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.

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.

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.

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 (FS_inf) 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 FS_inf using ERA-Interim. We also combine FS_inf 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 FS_inf 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 FS_inf. Moreover, our FS_inf product is temporally more stable than parameterized surface fluxes from ERA5 and previous FS_inf estimates using ERA-Interim, at least from 2000 onwards. The evaluation of the oceanic energy budget in the North Atlantic shows good agreement between FS_inf 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, FS_inf 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.

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.

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.

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-20^th 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 19^th 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.

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.

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.

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.

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.

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.

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 5^o 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 (SST_SPG) 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 SST_SPG 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 SST_SPG 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-SST_SPG 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-SST_SPG correlations and stronger JLI-SST_SPG 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.

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.

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.

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.

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.

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.

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.

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.

The Irminger Sea is one of the key region in the North Atlantic where deep winter convection develops.

We use ARMOR-3D dataset (0.25×0.25˚, from 1993) for calculating of heat (and freshwater) content, oceanic heat fluxes in the upper 500-m layer and the vertical heat exchange with the lower layers through the 500 m level. The air-sea heat exchange was derived from the OAFlux dataset (1×1˚), the radiation balance was obtained from the ERA-Interim reanalysis (0.25×0.25˚).

Computation of the heat balance was done over a closed region covering the central and western Irminger Sea (58-62˚ N and 36-44˚ W). The computations were repeated for several similar rectangular areas to analyze sensitivity of the analysis to the choice of boundaries of the region. However, the results of the analysis were largely independent from these variations.

The upper ocean heat advection in the study region was 37 TW (integrated along all boundaries of the region, sign «+» means that flux is directed to the study region), and was the dominant term in the annual mean heat balance. The annual mean latent heat flux (-21 TW) and sensible heat flux (-5 TW) were directed from the ocean. The annual mean radiation balance was 8 TW (to the ocean), while vertical heat exchange with the lower layers was low (-0.1 TW). On average, heat balance of upper 500-m layer was positive, and not all heat fluxes might be considered. For example, the contribution of horizontal mesoscale eddy exchange could be important. However, the amplitude of the interannual variability of the heat balance of about 15 TW was close to that of the heat content (about 20 TW), while the correlation between the parameters was significant and high (0.79) (after removal of the quadratic trend 0.80). This suggests that the main heat fluxes, which affect the interannual variability of the heat content in the upper 500-m layer were taken into account.

Interannual variability of maximum convection depth in the central Irminger Sea was found to significantly correlate with the upper ocean heat content mean over September-November (-0.73); both parameters showed a similar long-term tendency. The correlation of the convection depth with the freshwater content (September-November) was significantly less and positive (0.49). The latter is counterintuitive, as we expect a decrease of the convective depth with an increase of the upper ocean freshwater contents. It can be assumed this correlation was induced by a high negative correlation between the upper ocean heat and freshwater contents in the region (-0.64). The analysis, thus, suggests that the long-term variability of deep convection in the Irminger Sea was shaped by variability of the main heat fluxes, entering the region.

How to cite: Iakovleva, D. and Bashmachnikov, I.: The heat balance shapes deep convection in the Irminger Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1594, https://doi.org/10.5194/egusphere-egu21-1594, 2021.

The dense waters formed by wintertime convection in the Labrador Sea play a key role in setting the properties of the deep Atlantic Ocean. To understand how variability in their production might affect the Atlantic Meridional Overturning Circulation (AMOC) variability, it is essential to determine pathways and associated timescales of their export. In this study, we analyze the trajectories of Argo floats and of Lagrangian particles launched at 53^oN in the boundary current and traced backwards in time in a high‐resolution model, to identify and quantify the importance of upstream pathways. We find that 85% of the transport carried by the particles at 53^oN originates from Cape Farewell, and it is split between a direct route that follows the boundary current and an indirect route involving boundary‐interior exchanges. Although both routes contribute roughly equally to the maximum overturning, the indirect route governs its signal in denser layers. This indirect route has two branches: part of the convected water is exported rapidly on the Labrador side of the basin, and part follows a longer route towards Greenland and is then carried with the boundary current. Export timescales of these two branches typically differ by 2.5 years. This study thus shows that boundary‐interior exchanges are important for the pathways and the properties of water masses arriving at 53^oN. It reveals a complex three‐dimensional view of the convected water export, with implications for the arrival time of signals of variability therein at 53^oN and thus for our understanding of the AMOC.

How to cite: Georgiou, S., Ypma, S. L., Brüggemann, N., Sayol, J.-M., van der Boog, C. G., Spence, P., Pietrzak, J. D., and Katsman, C. A.: Direct and indirect pathways of convected water masses and their impacts on the overturning dynamics of the Labrador Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2395, https://doi.org/10.5194/egusphere-egu21-2395, 2021.

Temperature and salinity seasonal to interannual variability of Iceland Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) is investigated by combining two in-situ datasets in the Irminger Sea for the period 1997-2020: 12-yr of repeated hydrography (1997-2018) provided by the FOUREX, OVIDE and RREX sections and 4-yr of data (2016-2020) from 8 Deep Argo floats deployed in the region between 2016 and 2018.

In order to enable a consistent analysis of ocean temperature and salinity variability from unevenly distributed vertical profiles (both in space and time), it is necessary to estimate the appropriate regional climatology to be removed from every observation. Two independent procedures are followed to compute anomalies and quantify uncertainties related to the choice of climatology: First, the global 1°-resolution World Ocean Atlas 2018 (2005-2017 averages) climatology is retrieved from every observed profile (Deep Argo, hydrography). Second, the well-known and sampled OVIDE transect (2002-2018 average) is used to build a reference section of geographical anomalies that are subsequently propagated along potential vorticity contours in the Irminger Sea. Neutral density surfaces 28.02 kgm^-3 and 28.12 kgm^-3 are then chosen from mean OVIDE 2002-2018 gridded fields as representative of ISOW and DSOW levels, respectively. Significant decadal trends in water mass properties are revealed by repeated hydrography, whereas some striking boundary-interior spatial patterns are captured by Deep Argo floats. Property changes of ISOW and DSOW are discussed in terms of changes of source waters in the Nordic Seas, entrainment of Atlantic waters into the overflow waters and cascading events from the Greenland slope.

How to cite: Prieto, E., Desbruyères, D., and Thierry, V.: Interpreting the observed variability of deep waters in the Irminger Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3070, https://doi.org/10.5194/egusphere-egu21-3070, 2021.

Convection in the North Atlantic Ocean is a key component of the global overturning circulation (MOC) as it produces dense water at high latitudes. Recent work has highlighted the dominant role of the Irminger and Iceland basins in the production of the North Atlantic deep waters. Dense water formation in these basins is mainly explained by buoyancy forcing that transforms surface waters to the deep waters of the MOC lower limb. Air-sea fluxes and the surface density field are both key determinants of the buoyancy-driven transformation. To better understand the connection between atmospheric forcing and the Atlantic overturning circulation, we analyze the contributions of the air-sea fluxes and of the density structure to the transformation of surface water over the eastern subpolar gyre. More precisely, we consider the densification of subpolar mode water (SPMW) in the Iceland Basin that ‘pre-conditions’ the dense water formation downstream. Analyses using 40 years of observations (1980–2019) reveal that variability in transformation is only weakly sensitive to changes in the heat and freshwater fluxes. Instead, changes in SPMW transformation are largely driven by the variance in the surface density structure, as expressed by the outcropping area for those isopycnals that define SPMW.This large influence of the surface density on the SPMW transformation partly explains the unusually large SPMW transformation in winter 2014–15 over the Iceland Basin.  

How to cite: Petit, T., Lozier, M. S., Josey, S. A., and Cunningham, S. A.: Role of the density structure and air-sea fluxes on subpolar transformation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3500, https://doi.org/10.5194/egusphere-egu21-3500, 2021.

Iceland Scotland Overflow Water (ISOW), a component of the deep limb of the Atlantic Meridional Overturning Circulation (AMOC), is the equilibrated product of dense overflow into the eastern North Atlantic basin.  Modeling results and recent observations have suggested that a significant westward transport of ISOW (~1x10⁶ m³s^-1) may occur through the Bight Fracture Zone (BFZ) near 57°N, the first major channel through the Reykjanes Ridge where ISOW can cross into the Irminger Sea.  The remaining denser (and deeper) ISOW has been shown to leave the Iceland Basin westward via the Charlie-Gibbs Fracture Zone near 53°N, or southward into the West European Basin. Until now, there have been no measured time series in the BFZ to validate model results. Single moorings placed in the north and south channels of the BFZ from summer 2015 to summer 2017 were used to estimate a mean combined transport across the fracture zone of 0.8 ± 0.4 x10⁶ m³s^-1 westward, with each channel contributing about half of the mean transport. Variability between the two channels on shorter (month-long) times scales can be extreme: in March of 2016, for example, north channel transport was ~0.4 x10⁶ m³s^-1 eastward, while south channel transport was ~0.8 x10⁶ m³s^-1 westward.  For this 2-year period, transport is stronger in the summer (0.9-1.2 x10⁶ m³s^-1) than in winter (0.5-0.7 x10⁶ m³s^-1), where large fluctuations including complete reversals suggest transport variability may be affected by winter storms.  This mooring record also shows a fresh anomaly in ISOW beginning in early 2017, which has been shown by others to originate from the surface waters near the Grand Banks region of the western north Atlantic.  Transport variability in this two-year record is examined in the context of the transport variability of the OSNAP mooring arrays on the east and west flanks of the Reykjanes Ridge just north of BFZ during the same time period.  An observationally-based understanding of how the Iceland and Irminger basins communicate with each other via the deep limb of the AMOC through the BFZ will provide fundamental insight into the pathways and processes that define the subpolar AMOC system.

How to cite: Furey, H., Bower, A., Johns, B., Ramsey, A., and Houk, A.: Iceland-Scotland Overflow Water Transport Variability through the Deep Bight Fracture Zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7999, https://doi.org/10.5194/egusphere-egu21-7999, 2021.

Located south of Iceland, the Reykjanes Ridge is a major topographic structure of the North Atlantic Ocean that strongly influences the spatial distribution and circulation of the North Atlantic Subpolar Gyre water masses. Around the ridge, the circulation is composed of two main along-ridge currents, the southwestward East Reykjanes Ridge Current (ERRC) in the Iceland Basin and the northeastward Irminger Current (IC) in the Irminger Sea. To study the along Reykjanes Ridge flow variability and the inter-basin connection through the ridge and connections with the interior of each basin, volume and water mass transports over the Reykjanes Ridge during summer 2015, 2016 and 2017 are analyzed. Data used are velocity and hydrographic measurements carried out along and perpendicular to the crest of the Reykjanes Ridge during the RREX (Reykjanes Ridge Experiment Project) cruises in June–July 2015 and June–July 2017 and BOCATS cruise in July 2016. The new circulation scheme in the area described in 2015  by Petit et al. (J. Geophys. Res., 2018) with flows connecting the ERRC and IC branches at specific locations set by the bathymetry of the ridge is again observed  in 2016 and  2017, with variations concerning the connections with the interiors of the basins. The data set reveals remarkable changes in the hydrological properties and transports of the ERRC, IC and cross ridge flows. The westward transport across the ridge, which represents the subpolar gyre intensity, was estimated at -19.6±3.4 Sv in 2015 and -35.2±3 Sv in 2017. A freshening and a decline in density mainly affecting the Subpolar Mode Water was observed in 2017. It was associated with a lower mode water  transport partly compensated by a higher transport of intermediate and Arctic waters. We further document each water mass contribution to the westward flow of the gyre and the structure of the ERRC and IC.

How to cite: Salaün, I., Thierry, V., and Mercier, H.: The circulation near the Reykjanes Ridge in summers 2015, 2016 and 2017, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8250, https://doi.org/10.5194/egusphere-egu21-8250, 2021.

Deep convection and associated deep water formation are key processes for climate variability, since they impact the oceanic uptake of heat and trace gases and alter the structure and strength of the global overturning circulation. For long, deep convection in the subpolar North Atlantic was thought to be confined to the central Labrador Sea in the western subpolar gyre (SPG). However, there is increasing evidence that deep convection also occurs in the eastern SPG south of Cape Farewell and in the Irminger Sea. In particular, observations indicate gyre-scale intensified convection in 2015-2018. Here we assess this recent event in the context of the temporal evolution of the spatial deep convection pattern in the SPG since the mid-twentieth century, using realistic eddy-rich ocean model simulations. These reveal large interannual variability, including several periods with intensified deep convection in the eastern SPG. Notably, this happened in 2015-2018, but to a lesser degree in the late 1980s to early 1990s, the period with highest deep convection intensity in the Labrador Sea related to a persistent positive phase of the North Atlantic Oscillation. Our analyses further suggest that deep convection in 2015-2018 occurred with an unprecedented high (low) relative contribution of the eastern (western) SPG to the total deep convection volume. This is partly linked to a considerable smaller north-westward extent of deep convection in the Labrador Sea compared to previous periods of intensified deep convection, and may be a first fingerprint of strong near-surface freshening in the Labrador Sea associated with Greenland melting.

How to cite: Rühs, S., Oliver, E., Biastoch, A., Böning, C. W., Dowd, M., Getzlaff, K., and Myers, P. G.: Changing spatial patterns of deep convection in the subpolar North Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10729, https://doi.org/10.5194/egusphere-egu21-10729, 2021.

A series of global ocean - sea ice model simulations is used to investigate the spatial structure and temporal variability of the sinking branch of the meridional overturning circulation (AMOC) in the subpolar North Atlantic. The experiments include hindcast simulations of the last six decades based on the high-resolution (1/20°) VIKING20X-model forced by the CORE and JRA55-do reanalysis products, supplemented by sensitivity studies with a 1/4°-configuration (ORCA025) aimed at elucidating the roles of variations in the wind stress and buoyancy fluxes. The experiments exhibit different multi-decadal trends in the AMOC, reflecting the well-known sensitivity of ocean-only models to subtle details in the configuration of the subarctic freshwater forcing. All experiments, however, concur in that the dense, southward branch of the overturning is mainly fed by “sinking” (in density space) in the Irminger and Iceland Basins, in accordance with the first results of the OSNAP observational program. Remarkably, the contribution of the Labrador Sea has remained small throughout the whole simulation period, even during the phase of extremely strong convection in the early 1990s: i.e., the rate of deep water exported from the subpolar North Atlantic by the DWBC off Newfoundland never differed by more than O(1 Sv) from the DWBC entering the Labrador Sea at Cape Farewell. The model solutions indicate a particular concentration of the sinking along the deep boundary currents south of the Denmark Straits and south of Iceland, pointing to a prime importance for the AMOC of the outflows from the Nordic Seas and their subsequent enhancement by the entrainment of intermediate waters. Since these include the water masses formed by deep convection in the Labrador and southern Irminger Seas, our study offers an alternative interpretation of the dynamical role of decadal changes in Labrador Sea convection intensity in terms of a remote effect on the deep transports established in the outflow regimes.

How to cite: Böning, C. W., Biastoch, A., Getzlaff, K., Wagner, P., Rühs, S., Schwarzkopf, F. U., and Scheinert, M.: Decadal changes in the Atlantic Meridional Overturning Circulation in high-resolution simulations of the subpolar North Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7789, https://doi.org/10.5194/egusphere-egu21-7789, 2021.

The Atlantic Meridional Overturning Circulation (AMOC) at 26N has been measured since 2004 by the RAPID-MOCHA array. On a multi-year timescale it shows a decline with signs of a recovery since around 2012. This variability is likely to be part of longer decadal variability. We examine here the decadal variability of the AMOC and its drivers in a coupled model run nudged to observations from 1960-2017. Temperature and winds are nudged throughout the atmosphere and potential temperature and salinity are nudged in the ocean, but the ocean velocities are allowed to vary freely. We nudge an ensemble of 10 ocean analyses into the ocean model to get an ensemble of responses, the mean of which reproduces the observed AMOC. We use these ocean-atmosphere re-analyses to study the drivers of the AMOC. The North Atlantic Oscillation (NAO) is well known to have an impact on the AMOC and is an important driver here. We find that the tropical Pacific also has a strong impact on the subtropical AMOC on multi-annual to decadal timescales. Together these two factors can explain more than half of all variability of the AMOC at 26N through wind forcing associated with Rossby waves and western boundary waves. This Pacific impact, not reported on before, is from windstress curl anomalies close to the East Coast of the southern US due to changes in the Pacific storm track and the Walker Circulation. As both the NAO and tropical Pacific variability is associated with solar and volcanic forcing, it is possible that solar and volcanic forcing are important for multi-annual to multi-decadal AMOC variability. We use observations of the NAO and tropical Pacific to reconstruct the AMOC from 1870 to present day and predict a continued recovery in the future.

How to cite: Hermanson, L., Smith, D., Dunstone, N., and Eade, R.: A New Pacific Influence on the Atlantic Meridional Overturning Circulation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9989, https://doi.org/10.5194/egusphere-egu21-9989, 2021.

Ocean currents along the Southeast Greenland Coast play an important role in North Atlantic circulation and the global climate system. They carry dense water over the Denmark Strait sill, fresh water from the Arctic and the Greenland Ice Sheet into the subpolar ocean, and warm Atlantic water into Greenland’s fjords, where it can interact with outlet glaciers. Observational evidence from the OSNAP array and other mooring records shows that the circulation in this region displays substantial subinertial variability, typically with periods of several days. For the dense water flowing over the Denmark Strait sill, this variability augments the time-mean transport; on the shelf, the variability is large enough to occasionally reverse the mean transport direction of the coastal current, highlighting the importance of characterizing this variability when interpreting synoptic surveys. In this study, we used the output of a high-resolution realistic simulation to diagnose and characterize subinertial variability in sea surface height and velocity along the coast. The results show that the subinertial signals on the shelf and along the shelf break are coherent over hundreds of kilometers, and consistent with Coastal Trapped Waves in two subinertial frequency bands—at periods of 1–3 days and 5–18 days—portraying a combination of Mode I and higher modes waves. Furthermore, we find that northeasterly barrier winds may trigger the 5–18 day shelf waves, whereas the 1–3 day variability is linked to high wind speeds over Sermilik Deep.

How to cite: Gelderloos, R., Haine, T. W. N., and Almansi, M.: Coastal Trapped Waves along the Southeast Greenland Coast in a realistic numerical simulation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2855, https://doi.org/10.5194/egusphere-egu21-2855, 2021.

The large-scale system of ocean currents that transport warm surface (1000 m) waters northward and return cooler waters southward is known as the Atlantic meridional overturning circulation (AMOC). Variations in the AMOC have significant repercussions for the climate system, hence there is a need for long term monitoring of AMOC fluctuations. Currently the longest record of continuous directly measured AMOC changes is from the RAPID-MOCHA-WBTS programme, initiated in 2004. The RAPID programme, and other mooring programmes, have revolutionised our understanding of large-scale circulation, however, by design they are constrained to measurements at a single latitude.

High global coverage of surface ocean data from satellite altimetry is available since the launch of TOPEX/Poseidon satellite in 1992 and has been shown to provide reliable estimates of surface ocean transports on interannual time scales. Here we show that a direct calculation of ocean circulation from satellite altimetry compares well with transport estimates from the 26°N RAPID array on low frequency (18-month) time scales for the upper mid-ocean transport (UMO; r = 0.75), the Gulf Stream transport through the Florida Straits (r = 0.70), and the AMOC (r = 0.83). The vertical structure of the circulation is also investigated, and it is found that the first baroclinic mode accounts for 83% of the interior geostrophic variability, while remaining variability is explained by the barotropic mode. Finally, the UMO and the AMOC are estimated from historical altimetry data (1993 to 2018) using a dynamically based method that incorporates the vertical structure of the flow. The effective implementation of satellite-based method for monitoring the AMOC at 26°N lays down the starting point for monitoring large-scale circulation at all latitudes.

How to cite: Sanchez-Franks, A., Frajka-Williams, E., Moat, B., and Smeed, D.: A dynamically based method for estimating the Atlantic overturning circulation at 26°N from satellite altimetry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15886, https://doi.org/10.5194/egusphere-egu21-15886, 2021.

Pressure Inverted Echo Sounders, sited on the seabed, indirectly measure the density of the water above them by combining pressure and travel time of an echo-sound pulse to the surface. Where the approximate structure of the water column is locally known, they can be used to select between a number of typical TS profiles (a gravest empirical mode or GEM field), providing temperature and salinity. But how accurate is this profile, and can such an instrument replace the expensive tall moorings currently used to monitor the MOC? We evaluate PIES deployments at 26N on the western boundary of the Atlantic between 2006 and 2018. We find that high-frequency (around weekly) variations in temperature are well captured by this technique, and the geostrophic part of the AMOC could be estimated in this way. However the GEM databases don't account for all low frequency variations in temperature and salinity profiles. At 26N we see for example, the results from PIES with cold bias above the thermocline and with a compensatory warm bias below it, and these biases lasting months or years. The profiles are also inaccurate at the surface, although seasonally-varying GEM fields may be helpful here. However the technique shows promise, and if it is developed further incorporating additional data sources such ARGO or as sea-surface temperature it may be possible to use it for long term monitoring of the Atlantic at 26N.

How to cite: Moat, B., Frajka-Williams, E., Williams, J., and Meinen, C.: Can PIES (Pressure Inverted Echo Sounders) replace tall moorings to monitor the AMOC?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3122, https://doi.org/10.5194/egusphere-egu21-3122, 2021.

The Antilles Current is a narrow, northward flowing boundary current in the western Atlantic just east of the Bahamas. Its role in the larger scale circulation has been debated: alternately thought to be part of the western boundary closure of the gyre circulation or the northward flowing limb of the meridional overturning circulation (MOC). From 19 years of moored current meter observations (1987--1991, 2004--2018), we define the strength of the Antilles Current by the net transport between the Bahamas and 76.5°W (spanning about 45 km zonally) and in the thermocline (0–1000 m). We find a mean northward transport of 3.5 Sv, substantial interannual variability, and no discernable trend since 1987. The interannual variability of the AC transport is independent of the variability of the Florida Current (the Gulf Stream through the Florida Straits). Instead, the Antilles Current contributes to the interannual variability of the MOC at 26°N, while the trend in the strength of the gyre circulation (defined as the transbasin thermocline transport minus the AC) is responsible for the trend in the MOC. In particular, the 2009/10 slowdown of the MOC resulted from a weaker northward AC transport, rather than an intensified gyre transport. Using the recent 14 years of in situ transport records, we compare the interannual variability of the gyre circulation to that of wind stress curl forcing via a Sverdrup transport calculation, identifying a potential role for wind stress curl (WSC) forcing at 26°N with a ~2 year lag until 2016.  From 2016, the predicted gyre circulation using WSC diverges from the measured gyre strength.

How to cite: Frajka-Williams, E., Johns, W. E., Bryden, H. L., Smeed, D. A., Duchez, A., and Holton, L.: The Antilles Current and wind-driven gyre circulation at 26oN, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11132, https://doi.org/10.5194/egusphere-egu21-11132, 2021.

Fresh Arctic waters flowing into the Atlantic are thought to have two primary fates. They may be mixed into the deep ocean as part of the overturning circulation, or flow alongside regions of deep water formation without impacting overturning. Climate models suggest that as increasing amounts of fresh water enter the Atlantic, the overturning circulation will be disrupted, yet we lack an understanding of how much fresh water is mixed into the overturning circulation's deep limb in the present day. To constrain these fresh water pathways, we build steady-state volume, salt, and heat budgets east of Greenland that are initialized with observations and closed using inverse methods. Fresh water sources are split into oceanic Polar Waters from the Arctic and surface fresh water fluxes, which include net precipitation, runoff, and ice melt, to examine how they imprint the circulation differently. We find that 65 mSv of the total 110 mSv of surface fresh water fluxes that enter our domain participate in the overturning circulation, as do 0.6 Sv of the total 1.2 Sv of Polar Waters that flow through Fram Strait. Based on these results, we hypothesize that the overturning circulation is more sensitive to future changes in Arctic fresh water outflow and precipitation, while Greenland runoff and iceberg melt are more likely to stay along the coast of Greenland.

How to cite: Le Bras, I., Straneo, F., Muilwijk, M., Smedsrud, L. H., Li, F., Lozier, S., and Holliday, P.: How much Arctic fresh water participates in the subpolar overturning circulation?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2824, https://doi.org/10.5194/egusphere-egu21-2824, 2021.

How to cite: Zou, S., Bower, A., Furey, H., Pickart, R., Houpert, L., and Holliday, N. P.: Observed Denmark Strait Overflow Cyclones around Greenland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10059, https://doi.org/10.5194/egusphere-egu21-10059, 2021.

Denmark Strait, the channel located between Greenland and Iceland, is a critical gateway between the Nordic Seas and the North Atlantic. Mesoscale features crossing the strait regularly enhance the volume transport of the Denmark Strait overflow. They interact with the dense water masses descending into the subpolar North Atlantic and therefore are important for the Atlantic Meridional Overturning Circulation. Using a realistic numerical model, we find new evidence of the causal relationship between overflow surges (i.e., mesoscale features associated with high-transport events) and overflow cyclones observed downstream. Most of the cyclones form at the Denmark Strait sill during overflow surges and, because of potential vorticity conservation and stretching of the water column, grow as they move equatorward. A fraction of the cyclones form downstream of the sill, when anticyclonic vortices formed during high-transport events start collapsing. Regardless of their formation mechanism, the cyclones weaken starting roughly 150 km downstream of the sill, and potential vorticity is only materially conserved during the growth phase.

How to cite: Almansi, M., Haine, T., Gelderloos, R., and Pickart, R.: Evolution of Denmark Strait Overflow Cyclones and Their Relationship to Overflow Surges, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12690, https://doi.org/10.5194/egusphere-egu21-12690, 2021.

The Labrador Sea is an important site for deep convection, and the boundary current surrounding the Sea impacts the strength of this convection and the subsequent restratification. As part of the Overturning of the Subpolar North Atlantic Program, ten moorings have been maintained on the West Greenland shelf and slope that provide hourly, high-resolution renderings of the boundary current. These data reveal the presence and propagation of abundant mid-depth intensified cyclonic eddies, which have not previously been documented in the West Greenland boundary current system. This study quantifies these features and their structure and demonstrates that they are the downstream manifestation of Denmark Strait Overflow Water (DSOW) cyclones. Using the mooring data, the statistics of these features are presented, a composite eddy is constructed, and the velocity and transport structure are described. A synoptic survey of the region captured two of these features, and provides further insight into their structure and timing. This is the first time DSOW cyclones have been observed in the Labrador Sea, and their presence, propagation, and transport must be accounted for in order to assess their contribution to the heat and freshwater budgets of the Labrador Sea interior.

How to cite: Pacini, A., Pickart, R. S., Le Bras, I. A., Straneo, F., Holliday, N. P., and Spall, M. A.: Cyclonic eddies in the West Greenland boundary current system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13295, https://doi.org/10.5194/egusphere-egu21-13295, 2021.

The Labrador Sea’s surface circulation remains important for the large-scale thermohaline circulation due to its fast response to atmospheric forcing and strong links to the North Atlantic and the Arctic Ocean’s counterparts. Its role in redistribution of heat and momentum, as well as for the biochemical exchange with the atmosphere is crucial in several time and space scales. The region is characterised by advection of freshwater originating from the combined melt of the Arctic Ocean’s sea-ice and Greenland’s glaciers around and towards the interior of the Labrador Sea. The fate of surface freshwater is an important factor that modifies ocean stratification, deep water convection and thus, ocean climate. Despite the major role of surface freshwater in the Labrador Sea, the dominant mechanism responsible for its offshore transport remains debatable, whether it is due to wind-induced Ekman transport, particularly strong in winter, or to eddy advection.

To explore this disagreement, we use surface drifters deployed in three seasons: 50 in December 2019, 50 in March 2020 and 50 in August 2020 in the shelf/slope location off Cape Desolation and near Qaqortoq, a town in the south-west Greenland. The drifters are equipped with temperature sensors and underwater drogues allowing them to follow the cyclonic surface currents: first, the along-shelf, coastal current and along-slope, boundary current west of Greenland; then, if they are able to detach from the shelf edge, the interior circulation of the central Labrador Sea that directs them south-westward from the Davis Strait; eventually, joining the coastal and along-slope boundary currents east of Labrador before circulating into the Labrador Sea’s central basins or eventually leaving the study area.

To investigate the dominant force responsible for the surface transport we use a wind product (ERA5) in a combination with daily SST (OSTIA). Detachment from boundary current is defined as crossing of the 2500 m isobath. The number of crossings varies depending on the season and weather conditions, e.g. an abrupt change in wind direction. This, in turn, may create upwelling of deep-water masses near the shelf-break. However, trajectories of drifters superimposed on SST maps indicate that besides Ekman transport, eddies carry shelf-originating water offshore as well. Auxiliary data from below (Argo floats and other CTD profiles collected near the drifters) allow to distinguish how deep both processes can leave their signature or whether they can drive a return flow.

If any substantial changes in the North Atlantic wind field occur in the future, the fate of the surface water transport in the Labrador Sea will also change, both in respect to its volume and direction. This could potentially affect the balance between Ekman transport and eddies revealed by our analysis of surface drifters data.

How to cite: Goszczko, I., Frajka-Williams, E., Clement, L., and Holliday, N. P.: Wind driven Ekman transport vs eddies – ultimate fight or peaceful cooperation in the Labrador Sea?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13159, https://doi.org/10.5194/egusphere-egu21-13159, 2021.

The long-term response of the Atlantic meridional overturning circulation (AMOC) to anthropogenic climate change remains poorly understood in part, due to the computational expenses associated with running fully-coupled climate models to equilibrium. Here, we use a collection of millennial-length simulations from multiple state-of-the-art climate models to examine the transient and equilibrium responses of the AMOC to an abrupt quadrupling of atmospheric carbon-dioxide. All climate models exhibit a weakening of the AMOC on centennial timescales, but they disagree on the recovery of the AMOC over next millennia, despite the same greenhouse-gas forcing. In some models, the AMOC recovers after approximately 200 years, while in others the AMOC does not fully recover even after approximately 1000 years. To explain the behavior of the AMOC we relate the overturning circulation in the North Atlantic to the meridional density difference between the basin interior and the region of deep-water formation. This scaling both reproduces the initial decline and gradual recovery of the AMOC, and explains the inter-model spread of the AMOC responses. The initial shoaling and weakening occurs on centennial timescales and is attributed to the warming of the northern convection region. We argue that the AMOC weakens on a timescale linked to a combination of its initial depth and the global surface heat flux sensitivity. The recovery of the AMOC results from a pile-up of salinity in the Atlantic basin, when the AMOC is weakened, that propagates northward and reinvigorates convection. A weaker AMOC recovery is associated with a smaller salinity anomaly. We further show through surface water mass transformation that Southern Ocean processes may impact the salinity anomaly in the Atlantic basin. These results highlight the importance of considering the evolution of the AMOC and ocean heat transport beyond the 21st century as short-term changes are not indicative of long-term changes.

How to cite: Bonan, D., Thompson, A., Newsom, E., Sun, S., and Rugenstein, M.: Transient and equilibrium responses of the Atlantic meridional overturning circulation to warming in coupled climate models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10113, https://doi.org/10.5194/egusphere-egu21-10113, 2021.

North of Svalbard is a key region for the Arctic Ocean heat and salt budget as it is the gateway for one of the main branches of Atlantic Water to the Arctic Ocean. As the Atlantic Water layer advances into the Arctic, its core deepens from about 250 m depth around the Yermak Plateau to 350 m in the Laptev Sea, and gets colder and less saline due to mixing with surrounding waters. The complex topography in the region facilitates vertical and horizontal exchanges between the water masses and, together with strong shear and tidal forcing driving increased mixing rates, impacts the heat and salt content of the Atlantic Water layer that will circulate around the Arctic Ocean.

In September 2018, 6 moorings organized in 2 arrays were deployed across the Atlantic Water Boundary current for more than one year (until November 2019), within the framework of the Nansen Legacy project to investigate the seasonal variations of this current and the transformation of the Atlantic Water North of Svalbard. The Atlantic Water inflow exhibits a large seasonal signal, with maxima in core temperature and along-isobath velocities in fall and minima in spring. Volume transport of the Atlantic Water inflow varies from 0.7 Sv in spring to 3 Sv in fall. An empirical orthogonal function analysis of the daily cross-isobath temperature sections reveals that the first mode of variation (explained variance ~80%) is the seasonal cycle with an on/off mode in the temperature core. The second mode (explained variance ~ 15%) corresponds to a short time scale (less than 2 weeks) variability in the onshore/offshore displacement of the temperature core. On the shelf, a counter-current flowing westward is observed in spring, which transports colder (~ 1°C) and fresher (~ 34.85 g kg^-1) water than Atlantic Water (θ > 2°C and S_A > 34.9 g kg^-1). The processes driving the dynamic of the counter-current are under investigation. At greater depth (~1000 m) on the offshore part of the slope, a bottom-intensified current is noticed that seems to covary with the wind stress curl. Heat loss of the Atlantic Water between the two mooring arrays is maximum in winter reaching 250 W m^-2 when the current is the largest and the net radiative flux from the atmosphere to the ocean is the smallest (only 50 W m^-2 compared to about 400 W m^-2 in summer).

How to cite: Koenig, Z., Kalhagen, K., Kolås, E., Fer, I., Nilsen, F., and Cottier, F.: Atlantic Water properties, transport, and water mass transformation north of Svalbard from one-year-long mooring observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2505, https://doi.org/10.5194/egusphere-egu21-2505, 2021.

Kongsfjorden is an Arctic fjord situated in the Svalbard archipelago. The fjord hydrography is influenced by the warm and saline Atlantic water from West Spitsbergen Current (WSC) flowing northward on the shelf slope and the cold and fresh polar waters circulating on the shelf. Once several conditions are satisfied, Atlantic waters from the WSC can extensively flood the fjord. These intrusions are majorly confined to the summer season although some strong events have been identified also in winter. In this study, a decade of continuous observations is used to examine changes in water properties and water masses variability in inner Kongsfjorden, with a special focus on Atlantic water intrusions. Data have been gathered by the National Research Council of Italy (CNR) through the Mooring Dirigibile Italia (MDI) in addition to summer CTD surveys. MDI was deployed in September 2010 at 100m depth and comprises various temperature and salinity sensors placed at different depths. Analysis of the longest temperature series reveals a positive linear trend since 2010. However, both temperature and salinity present a peak at the beginning of 2017 and decreasing values toward the end of the series. No significant trends were found when considering the monthly water column temperature as average of few sensors’ measurements. Yet, differentiating the seasonal contributions reveals that summer temperatures feature a fast warming (0.26 °C/yr) whereas winters do not show a statistically significant linear trend. Temperature and salinity observations gathered at 25 and 85m depth are used to depict water masses variability accorfing to previous water masses classifications. Some evidences are noted: first, events of Atlantic water intrusions are always confined near the bottom and they are never seen at 25m, whilst summer freshwater is found only in the near surface. Second, the timing of occurrence of these two water types seems to be related: the presence of large freshwater volumes close to the surface are preceded by the arrival of warm and saline waters. This evidence is interpreted as the melting signal of Kronebreen, the largest tidewater glacier in Kongsfjorden, triggered by the intrusion of Atlantic water. As a result, the large freshwater input manages to dilute the Atlantic water settled near the bottom. Third, the temperature and salinity peaks at the beginning of 2017 are associated to a massive Atlantic water flooding in the inner fjord lasting several months in summer/autumn 2016 and 2017. After this period, Atlantic water is seen only for few months in summer 2019. CTD measurements are used to depict the summer hydrography of Kongsfjorden and the focus is drawn on the characterisation of the seasonal cycle for each available year of measurements. Finally, the drivers of Atlantic water intrusion events are examined, as the presence of low pressure systems and the wind patterns in the region.

How to cite: De Rovere, F., Chiggiato, J., Langone, L., Schröeder, K., Miserocchi, S., and Giglio, F.: Water masses variability in inner Kongsfjorden during 2010-2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2121, https://doi.org/10.5194/egusphere-egu21-2121, 2021.