Improved Understanding of Ocean Variability and Climate

This session will focus on variability in the ocean and its role in the wider climate system using both observations and models. Areas to be considered will include both ocean heat uptake and circulation variability as well as exploring the use of sustained ocean observing efforts and models to make progress in understanding the ocean’s role in the climate system. More than 90% of the excess heat in the climate system has been stored in the ocean, which mitigates the rate of surface warming. Better understanding of ocean ventilation mechanisms, as well as the uptake, transport, and storage of oceanic heat are therefore essential for reducing the uncertainties on global warming projections. Circulation variability and connectivity, particularly from the South Atlantic to the North Atlantic and Arctic Ocean, are also of interest as well as how they are driven by local-, large- or global-scale processes or teleconnections. Sustained observations at sea are being made within a wide variety of programmes and are leading to significant advances in our ability to understand and model climate. Thus, this session will also explore ongoing and planned sustained ocean observing efforts and illuminate their roles in improving understanding of the ocean’s role in the climate system. For example, air-sea flux moorings are being maintained at select sites to assess models and air-sea flux fields. Deep temperature and salinity measurements are being made at time series moorings and will be made by deep Argo floats. Significant advances are also being made using Argo floats for biogeochemistry and carbon measurements. Such observations provide the means to develop linkages between sustained ocean observing and climate modelling. In conclusion, the session will consider key aspects of ocean variability and its climate relevance, as well as encouraging the use of observations and models to enhance understanding of these areas.

Convener: Simon Josey | Co-conveners: Léon Chafik, Damien Desbruyeres, Yavor Kostov, Iselin Medhaug
vPICO presentations
| Mon, 26 Apr, 09:00–12:30 (CEST), 13:30–15:00 (CEST)

vPICO presentations: Mon, 26 Apr

Chairpersons: Yavor Kostov, Iselin Medhaug, Simon Josey
The Ocean within a Changing Climate
Jan Zika, Abhishek Savita, Ryan Holmes, and Taimoor Sohail

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

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

Maurice Huguenin, Ryan Holmes, and Matthew England

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

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

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

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


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

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

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

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

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

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

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

Emily Newsom, Laure Zanna, and Samar Khatiwala

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

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

Jennifer Mecking and Sybren Drijfhout

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

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

Johannes Mayer, Michael Mayer, and Leopold Haimberger

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

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


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

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

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

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

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

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

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

Benjamin Schmiedel and Fabien Roquet

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

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

W John Gould and Stuart Cunningham

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Robin Waldman, Christophe Cassou, and Aurore Voldoire

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

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