia creating an anomalous divergence circulation. It favours the moisture transport towards the south-eastern region of the continent. Consequently, the specific humidity increases and causes an above-normal rainfall in a SE-NW axis over Australia. An opposite process is seen in case of a negative W4 event.

How to cite: Senapati, B., Dash, M., and Behera, S.: Global wave number-4 pattern in the southern subtropics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1536, https://doi.org/10.5194/egusphere-egu21-1536, 2021.

EGU21-13163 | vPICO presentations | OS1.1

Pathways connecting the North Brazil Current and the RAPID line

Kimberley Drouin, M Susan Lozier, F Javier Beron-Vera, Phillip Miron, and M Josefina Olascoaga

The North Brazil Current is considered a bottleneck in the South Atlantic, responsible for funneling upper-ocean waters into the North Atlantic. This work explores the surface and subsurface pathways that connect the North Brazil Current to the RAPID line. To that extent, observational trajectories from surface drifters and Argo floats are used in conjunction with Markov chain theory and tools from dynamical systems analysis to compute probable pathways. More specifically, these pathways are computed as ensembles of paths transitioning directly between the North Brazil Current and the RAPID line. In addition, simulated trajectories will be used (1) to assess how representative the two-dimensional observational trajectories are of the three-dimensional circulation, and (2) to compute the associated volume transport of different pathways. Preliminary results suggest that two dominant pathways connect the North Brazil Current and the RAPID line. First, is the traditional pathway through the Caribbean Sea and Gulf of Mexico, which carries waters to the Florida Current, and second is a more direct route east of the Caribbean that supplies waters to the Antilles Current and the basin interior.  

How to cite: Drouin, K., Lozier, M. S., Beron-Vera, F. J., Miron, P., and Olascoaga, M. J.: Pathways connecting the North Brazil Current and the RAPID line, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13163, https://doi.org/10.5194/egusphere-egu21-13163, 2021.

EGU21-12070 | vPICO presentations | OS1.1

Inter-decadal variations of surface winds off Peruvian coast

Sadegh Yari and Volker Mohrholz

The Peruvian upwelling system (PUS) is the most productive marine ecosystem among the Eastern Boundary upwelling Systems (EBUS). The trade wind system drives a nearly continuous upwelling which is subjected to variations on a wide range of times scales. The wind forced upwelling controls crucially the nutrient supply to the euphotic surface layer and thus, the overall productivity of the system.

Using long term data from ERA5 (1979-2019) the wind forcing in the PUS was analyzed to obtain information about long term trends in the mean state and its variability.

Beside the strong annual cycle, the wind forcing is dominated by interannual and a long term interdecadal oscillation.

The interannual fluctuations with a period of 2-5 years are related to the known events of El Niño and La Niña. The wind anomaly shows a good correlation with Oceanic Niño Index (ONI). Interdecadal variation of wind depict a main period of 15-20 years whit negative anomaly values from 1979 to 1996, and positive anomaly values for 1996-2014. These long term variations can be attributed to the Interdecadal Pacific Oscillations (IPO). The spatial distribution of wind stress along the Peruvian coast is not uniform. The highest values are observed in Lima-Marcona area (12º-15.4º S) while it decreases sharply southward and gradually northward. Additionally the coastal upwelling area is modulated locally by the coupling of wind and SST.

How to cite: Yari, S. and Mohrholz, V.: Inter-decadal variations of surface winds off Peruvian coast, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12070, https://doi.org/10.5194/egusphere-egu21-12070, 2021.

EGU21-1225 | vPICO presentations | OS1.1

Monitoring the Ocean Heat Content and the Earth Energy imbalance from space altimetry and space gravimetry: the MOHeaCAN product

Marti Florence, Ablain Michaël, Fraudeau Robin, Jugier Rémi, Meyssignac Benoît, Blazquez Alejandro, Restano Marco, and Benveniste Jérôme

The Earth Energy Imbalance (EEI) is a key indicator to understand climate change. However, measuring this indicator is challenging since it is a globally integrated variable whose variations are small, of the order of several tenth of W.m-2, compared to the amount of energy entering and leaving the climate system of ~340 W.m-2. Recent studies suggest that the EEI response to anthropogenic GHG and aerosols emissions is 0.5-1 W.m-2. It implies that an accuracy of <0.3 W.m-2 at decadal time scales is necessary to evaluate the long term mean EEI associated with anthropogenic forcing. Ideally an accuracy of <0.1 W.m-2 at decadal time scales is desirable if we want to monitor future changes in EEI.

In the frame of the MOHeaCAN project supported by ESA, the EEI indicator is deduced from the global change in Ocean Heat Content (OHC) which is a very good proxy of the EEI since the ocean stores 93% of the excess of heat  gained by the Earth in response to EEI. The OHC is estimated from space altimetry and gravimetry missions (GRACE). This “Altimetry-Gravimetry'' approach is promising because it provides consistent spatial and temporal sampling of the ocean, it samples nearly the entire global ocean, except for polar regions, and it provides estimates of the OHC over the ocean’s entire depth. Consequently, it complements the OHC estimation from the ARGO network. 

The MOHeaCAN product contains monthly time series (between August 2002 and June 2017) of several variables, the main ones being the regional OHC (3°x3° spatial resolution grids), the global OHC and the EEI indicator. Uncertainties are provided for variables at global scale, by propagating errors from sea level measurements (altimetry) and ocean mass content (gravimetry). In order to calculate OHC at regional and global scales, a new estimate of the expansion efficiency of heat at global and regional scales have been performed based on the global ARGO network. 

A scientific validation of the MOHeaCAN product has also been carried out performing thorough comparisons against independent estimates based on ARGO data and on the Clouds and the Earth’s Radiant energy System (CERES) measurements at the top of the atmosphere. The mean EEI derived from MOHeaCAN product is 0.84 W.m-2 over the whole period within an uncertainty of ±0.12 W.m-2 (68% confidence level - 0.20 W.m-2 at the 90% CL). This figure is in agreement (within error bars at the 90% CL) with other EEI indicators based on ARGO data (e.g. OHC-OMI from CMEMS) although the best estimate is slightly higher. Differences from annual to inter-annual scales have also been observed with ARGO and CERES data. Investigations have been conducted to improve our understanding of the benefits and limitations of each data set to measure EEI at different time scales.

The MOHeaCAN product from “altimetry-gravimetry” is now available and can be downloaded at https://doi.org/10.24400/527896/a01-2020.003. Feedback from interested users on this product are welcome.

How to cite: Florence, M., Michaël, A., Robin, F., Rémi, J., Benoît, M., Alejandro, B., Marco, R., and Jérôme, B.: Monitoring the Ocean Heat Content and the Earth Energy imbalance from space altimetry and space gravimetry: the MOHeaCAN product, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1225, https://doi.org/10.5194/egusphere-egu21-1225, 2021.

EGU21-3619 | vPICO presentations | OS1.1

GO-SHIP Easy Ocean: Formatted and gridded ship-based hydrographic section data

Katsuro Katsumata, Sarah Purkey, Rebecca Cowley, Bernadette Sloyan, Diggs Stephen, Thomas Moore, Lynne Talley, and James Swift

Despite numerous technological advances over the last several decades, ship-based hydrography remains the only method for obtaining high-quality, high spatial and vertical resolution measurements of a suite of physical, chemical, and biological parameters over the full water column essential for physical, chemical, and biological oceanography and climate science. The Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) coordinates a network of globally sustained hydrographic sections as part of the global ocean observing system, building on previous programs. These data provide a unique data set that spans four decades, comprised of  more than 40 cross-ocean transects, many with multiple repeats. The section data are, however, difficult to use owing to inhomogeneous format. The purpose of this data product is to increase the value of these data by better combining, reformatting and gridding in order to facilitate their use with less effort by a wider audience. The product is machine readable and readily accessible by many existing visualisation and analysis software packages. The data processing can be repeated with modifications to suit various applications such as analysis of deep ocean , validation of numerical simulation output, and calibration of autonomous platforms. This initial release includes temperature, salinity, and dissolved oxygen data from Conductivity-Temperature-Depth profiles, but the product will include other properties in future releases.

How to cite: Katsumata, K., Purkey, S., Cowley, R., Sloyan, B., Stephen, D., Moore, T., Talley, L., and Swift, J.: GO-SHIP Easy Ocean: Formatted and gridded ship-based hydrographic section data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3619, https://doi.org/10.5194/egusphere-egu21-3619, 2021.

EGU21-9175 | vPICO presentations | OS1.1

A New Estimate of Global Ocean Climatology

Kanwal Shahzadi, Nadia Pinardi, Marco Zavaterelli, and Simona Simoncelli

The estimation of climatology is a key element for improving our understanding of the ocean state. Historical data sets available today enables an almost complete reconstruction of global ocean fields. In this study, a new global ocean climatological estimate of basic physical parameters such as temperature, salinity, density, dissolved oxygen, and apparent oxygen utilization is computed using the World Ocean Database (WOD18). The reliability of estimate is closely tied to the quality assurance of the in-situ observations and statistical interpolation schemes of the mapping. Therefore, in this context, WOD18 used for this study has gone through a non-linear quality control procedure developed by Shahzadi (2020) on a global domain. The mapping of resulting data is carried out using Data Interpolating Variational Analysis (DIVA). Sensitivity experiments are carried out to choose the key parameters of DIVA, namely the horizontal correlation lengths, and the Noise to Signal ratio (N/S). Furthermore, two new indices such as roughness index, and root mean square of residuals are designed to show the impact of the correlation length, and N/S ratio choices. For temperature and salinity, two different versions of the climatological estimates are produced: (i) a long-term (1900 to 2017) climatology using multiple platforms in-situ data, and (ii) a shorter time estimate (2003-2017) using data from ocean drifting platforms such as profiling floats. The two versions are intercompared and differences are evaluated.  Similar procedures are applied for global mapping of Density, Oxygen, and Apparent Oxygen utilization. The new climatological estimates are compared with previous estimates such as World Ocean Atlas and World Argo Global Hydrographic climatological estimates, and thereby the differences are analysed.

Keywords: WOD18, temperature, salinity, apparent oxygen, DIVA, climatology, non-linear quality control.

Shahzadi, K., (2020): “A New Global Ocean Climatology”, Ph.D. Thesis (under evaluation), University of Bologna, Italy, pp. (19-35. of pages)

How to cite: Shahzadi, K., Pinardi, N., Zavaterelli, M., and Simoncelli, S.: A New Estimate of Global Ocean Climatology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9175, https://doi.org/10.5194/egusphere-egu21-9175, 2021.

EGU21-9651 | vPICO presentations | OS1.1 | Highlight

The Greenland Clipper: a fast ocean connection between Greenland and the Southern Ocean

Laurits Andreasen, Markus Jochum, Anna von der Heydt, Guido Vettoretti, and Roman Nuterman

The glacial Dansgaard-Oeschger (DO) events are thought to result in a global reorganization of oceanic heat fluxes and heat content.

DO events originate in the North Atlantic, but are communicated all the way to the pole of the other hemisphere. This interhemispheric coupling is known as the bipolar seesaw. A striking feature of the bipolar seesaw is the ~100 year time lag between the initial onset at high northern latitudes and the following adjustments at high southern latitudes.

Here, we focus on this time lag.

Ultimately high southern latitudes are expected to begin their adjustment, when the sea ice margin in the Southern Ocean (SO) shift position due to cooling/warming in the ocean below. But how is the northern signal propagated into the SO, and what processes control the time it takes the SO to change its state?

We expect the SO adjustment to have four components: Planetary waves, geostrophic adjustments in the Atlantic, vertical mixing and finally heat fluxes from baroclinic eddies in the SO.

To investigate the relative importance of these components on the adjustment time in the SO, we apply a fresh water perturbation at high northern latitude in an idealized setup of the Atlantic basin and the Southern Ocean using the newly developed OGCM VEROS. We measure the time it takes the model's Southern Ocean to adjust to the perturbation as a function of different model parameters associated with the components mentioned above.

We find that the adjustment time - which we believe is related to the bipolar seesaw time lag - is dominated by two components. The first is associated with geostrophic adjustment in the South Atlantic, and the second with the eddy heat fluxes in the Southern Ocean. Interestingly we find that in the limit of a high (realistic) eddy transfer (Gent-McWilliams) coefficient, the geostrophic component constitutes the main part of the the adjustment time and quantitatively matches the observed time lag in the bipolar seesaw.

This make us suggest that the bipolar seesaw time lag could be caused mainly by adjustments in the South Atlantic.

How to cite: Andreasen, L., Jochum, M., von der Heydt, A., Vettoretti, G., and Nuterman, R.: The Greenland Clipper: a fast ocean connection between Greenland and the Southern Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9651, https://doi.org/10.5194/egusphere-egu21-9651, 2021.

EGU21-15807 | vPICO presentations | OS1.1 | Highlight

Linking submesoscale fronts and air-sea heat fluxes in the Southern Ocean: Results from the first Saildrone circumnavigation of Antarctica

Hanna S. Rosenthal, Louise C. Biddle, Sebastiaan Swart, Sarah T. Gille, and Matthew R. Mazloff

The role of the Southern Ocean in the global heat and carbon cycle is fundamental towards our climate, but observational data to quantify air-sea fluxes, such as surface  heat  fluxes, are  still  scarce. In  order  to  investigate  the  effects  of  fine- scale oceanic fronts (0.1 km–10 km) on air-sea fluxes in the Southern Ocean, high-resolution  hydrographic  and  meteorological  data  collected  by  three  un-crewed surface vehicles (Saildrones) during their first Circumnavigation of Antarctica in 2019 was assessed. Comparisons of key variables from the in situ Saildrones datasets with those from ERA5 and a stationary mooring show good  agreement.  Temperature-driven density fronts were detected in the Saildrone data and their impact on the turbulent heat flux was quantified during steady atmospheric conditions.  Over 2000 surface ocean temperature dominated density fronts were detected at length-scales (i.e.  front width) ranging from sub-kilometer to mesoscale (order of 0.1 km–100 km). 
Temperature-driven density fronts with a length scale (as seen from the Saildrones perspective ) smaller than 1 km contributed 75% and 51% of the sensible and latent heat flux changes, respectively. The direct link between the fronts and the impact on the heat fluxes decreases sharply  when the front length increases. This suggests that smaller (submesoscale) fronts have a larger impact on heat flux variability than larger (balanced) fronts . The parametrization of  these  fine-scale ocean-atmospheric processes  in  global climate  models  could  lead to more accurate  representations  of  the  heat  flux  variability both at local and global scale.

How to cite: Rosenthal, H. S., Biddle, L. C., Swart, S., Gille, S. T., and Mazloff, M. R.: Linking submesoscale fronts and air-sea heat fluxes in the Southern Ocean: Results from the first Saildrone circumnavigation of Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15807, https://doi.org/10.5194/egusphere-egu21-15807, 2021.

Water cycle have prevailed on upper ocean salinity acting as the climate change fingerprint in the numerous observation and simulation works. Water mass in the Southern Ocean accounted for the increasing importance associated with the heat and salt exchanges between Subantarctic basins and tropical oceans. The circumpolar deep water (CDW), the most extensive water mass in the Southern Ocean, plays an indispensable role in the formation of Antarctic Bottom Water. In our study, the observed CTDs and reanalysis datasets are examined to figure out the recent salinity changes in the three basins around the Antarctica. Significant surface salinity anomalies occurred in the South Indian/Pacific sectors south of 60ºS since 2008, which are connected with the enhanced CDW incursion onto the Antarctic continental shelf. Saltier shelf water was found to expand northward from the Antarctica coast. Meanwhile, the freshening of Upper Circumpolar Deep Water(UCDW), salting and submergence of Subantarctic Mode Water(SAMW) were also clearly observed. The modified vertical salinity structures contributed to the deepen mixed layer and enhanced intermediate stratification between SAMW and UCDW. Their transport of salinity flux attributed to the upper ocean processes responding to the recent atmospheric circulation anomalies, such as the Antarctic Oscillation and Indian Ocean Dipole. The phenomena of SAMW and UCDW salinity anomalies illustrated the contemporaneous changes of the subtropical and polar oceans, which reflected the meridional circulation fluctuation. Salinity changes in upper southern ocean (< 2000m) revealed the influence of global water cycle changes, from the Antarctic to the tropical ocean, by delivering anomalies from high- and middle-latitudes to low-latitudes oceans.

How to cite: Du, L. and Ni, X.: Upper ocean salinity variabilities in the Southern Ocean responding to the recent surface salting in perspective of climate changes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4727, https://doi.org/10.5194/egusphere-egu21-4727, 2021.

EGU21-6308 | vPICO presentations | OS1.1

Characteristics and Variability of Antarctic Intermediate Water in the UKESM1-0-LL CMIP6 model

Ophélie Meuriot, Yves Plancherel, and Camille Lique

Antarctic Intermediate Water (AAIW) is the dominant intermediate water mass in the Southern Hemisphere. AAIW plays a key role in the hydrological cycle and also contributes to the replenishment of nutrients at low latitudes. It is characterised by a mid-depth salinity minimum. Although its salinity minimum signature can be clearly identified, the formation mechanisms and how its properties evolve with climate change are unclear.

The aim of this study is to assess the ability of the UKESM1-0-LL CMIP6 model to represent the key characteristics and variability of AAIW and to evaluate its evolution under radiative forcing (with the SSP5-8.5 and SSP2-4.5 scenarios).

A diagnostic is developed to identify the core of AAIW in the different basins and scenarios. AAIW can be identified in the UKESM1-0-LL model but it is lighter than in observations. The Pacific, Atlantic and Indian type of AAIW have core density values of 26.5 kg/m3, 26.6 kg/m3 and 26.9 kg/m3 respectively. AAIW presents different properties across each basin with different depth, temperature and salinity properties. The Pacific type of AAIW is lighter and fresher than the Atlantic and Indian types of AAIW. Under radiative forcing, it is found that AAIW shoals and becomes warmer. The largest changes in temperature, salinity and density are found in the Pacific. The outcrop location of the salinity minimum remains constant in the different scenarios in spite of the surface conditions changing with climate change.

A change in depth could have major implications on the overturning circulation. Ongoing and future work focuses on identifying which mechanisms need to be improved in CMIP6 models to reduce the bias observed in AAIW.

How to cite: Meuriot, O., Plancherel, Y., and Lique, C.: Characteristics and Variability of Antarctic Intermediate Water in the UKESM1-0-LL CMIP6 model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6308, https://doi.org/10.5194/egusphere-egu21-6308, 2021.

EGU21-9448 | vPICO presentations | OS1.1

A Lagrangian view of the transfer of Southern waters to the South Atlantic Ocean

Anna Olivé Abelló, Josep L. Pelegrí, and Ignasi Vallès-Casanova

The Atlantic Meridional Overturning Circulation (AMOC), a key component of the Earth's climate system, is sustained through the northward transport of Southern Ocean waters to high latitudes. This returning limb of the AMOC consists largely of relatively cold waters entering from the Pacific Ocean through the Drake Passage, what is commonly referred to as cold-water route. Here, we explore the pathways and transit times of these Antarctic waters that are incorporated to the South Atlantic, with special attention to their recirculation in the subtropical gyre and their escape northward through the North Brazil Current. For this purpose, we use daily values of the climatological GLORYS12v1 velocity field, as obtained using data for 2002-2018 and track the trajectories with the help of the OceanParcels software. We trace the particles transiting through four sections in the Southern and South Atlantic Oceans: 64°W and 27°E, crossing entire Antarctic Circumpolar Current (ACC) through the Drake Passage and off South Africa, respectively; 32°S, from the African coast out to 5°S, sampling the eastern boundary current system; and 21°S, from the American coast out to 30°W, sampling the North Brazil Current.

Particles are released daily in the Drake Passage down to 1800 m during one full year, its spatial distribution and number being proportional to the transport crossing each vertical portion of the section. This represents an annual-mean of 116.3 Sv entering the Atlantic sector through the Drake Passage, split into 13.3 Sv for surface (Subantarctic Surface Water, SASW, and Subantarctic Mode Water, SAMW), 40.2 Sv for intermediate (Antarctic Surface Water, AASW, and Antarctic Intermediate Water, AAIW) and 62.8 Sv for deep (Upper Circumpolar Deep Water, UCDW) water masses. The particles are then tracked forward, with a one-day resolution, during 20 years. The simulation shows that about 83% of the SASW/SAMW transport follow the ACC past South Africa while the remaining 17% are incorporated to the subtropical gyre. Among the latter, only 13% veer northward and cross the 21°S section. Regarding the intermediate waters, AASW/AAIW, 93% of transport follows the ACC, and 7% join the subtropical gyre. Finally, for the UCDW transport, which remains part of ACC, about 97% follow eastward as the ACC and only 3% drift cross the 32°S section, and only 4% of the latter reach through the 21°S section. The median times for the Drake Passage water particles to get to the 27°E, 32°S and 21°S sections are: 1.7, 2.1 and 5.7 yr for the SASW/SAMW; 2.3, 5.3 and 6.5 yr for the AASW/AAIW; and 3.3, 6.0 and 11.7 yr for the UCDW, respectively. Long tails in the age distributions reflect a high degree of recirculation, being remarkable the high presence of mesoscale eddies around 32°S over Cape Basin.

How to cite: Olivé Abelló, A., Pelegrí, J. L., and Vallès-Casanova, I.: A Lagrangian view of the transfer of Southern waters to the South Atlantic Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9448, https://doi.org/10.5194/egusphere-egu21-9448, 2021.

EGU21-9693 | vPICO presentations | OS1.1

Revisiting the Malvinas Current upper circulation and water masses using a HR ocean reanalysis

Camila Artana, Christine Provost, Léa Poli, Ramiro Ferrari, and Jean-Michel Lellouche

We use 25 years of ocean reanalysis to revisit the Malvinas Current, a major route of Antarctic Intermediate Waters, from the South (Drake Passage) to the North (Brazil-Malvinas Confluence) from the synoptic to interannual time scales. The MC mean surface velocity structure evolves as the geometry of the continental slope changes. Over the Malvinas Plateau, the slope is gentle, the MC is rather wide and is organized in two jets. As the slope steepens further north, the jets narrow, intensify and merge at 45°S. The MC appears as stable current over the 25 years connecting two of the major regions with high eddy kinetic energy (Drake Passage and the Brazil Malvinas Confluence). The MC plays a minor role in the velocity variations observed at the confluence at seasonal and interannual scales. Velocity variations at the confluence are related with changes in the intensity of the Brazil Current (BC), in particular, the summer intensification (+15 cm/s at the surface) of the BC (34°-36°S over the slope) advects into a winter intensification and southward displacement of the BC overshoot (40/44°S-54°W).  

The Malvinas Plateau is a key region for eddy activity dissipation and for water mass properties modification. Winter deep mixed layers occasionally reach 600 m south of 50°S on the Malvinas Plateau, and show large interannual variations. We compute the volume transport in the layer associated with the Subantarctic Surface Waters (SASW), Subantarctic Mode Waters (SAMW) and Antarctic Intermediate waters (AAIW) over sections spanning the 55 -41°S latitudinal range. The transport time series along the Patagonian slope have a mean of 27.1+- 0.1 Sv and a standard deviation decreasing from south (51°S) to north (45°S) from 4.6 and 3.4 Sv. Variations of SASW/SAMW/AAIW transport are small at the seasonal scale; in contrast, the transport times series vary over a range of 5 Sv at the interannual scale.  In general the transport time series covary and show an absolute minimum in 2004 of the order of 23+-2 Sv. This minimum was associated with a unique southward displacement of the BC overshoot leading to a blocking event at 48°S disconnecting the MC from its source in March, followed by a feeding event in May supplying polar waters reducing the SASW/SAMW/AAIW layer volume. Over the 25 years there is a significant freshening trend and no trend in volume transport. 

How to cite: Artana, C., Provost, C., Poli, L., Ferrari, R., and Lellouche, J.-M.: Revisiting the Malvinas Current upper circulation and water masses using a HR ocean reanalysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9693, https://doi.org/10.5194/egusphere-egu21-9693, 2021.

EGU21-9784 | vPICO presentations | OS1.1

Anatomy of subinertial waves along the Patagonian shelf break in a 1/12° global operational model

Lea Poli, Camila Artana, Christine Provost, Jérôme Sirven, Nathalie Sennéchael, Yannis Cuypers, and Jean-Michel Lellouche

The Patagonian slope hosts a variety of waves. We used a state of the art ocean reanalysis to examine waves at the shelf break and in the core of the Malvinas Current (MC) at periods larger than 10 days. Statistics over 25 years indicated three types of signals: in phase signals at specific locations of the shelf break to the south of 47°S, fast propagating signals all along the shelf break (phase speed from 140 cm/s to 300cm/s) at periods between  5 and  110 days, and slower signals in the core of the MC (phase speeds from 10 cm/s to 30cm/s) at 20-day, 60-day and 100-day periods. 

        The large zonal wind stress variations south of 47°S forced in-phase along-slope velocity variations and triggered fast propagating waves at distinct sites corresponding to abrupt changes in the shelf break orientation. The shelf break waves modulated the intensity of the inshore jet, which varied from 0 to 30 cm/s at 100 m depth, and had spatial and temporal structures and scales consistent with those of observed upwelling events. Slow propagating waves in the core of the MC had along-slope wavelengths between 450 and 1200 km and were not forced by the local winds. They were tracked back to the Drake Passage and the Malvinas Escarpment.

How to cite: Poli, L., Artana, C., Provost, C., Sirven, J., Sennéchael, N., Cuypers, Y., and Lellouche, J.-M.: Anatomy of subinertial waves along the Patagonian shelf break in a 1/12° global operational model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9784, https://doi.org/10.5194/egusphere-egu21-9784, 2021.

EGU21-14301 | vPICO presentations | OS1.1

Sea Level Anomalies in the Southern Ocean due to Thermohaline Variability

Fabien Roquet, Marlen Kolbe, Etienne Pauthenet, and David Nerini

The Southern Ocean is responsible for the majority of the global oceanic heat uptake which contributes to global sea level rise. At the same time, ocean temperature does not change everywhere at the same rate and salinity changes are also associated with sea level variability. Changes in heat and salt content drive together variations in the steric height that differ importantly in both time and space. This study investigates steric height variability in the Southern Ocean from 2008 to 2017 by analysing temperature and salinity variations obtained from global ocean reanalyses. The thermohaline variability is decomposed on so-called thermohaline modes using a functional Principal Component Analysis (fPCA). Thermohaline modes provide a natural basis on which to decompose the joint temperature-salinity vertical profiles into a sum of vertical modes weighted by their respective principal components. Steric height was computed in the reanalyses and related to the principal component using a Multiple Linear Regression (MLR) model. Trends in steric height are found to differ significantly between subtropical and subpolar regions, simultaneously which with a shift from a thermohaline stratification dominated by the first "thermocline" mode in the North to the second "saline" mode in the South. The Polar Front appears as a natural boundary between the two regions, where steric height variations are minimized. Since 2008, steric height has dropped close to the Antarctic continent, while subtropical waters farther north have mostly risen due to increased heat storage. While the dominant cause for the significant sea level rise south of 30S remains freshwater discharge from glaciers and ice sheets, thermohaline variability produces sizeable regional variability in the rate of sea level rise.

How to cite: Roquet, F., Kolbe, M., Pauthenet, E., and Nerini, D.: Sea Level Anomalies in the Southern Ocean due to Thermohaline Variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14301, https://doi.org/10.5194/egusphere-egu21-14301, 2021.

OS1.2 – The North Atlantic: natural variability and global change

EGU21-2928 | vPICO presentations | OS1.2

How confident are predictability estimates of the winter North Atlantic Oscillation?

Antje Weisheimer, Damien Decremer, David MacLeod, Chris O'Reilly, Tim Stockdale, Stephanie Johnson, and Tim Palmer

Predictions of the winter NAO and its small signal-to-noise ratio have been a matter of much discussion recently. Here we look at the problem from the perspective of 110-year-long historical hindcasts over the period 1901-2010 performed with ECMWF’s coupled model. Seasonal forecast skill of the NAO can undergo pronounced multidecadal variations: while skill drops in the middle of the century, the performance of the reforecasts recovers in the early twentieth century, suggesting that the mid-century drop in skill is not due to a lack of good observational data. We hypothesize instead that these changes in model predictability are linked to intrinsic changes of the coupled climate system. 

The confidence of these predictions, and thus the signal-to-noise behaviour, also strongly depends on the specific hindcast period. Correlation-based measures like the Ratio of Predictable Components are shown to be highly sensitive to the strength of the predictable signal, implying that disentangling of physical deficiencies in the models on the one hand, and the effects of sampling uncertainty on the other hand, is difficult. These findings demonstrate that relatively short hindcasts are not sufficiently representative for longer-term behaviour and can lead to skill estimates that may not be robust in the future.

See also: Weisheimer, A., D. Decremer, D. MacLeod, C. O'Reilly, T. Stockdale, S. Johnson and T.N. Palmer (2019). How confident are predictability estimates of the winter North Atlantic Oscillation? Q. J. R. Meteorol. Soc., 145, 140-159, doi:10.1002/qj.3446.

How to cite: Weisheimer, A., Decremer, D., MacLeod, D., O'Reilly, C., Stockdale, T., Johnson, S., and Palmer, T.: How confident are predictability estimates of the winter North Atlantic Oscillation?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2928, https://doi.org/10.5194/egusphere-egu21-2928, 2021.

EGU21-12580 | vPICO presentations | OS1.2

Variability of the North Atlantic Oscillation in the 20th century

André Düsterhus, Leonard Borchert, Vimal Koul, Holger Pohlmann, and Sebastian Brune

The North Atlantic Oscillation (NAO) has over the year a major influence on European weather. In many applications, being it in modern or paleo climate science, the NAO is assumed to varying in strength, but otherwise often understood as being a constant feature of the pressure system over the North Atlantic. In recent years investigations on the seasonal-predictability of the winter NAO has shown that the prediction skill is varying over time. This opens the question, why this is the case and how well models are able to represent the NAO in all its variability over the 20th century.

To investigate this further we take a look at a seasonal prediction of the NAO with the Max Planck Institute Earth System Model (MPI-ESM) seasonal prediction system, with 30 members over the 20th century. We analyse its dependence of prediction skill on various features of the NAO and the North Atlantic system, like the Atlantic Multidecadal Variability (AMV). As such we will demonstrate, that the NAO is a much less stable system over time as currently assumed and that models may not be in the position to predict its full variability appropriately.

How to cite: Düsterhus, A., Borchert, L., Koul, V., Pohlmann, H., and Brune, S.: Variability of the North Atlantic Oscillation in the 20th century, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12580, https://doi.org/10.5194/egusphere-egu21-12580, 2021.

EGU21-9501 | vPICO presentations | OS1.2

The Atlantic Multidecadal Oscillation controls the impact of the North Atlantic Oscillation on North European climate

Florian Börgel, Claudia Frauen, Thomas Neumann, and H. E. Markus Meier

European climate is heavily influenced by the North Atlantic Oscillation (NAO). However, the spatial structure of the NAO is varying with time, affecting its regional importance. By analyzing an 850-year global climate model simulation of the last millennium it is shown that the variations in the spatial structure of the NAO can be linked to the Atlantic Multidecadal Oscillation (AMO). The AMO changes the zonal position of the NAO centers of action, moving them closer to Europe or North America. During AMO+ states, the Icelandic Low moves further towards North America while the Azores High moves further towards Europe and vice versa for AMO- states. The results of a regional downscaling for the East Atlantic/European domain show that AMO-induced changes in the spatial structure of the NAO reduce or enhance its influence on regional climate variables of the Baltic Sea such as sea surface temperature, ice extent, or river runoff.

How to cite: Börgel, F., Frauen, C., Neumann, T., and Meier, H. E. M.: The Atlantic Multidecadal Oscillation controls the impact of the North Atlantic Oscillation on North European climate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9501, https://doi.org/10.5194/egusphere-egu21-9501, 2021.

EGU21-2671 | vPICO presentations | OS1.2

Response of Extratropical Cyclones when Crossing a Sea Surface Temperature Front

Hai Bui and Thomas Spengler

Sea surface temperatures (SSTs) can influence the development of extratropical cyclones by providing latent and sensible heat through surface fluxes as well as by modifying the environmental low-level baroclinicity. As surface fluxes as well as low-level baroclinicity maximize along the prominent SST fronts associated with the Gulf Stream and Kuroshio, the influence of these mechanisms on cyclone development is anticipated to be strongest along SST fronts. To map the sensitivity to the structure and position of SST fronts during the development of extratropical cyclones, we examine the response of cyclones when they cross an SST front at different directions and speeds. The results are based on idealized numerical simulations with the WRF model, where we prescribe moving SST fronts and a baroclinically unstable environment with an incipient cyclone. Cyclones moving towards the warmer side of the SST front deepen faster and have a faster crossing speed. The diabatic production of eddy available potential energy through latent heating, mainly associated with convection, plays a dominant role in the deepening. Cyclones that move to the colder side of the SST front weaken due to a reduction of available moisture for diabatic processes. However, before these cyclones weaken, they experience a brief period of faster deepening attributable to the enhanced environmental low-level baroclinicity associated with the SST gradient.

How to cite: Bui, H. and Spengler, T.: Response of Extratropical Cyclones when Crossing a Sea Surface Temperature Front, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2671, https://doi.org/10.5194/egusphere-egu21-2671, 2021.

EGU21-10219 | vPICO presentations | OS1.2

Atmospheric response to Gulf Stream SST front shifting: impact of horizontal resolution in an ensemble of global climate models

Luca Famooss Paolini, Alessio Bellucci, Paolo Ruggieri, Panos Athanasiadis, and Silvio Gualdi

Western boundary currents transport a large amount of heat from the Tropics toward higher latitudes; furthermore they are characterized by a strong sea surface temperature (SST) gradient, which anchors zones of intense upward motion extending up to the upper-troposphere and shapes zones of intense baroclinic eddy activity (storm tracks). For such reasons they have been shown to be fundamental in influencing the climate of the Northern Hemisphere and its variability, and a potentially relevant source of atmospheric predictability. 

 

General circulation models show deficiencies in simulating the observed atmospheric response to SST front variability. The atmospheric horizontal resolution has been recently proposed as a key element in understanding such differences. However, the number of studies on this subject is still limited. Furthermore, a multi-model analysis to systematically investigate differences between low-resolution and high-resolution atmospheric response to oceanic forcing is still lacking. 

 

The present work has the objective to fill this gap, analysing the atmospheric response to Gulf Stream SST front shifting using data from recent High Resolution Model Intercomparison Project (HighResMIP). This project was designed with the specific objective of investigating the impact of increased model horizontal resolution on the representation of the observed climate. Ensembles of historical simulations performed with three atmospheric general circulation models (AGCMs) have been analysed, each conducted with a low-resolution (LR, about 1°) and a high-resolution (HR, about 0.25°) configuration. AGCMs have been forced with observed SSTs (HadISST2 dataset), available at daily frequency on a 0.25° grid, during 1950–2014. 


Results show atmospheric responses to the SST-induced diabatic heating anomalies that are strongly resolution dependent. In LR simulations a low-pressure anomaly is present downstream of the SST anomaly, while the diabatic heating anomaly is mainly balanced by meridional advection of air coming from higher latitudes, as expected for an extra-tropical shallow heat source. In contrast, HR simulations generate a high-pressure anomaly downstream of the SST anomaly, thus driving positive temperature advection from lower latitudes (not balancing diabatic heating). Along the vertical direction, both in LR and HR simulation, the diabatic heating in the interior of the atmosphere is balanced by upward motion south of GS SST front and downward motion north and further south of the Gulf Stream. Finally, LR simulations show a reduction in storm-track activity over the North Atlantic, whereas HR simulations show a meridional displacement of the storm-track considerably larger (yet in the same direction) than that of the SST front. HR simulations reproduce the atmospheric response obtained from observations, albeit weaker. This is a hint for the existence of a positive feedback between ocean and atmosphere, as proposed in previous studies. These findings are qualitatively consistent with previous results in literature and, leveraging on recent coordinated modelling efforts, shed light on the effective role of atmospheric horizontal resolution in modelling the atmospheric response to extra-tropical oceanic forcing.

How to cite: Famooss Paolini, L., Bellucci, A., Ruggieri, P., Athanasiadis, P., and Gualdi, S.: Atmospheric response to Gulf Stream SST front shifting: impact of horizontal resolution in an ensemble of global climate models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10219, https://doi.org/10.5194/egusphere-egu21-10219, 2021.

Decadal variability in indices of North Atlantic (NA) atmospheric circulation plays a major role in changing climate over western Europe. However, reproducing characteristics of this variability in climate models presents a major challenge. Climate models broadly exhibit weaker-than-observed multi-decadal variability in atmospheric circulation indices. A prominent explanation for this is that model-simulated links between anomalous sea-surface temperatures (SSTs) and atmospheric variability are too weak. The dominant mode of basin-wide NA SST variability is Atlantic multi-decadal variability (AMV), which on multi-decadal timescales is expressed more strongly over the NA sub-polar gyre (SPG). SSTs over the SPG region (SSTSPG) are therefore the main focus here.

Studies to date have shown that variability in the North Atlantic Oscillation (NAO) exhibits strongest correlations with AMV indices in late winter, but the reasons for this are not clear. Here we show that this stronger late-winter correlation is particularly clear for SSTSPG and coincides with a climatological equatorward shift of the eddy-driven NA westerly jet from early-to-late winter. To help gain dynamical insight, indices of eddy-driven jet latitude (JLI) and speed (JSI) were correlated with SSTSPG and it was found that they exhibit more pronounced early-to-late winter shifts in correlations than for the NAO; In particular,  correlations strengthen from early-to-late winter for JLI while weaken for JSI. Our results suggest that the jet-SSTSPG linkages progress through winter from JSI dominant in early winter to JLI dominant in late winter.

CMIP5 and CMIP6 models were then evaluated for representation of these observed characteristics in ocean-atmosphere linkages. Consistent with the observed sub-seasonal links between climatological jet latitude and atmosphere-ocean correlation strength, CMIP models with larger equatorward jet biases exhibit weaker JSI-SSTSPG correlations and stronger JLI-SSTSPG correlations. A pronounced early-winter equatorward bias in jet latitude in CMIP models could partially explain the weaker-than observed linkage between SSTs and atmospheric variability.  

How to cite: Bracegirdle, T., Lu, H., and Robson, J.: Observed and CMIP-simulated links between North Atlantic climatological winter jet latitude and inter-annual to decadal ocean-atmosphere coupling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7685, https://doi.org/10.5194/egusphere-egu21-7685, 2021.

EGU21-1310 | vPICO presentations | OS1.2

North Atlantic Ocean Circulation Response to Stochastic Mesoscale Weather Systems

Shenjie Zhou, Xiaoming Zhai, and Ian Renfrew

The ocean is forced by the atmosphere on a range of spatial and temporal scales. In ocean and climate models the resolution of the atmospheric forcing sets a limit on the scales that are represented. For typical climate models this means mesoscale (< 400 km) atmospheric forcing is absent. Previous studies have demonstrated that mesoscale forcing significantly affects key ocean circulation systems such as the North Atlantic Subpolar gyre and the Atlantic Meridional Overturning Circulation (AMOC). However, the approach of these studies has either been ad hoc or limited in resolution. Here we present ocean model simulations with and without realistic mesoscale atmospheric forcing that represents scales down to 10 km. We use a novel stochastic parameterization – based on a cellular automaton algorithm that is common in weather forecasting ensemble prediction systems – to represent spatially coherent weather systems over a range of scales, including down to the smallest resolvable by the ocean grid. The parameterization is calibrated spatially and temporally using marine wind observations. The addition of mesoscale atmospheric forcing leads to coherent patterns of change in the sea surface temperature and mixed-layer depth. It also leads to non-negligible changes in the volume transport in the North Atlantic subtropical gyre (STG) and subpolar gyre (SPG) and in the AMOC. A non-systematic basin-scale circulation response to the mesoscale wind perturbation emerges – an in-phase oscillation in northward heat transport across the gyre boundary, partly driven by the constantly enhanced STG, correspoding to an oscillatory behaviour in SPG and AMOC indices with a typical time scale of 5-year, revealing the importance of ocean dynamics in generating non-local ocean response to the stochastic mesoscale atmospheric forcing. Atmospheric convection-permitting regional climate simulations predict changes in the intensity and frequency of mesoscale weather systems this century, so representing these systems in coupled climate models could bring higher fidelity in future climate projections.

How to cite: Zhou, S., Zhai, X., and Renfrew, I.: North Atlantic Ocean Circulation Response to Stochastic Mesoscale Weather Systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1310, https://doi.org/10.5194/egusphere-egu21-1310, 2021.

The southward flow of North Atlantic Deep Water makes up the major component of the AMOC's deepwater limb. In the subtropical North Atlantic, it's flow is concentrated along the continental slope, forming a coherent Deep Western Boundary Current (DWBC). Both, observations and models show a high variability of the flow in this region.
We use an eddy-rich ocean model to show that this variability is mainly caused by eddies and meanders that are generated by barotropic instability. They occur along the entire DWBC pathway and introduce several reciruculation gyres that result in a decorrelation of DWBC transport at 26.5°N and 16°N, despite the fact that a considerable mean transport of 20 Sv connects the two latitudes. Water in the DWBC at 26.5°N is partly returned northward. Because the amount of water returned depends on the DWBC transport itself, a stronger DWBC does not necessarily lead to an increased amount of water that reaches 16°N. 
Along the pathway to 16°N, the transport signal is altered by a broad and temporally variable transit time distribution. Thus, advection in the DWBC cannot account for coherent AMOC changes on interannual timescales seen in the model.

How to cite: Schulzki, T., Getzlaff, K., and Biastoch, A.: On the variability of the DWBC transport between 26.5°N and 16°N in an eddy-rich ocean model and its implications for meridionally coherent changes., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2403, https://doi.org/10.5194/egusphere-egu21-2403, 2021.

EGU21-5071 | vPICO presentations | OS1.2

AMOC Evolution at 47°N in the Last Decades in Observations and a High-Resolution Ocean Model

Simon Wett, Monika Rhein, Arne Biastoch, Claus Böning, and Klaus Getzlaff

The Atlantic Meridional Overturning Circulation (AMOC) plays an important role for the climate system of Europe and the Arctic. It is responsible for the northward transport of warm and saline water in the upper water column and the southward transport of cold and fresh water in the deep.

Since the early 2000s, observations from ship-based measurements and moorings are available which allow estimates of the individual components of the AMOC. However, the spatial resolution of mooring measurements is coarse and ship-based surveys are mostly done only once a year, adding to the uncertainty of these measurements. Earlier observational studies in the subpolar North Atlantic have found decadal trends of individual AMOC components. However, whether the entirety of the AMOC exhibits a trend remains unclear. Due to the observational limitations, most knowledge about the recent AMOC development is based on model simulations. Comparing these model simulations with observations remains an important task to understand the changes in the AMOC strength in the last decades and improve model representations of the AMOC.

We analyze a realization of the high-resolution VIKING20X ocean model from 1980 to 2019 offering a large overlap with the available observations. We compare it to measurements of the NOAC array at 47°N and sections obtained from repeated ship surveys. We aim to merge observations and model simulation to better estimate recent AMOC changes and increase our understanding of the underlying processes.

How to cite: Wett, S., Rhein, M., Biastoch, A., Böning, C., and Getzlaff, K.: AMOC Evolution at 47°N in the Last Decades in Observations and a High-Resolution Ocean Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5071, https://doi.org/10.5194/egusphere-egu21-5071, 2021.

EGU21-11010 | vPICO presentations | OS1.2

The Irminger Sea – air-ice-ocean interactions in a 5 km coupled simulation with ICON-ESM

Oliver Gutjahr, Johann H. Jungclaus, Nils Brüggemann, Helmuth Haak, and Jochem Marotzke

Recent observations suggest that deep convection and water mass transformation in the Irminger Sea southeast of Greenland, together with overflows from the Nordic Seas, may be more important for the variability of the Atlantic meridional overturning circulation (AMOC) than the Labrador Sea. The preconditioning for and triggering of deep convection in the Irminger Sea is strongly associated with topography-induced mesoscale wind phenomena, such as Greenland tip jets, katabatic winds and marine cold air outbreaks. However, the resolution of current coupled climate models is too coarse to capture all the properties of these wind systems or to capture them at all. Here we explore the air-ice-ocean interactions induced by mesoscale wind phenomena in the Irminger Sea in a 1-year global coupled 5km simulation with ICON-ESM. The model is able to capture the complex interactions of the wind field and the ocean. We find that strong downward katabatic winds cause substantial heat loss from the Irminger Sea in addition to Greenland tip jets. The outflowing katabatic winds form narrow streaks of cold air that extend across the entire Irminger basin from southeast Greenland to Iceland. In addition, cold air outbreaks from the sea ice lead to the genesis of mesoscale cyclones, which in turn can cause Greenland tip jets before moving off to the east. All these wind phenomena cause substantial heat loss that preconditions the ocean for deep convection. If these wind systems are not resolved, the water mass transformation in the Irminger Sea could be too weak, contributing to why the Labrador Sea dominates AMOC variability in models. We conclude that resolving these mesoscale wind systems in an Earth system model could have significant implications for deep convection and water mass transformation in the Irminger Sea, and thus for AMOC variability.

How to cite: Gutjahr, O., Jungclaus, J. H., Brüggemann, N., Haak, H., and Marotzke, J.: The Irminger Sea – air-ice-ocean interactions in a 5 km coupled simulation with ICON-ESM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11010, https://doi.org/10.5194/egusphere-egu21-11010, 2021.

There is a controversy about the nature of multidecadal climate variability in the North Atlantic (NA) region, concerning the roles of ocean circulation and atmosphere-ocean coupling. Here we describe NA multidecadal variability from a version of the Kiel Climate Model, in which both subpolar gyre (SPG)-Atlantic Meridional Overturning Circulation (AMOC) and atmosphere-ocean coupling are essential. The oceanic barotropic streamfuntions, meridional overturning streamfunctions, and sea level pressure are jointly analyzed to derive the leading mode of Atlantic variability. This mode accounting for about 23.7 % of the total combined variance is oscillatory with an irregular periodicity of 25-50 years and an e-folding time of about a decade. SPG and AMOC mutually influence each other and together provide the delayed negative feedback necessary for maintaining the oscillation. An anomalously strong SPG, for example, drives higher surface salinity and density in the NA’s sinking region. In response, oceanic deep convection and AMOC intensify, which, with a time delay of about a decade, reduces SPG strength by enhancing upper-ocean heat content. The weaker gyre circulation leads to lower surface salinity and density in the sinking region, which eventually reduces deep convection and AMOC strength. There is a positive ocean-atmosphere feedback between the sea surface temperature and low-level atmospheric circulation over the Southern Greenland area, with related wind stress changes reinforcing SPG changes, thereby maintaining the (damped) multidecadal oscillation against dissipation. Stochastic surface heat-flux forcing associated with the North Atlantic Oscillation drives the eigenmode.

How to cite: Sun, J., Latif, M., and Park, W.: Subpolar Gyre – AMOC – Atmosphere Interactions on Multidecadal Timescales in a Version of the Kiel Climate Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4608, https://doi.org/10.5194/egusphere-egu21-4608, 2021.

EGU21-2725 | vPICO presentations | OS1.2

OSNAP and water mass transport in CMIP6 models

Laura Jackson

The Atlantic Meridional Overturning Circulation (AMOC) influences our climate by transporting heat northwards in the Atlantic ocean. The subpolar North Atlantic plays an important role in this circulation, with transformation of water to higher densities, deep convection and formation of deep water. Recent OSNAP observations have shown that the overturning is stronger to the east of Greenland than the west.

Here we analyse a CMIP6 climate model at two resolutions (HadGEM3 GC3.1 LL and MM) and show both compare well with the OSNAP observations. We explore the source of low frequency variability of the AMOC and how it is related to the surface water mass transformation in different regions. We also investigate time-mean and low frequency water mass transformations in other CMIP6 climate models.

How to cite: Jackson, L.: OSNAP and water mass transport in CMIP6 models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2725, https://doi.org/10.5194/egusphere-egu21-2725, 2021.

EGU21-8434 | vPICO presentations | OS1.2

Labrador Slope Water Connects the Subarctic with the Gulf Stream

Adrian New, David Smeed, Arnaud Czaja, Adam Blaker, Jenny Mecking, Jamie Mathews, and Alejandra Sanchez-Franks

Labrador Slope Water (LSLW) is found in the Slope Sea on the US-Canadian eastern shelf-slope as a relatively fresh and cool water mass, lying between the upper layer water masses and those carried by the Deep Western Boundary Current. It originates from the Labrador Current and has previously only been reported in the Eastern Slope Sea (east of 66°W). We here use the EN4 gridded database and the Line W hydrographic observations to show for the first time that the LSLW also penetrates into the Western Slope Sea, bringing it into close contact with the Gulf Stream. We also show that the LSLW spreads across the entire Slope Sea north of the Gulf Stream, and is both fresher and thicker when the Atlantic Meridional Overturning Circulation (AMOC) is high at the RAPID array at 26°N. The fresher, thicker LSLW is likely to contribute an additional 1.5 Sv of Gulf Stream transport. The spreading of the LSLW is also investigated in a high-resolution ocean general circulation model (NEMO), and is found to occur both as a western boundary current and through the extrusion of filaments following interaction with Gulf Stream meanders and eddies. The mechanism results in downward vertical motion as the filaments are entrained into the Gulf Stream. We conclude that the LSLW (rather than the deeper Labrador Sea Water) provides the intermediate depth water masses which maintain the density contrast here which partly drives the Gulf Stream, and that the transport of the LSLW from the Labrador shelf-slope offers a potential new mechanism for decadal variability in the Atlantic climate system, through connecting high latitude changes in the Subarctic with subsequent variability in the Gulf Stream and AMOC.

How to cite: New, A., Smeed, D., Czaja, A., Blaker, A., Mecking, J., Mathews, J., and Sanchez-Franks, A.: Labrador Slope Water Connects the Subarctic with the Gulf Stream, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8434, https://doi.org/10.5194/egusphere-egu21-8434, 2021.

EGU21-7770 | vPICO presentations | OS1.2

Oceanic Primary production decline halved in eddy-resolving simulations of global warming

Damien Couespel, Marina Lévy, and Laurent Bopp

The decline in ocean primary production is one of the most alarming consequences of anthropogenic climate change. This decline could indeed lead to a decrease in marine biomass and fish catch, as highlighted by recent policy-relevant reports. Because of computational constraints, current Earth System Models used to project ocean primary production under global warming scenarios have to parameterize flows occurring below the resolution of their computational grid (typically 1°). To overcome these computational constraints, we use an ocean biogeochemical model in an idealized configuration representing a mid-latitude double-gyre circulation, and perform global warming simulations under increasing horizontal resolution  (from 1° to 1/27°) and under a large range of parameter values for the eddy parameterization employed in the coarse resolution configuration. In line with projections from Earth System Models, all our simulations project a marked decline in net primary production in response to the global warming forcing. Whereas this decline is only weakly sensitive to the eddy parameters in the eddy-parametrized coarse resolution, the simulated decline in primary production is halved at the finest eddy-resolving resolution (-12% at 1/27° vs -26 at 1°). This difference stems from the high sensitivity of the subsurface nutrient transport to model resolution. Our results call for improved representation of the role of eddies on nutrient transport below the seasonal mixed-layer to better constrain the future evolution of marine biomass and fish catch potential for decision-making.

How to cite: Couespel, D., Lévy, M., and Bopp, L.: Oceanic Primary production decline halved in eddy-resolving simulations of global warming, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7770, https://doi.org/10.5194/egusphere-egu21-7770, 2021.

EGU21-12539 | vPICO presentations | OS1.2

Variable response of North Atlantic deep-sea benthic ecosystems to industrial-era climate change

Charlotte O'Brien, Peter Spooner, David Thornalley, Jack Wharton, Eirini Papachristopoulou, Francesco Pallottino, Svetlana Radionovskaya, Nicolas Dutton, Tianying Li, Rebecca Garratt, and Delia Oppo

Traditionally, deep-sea ecosystems have been considered to be insulated from the effects of modern climate change. Yet, with the recognition of the importance of food supply from the surface ocean and deep-sea currents to sustaining these systems, the potential for rapid response of benthic systems to climate change is gaining increasing attention. North Atlantic benthic responses to past climate change have been well-documented using marine sediment cores on glacial-interglacial timescales, and ocean sediments have also begun to reveal that planktic species assemblages are already being influenced by global warming. However, very few ecological time-series exist for the deep ocean covering the Holocene-through-industrial era. Here, we use benthic and planktic foraminifera found in Northeast Atlantic (EN539-MC16-A/B and RAPID-17-5P), Northwest Atlantic (KNR158-4-10MC and KNR158-4-9GGC) and Labrador Sea (RAPID-35-25B and RAPID-35-14P) sediments to show that, in locations beneath areas of major North Atlantic surface water change, benthic ecosystems have also changed significantly over the industrial era relative to the Holocene. We find that the response of the benthos is dependent on changes in the surface ocean near to the study sites. Our work highlights the spatial heterogeneity of these benthic ecosystem changes and therefore the need for local-regional scale modelling and observations to better understand responses to deep-sea circulation changes and modern surface climate change. 

How to cite: O'Brien, C., Spooner, P., Thornalley, D., Wharton, J., Papachristopoulou, E., Pallottino, F., Radionovskaya, S., Dutton, N., Li, T., Garratt, R., and Oppo, D.: Variable response of North Atlantic deep-sea benthic ecosystems to industrial-era climate change, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12539, https://doi.org/10.5194/egusphere-egu21-12539, 2021.

EGU21-940 | vPICO presentations | OS1.2

Shifting ocean circulation warms the Subpolar North Atlantic since 2016

Damien Desbruyères, Léon Chafik, and Guillaume Maze

The Subpolar North Atlantic (SPNA) is known for rapid reversals of decadal temperature trends, with ramifications encompassing the large-scale meridional overturning and gyre circulations, Arctic heat and mass balances, or extreme continental weather. Here, we combine datasets derived from sustained ocean observing systems (satellite and in situ), and idealized observation-based modelling (advection-diffusion of a passive tracer) and machine learning technique (ocean profile clustering) to document and explain the most-recent and ongoing cooling-to-warming transition of the SPNA. Following a gradual cooling of the region that was persisting since 2006, a surface-intensified and large-scale warming sharply emerged in 2016 following an ocean circulation shift that enhanced the northeastward penetration of warm and saline waters from the western subtropics. Driving mechanisms and ramification for deep ocean heat uptake will be discussed.

How to cite: Desbruyères, D., Chafik, L., and Maze, G.: Shifting ocean circulation warms the Subpolar North Atlantic since 2016, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-940, https://doi.org/10.5194/egusphere-egu21-940, 2021.

EGU21-1982 | vPICO presentations | OS1.2

Linking variable Nordic Seas inflow to upstream circulation anomalies

Helene Asbjørnsen, Helen Johnson, and Marius Årthun

The inflow across the Iceland-Scotland Ridge determines the amount of heat supplied to the Nordic Seas from the subpolar North Atlantic (SPNA). Variability in inflow properties and volume transport at the ridge influence marine ecosystems and sea ice extent further north. The predictability of such downstream impacts depends on how variability at the ridge relate to large-scale ocean circulation changes in the North Atlantic. Here, we identify the upstream pathways of the Nordic Seas inflow, and assess the mechanisms responsible for interannual inflow variability. Using an eddy-resolving ocean model hindcast and a Lagrangian analysis tool, numerical particles are released at the ridge during 1986-2015 and tracked backward in time. Overall, 64% of the mean inflow volume transport has a subtropical origin and 26% has a subpolar or Arctic origin. The local instantaneous response to the NAO is important for the overall transport of both subtropical and Arctic-origin waters at the ridge. In the years before reaching the ridge, the subtropical particles are influenced by atmospheric circulation anomalies in the gyre boundary region and over the SPNA, forcing shifts in the North Atlantic Current (NAC) and the subpolar front. An equatorward shifted NAC and westward shifted subpolar front correspond to a warmer, more saline inflow. Wind stress curl anomalies over the SPNA also affect the amount of Arctic-origin water re-routed from the Labrador Current toward the Nordic Seas. A high transport of Arctic-origin water is associated with a colder, fresher inflow across the Iceland-Scotland Ridge. The results thus demonstrate the importance of gyre dynamics and wind forcing in affecting the Nordic Seas inflow properties and volume transport.

How to cite: Asbjørnsen, H., Johnson, H., and Årthun, M.: Linking variable Nordic Seas inflow to upstream circulation anomalies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1982, https://doi.org/10.5194/egusphere-egu21-1982, 2021.

EGU21-8089 | vPICO presentations | OS1.2

A climatology of the North Atlantic subpolar gyre boundary

Sam Jones, Stuart Cunningham, Neil Fraser, and Mark Inall

Circulation at the boundary of the subpolar North Atlantic influences both the horizontal (gyre) and vertical (overturning) components of the flow structure. While boundary current transport projects directly onto subpolar gyre strength, recent modelling studies have highlighted that buoyancy fluxes between the basin interior and the boundary, followed by rapid buoyancy export by boundary currents, are crucial steps in projecting air-sea interaction onto the strength of the Atlantic Meridional Overturning Circulation (AMOC). This work seeks observational insights into these key boundary processes.
To achieve this, we have constructed a robust boundary climatology from quality controlled CTD and Argo hydrography since the turn of the millennium. Following the 1000 m isobath north of 47 °N and aggregating data into 100 km bins, we build a picture of the typical large-scale temperature and salinity structure for each month.
This product will allow us to identify where and when important interior-boundary buoyancy fluxes take place over a seasonal cycle. A first step is to evaluate geostrophic flow into the boundary, and hence describe the vertical structure of advective buoyancy exchange. By appealing to satellite altimetry and Argo trajectories, we can also estimate turbulent eddy fluxes both at the surface and 1000 m depth. Models indicate these parameters are key in dictating the pathways for the AMOC lower limb, and we will place our observational findings in the context of these results.
Boundary current strength is another key parameter dictating the export of dense water from the subpolar gyre. We will appeal to satellite altimetry to build corresponding climatologies for barotropic boundary flow. Furthermore, along-slope density gradients give rise to a baroclinic boundary current forcing term, which we aim to investigate here. Water density generally increases as we follow the gyre counter-clockwise, with the notable exception of the West Greenland Current section, and our product allows us to partition the spatially-varying contribution of temperature and salinity towards these density gradients. For example, we can evaluate the impact of cooling along the eastern boundary, or surface freshening around southern Greenland, on the dynamics of boundary flow. Ultimately, we would like to understand the evolution of the dynamical balance experienced by a hypothetical fluid parcel traversing the entire subpolar gyre.

How to cite: Jones, S., Cunningham, S., Fraser, N., and Inall, M.: A climatology of the North Atlantic subpolar gyre boundary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8089, https://doi.org/10.5194/egusphere-egu21-8089, 2021.

EGU21-15972 | vPICO presentations | OS1.2

Drivers of the water mass transformations that set the overturning circulation in the subpolar North Atlantic

D. Gwyn Evans, N. Penny Holliday, and Marilena Oltmanns

The OSNAP (Overturning in the Subpolar North Atlantic Program) array at ~60°N has provided new and unprecedented insight into the strength and variability of the meridional overturning circulation in the subpolar North Atlantic. OSNAP has identified the region of the subpolar North Atlantic east of Greenland as a key region for the water mass transformation and densification that sets the strength and variability of the overturning circulation. Here, we will investigate the drivers of this water mass transformation and their roles in driving the overturning circulation at OSNAP. Using a water mass analysis on both model-based and observational-based datasets, we isolate diathermal (across surfaces of constant temperature) and diahaline (across surfaces of constant salinity) transformations due to air-sea buoyancy fluxes, and mixing. We show that the time-mean overturning strength is set by both the air-sea buoyancy fluxes and the strength of subsurface mixing. This balance is apparent on a seasonal timescale, where we resolve large seasonal fluctuations in the both the air-sea buoyancy fluxes and mixing. The residual of this seasonal cycle then corresponds to the mean overturning strength. On interannual timescales, mixing becomes the dominant driver of variability in the overturning circulation. To determine the location of these water mass transformations and the dynamical processes responsible for the mixing-driven variability, our water mass analysis is projected onto geographical coordinates.

How to cite: Evans, D. G., Holliday, N. P., and Oltmanns, M.: Drivers of the water mass transformations that set the overturning circulation in the subpolar North Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15972, https://doi.org/10.5194/egusphere-egu21-15972, 2021.

EGU21-847 | vPICO presentations | OS1.2

Long-term ventilation changes of Subpolar Mode Waters in the North Atlantic Ocean and its impact on the oxygen distribution

Ilaria Stendardo, Bruno Buongiorno Nardelli, and Sara Durante

In the subpolar North Atlantic Ocean, Subpolar Mode Waters (SPMWs) are formed during late winter convection following the cyclonic circulation of the subpolar gyre. SPMWs participate in the upper flow of the Atlantic overturning circulation (AMOC) and provide much of the water that is eventually transformed into several components of the North Atlantic deep water (NADW), the cold, deep part of the AMOC. In a warming climate, an increase in upper ocean stratification is expected to lead to a reduced ventilation and a loss of oxygen. Thus, understanding how mode waters are affected by ventilation changes will help us to better understand the variability in the AMOC. In particular, we would like to address how the volume occupied by SPMWs has varied over the last decades due to ventilation changes, and what are the aspects driving the subpolar mode water formation, their interannual variations as well as the impact of the variability in the mixing and subduction and vertical dynamics on ocean deoxygenation. For this purpose, we use two observation-based 3D products from Copernicus Marine Service (CMEMS), the ARMOR3D and the OMEGA3D datasets. The first consists of 3D temperature and salinity fields, from the surface to 1500 m depth, available weekly over a regular grid at 1/4° horizontal resolution from 1993 to present. The second consists of observation-based quasi-geostrophic vertical and horizontal ocean currents with the same temporal and spatial resolution as ARMOR3D.

How to cite: Stendardo, I., Buongiorno Nardelli, B., and Durante, S.: Long-term ventilation changes of Subpolar Mode Waters in the North Atlantic Ocean and its impact on the oxygen distribution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-847, https://doi.org/10.5194/egusphere-egu21-847, 2021.

EGU21-1594