Over the past decade, Antarctic sea ice extent exhibited a sequence of record maxima, followed by a rapid decline in 2015/16, and record minima since. In this presentation, we show that this sudden and remarkable ice loss marks an abrupt transition from a high to a low ice state that cannot be explained by year-to-year variability. Instead, it is most likely associated with a longer term variability arising from ice–ocean feedbacks. The abrupt transition was preceded by a multi-decadal increase in persistence and variance of the sea ice anomalies, an increasing upper Southern Ocean density stratification, and an accumulation of heat at the subsurface; suggesting a decoupling of the surface from the subsurface ocean. During this period, the sea ice anomalies shifted from being structured predominantly regionally and seasonally to a largely circumpolar and interannual regime. In 2015/16, the upper ocean density stratification in the ice-covered region suddenly weakened, leading to a release of heat from the subsurface, contributing to the sea ice decline during winter. Our analysis suggests that the sudden sea ice loss in 2015/16, and the persisting low ice conditions since, arose from a systematic change in the physical state of the coupled circumpolar ice–ocean system. This change will have wide implications for global climate, ecosystems, and the Antarctic Ice Sheet.

How to cite: Haumann, F. A., Massonnet, F., Holland, P. R., Bushuk, M., Maksym, T., Hobbs, W., Meredith, M. P., Cerovečki, I., Lavergne, T., Meier, W. N., Raphael, M., and Stammerjohn, S.: An abrupt transition in the Antarctic sea ice–ocean system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8749, https://doi.org/10.5194/egusphere-egu23-8749, 2023.

Antarctic coastal polynyas produce Dense Shelf Water, a precursor to Antarctic Bottom Waters (AABW) that supply the global abyssal circulation. Recent studies suggest that increasing atmospheric CO₂ concentrations will weaken AABW export by suppressing heat loss to the atmosphere. However, future projections of DSW formation are hindered by the small spatial scales of atmosphere-sea ice-ocean interactions in polynyas. Here, using a high-resolution ocean-ice-atmosphere coupled model, this study shows that wintertime sea ice production rates are still active under elevated CO₂ concentrations, although delayed freeze-up decreases autumn sea ice production. In winter, Antarctic coasts exhibit a nonlinear response CO₂ concentration: doubling CO₂ decreases sea ice production only by around 6–8%, versus 10–30% under CO₂ quadrupling. Despite continued sea ice production in winter, doubling or quadrupling CO₂ substantially freshens Dense Shelf Water, primarily due to increased precipitation, implying a shutdown of AABW formation.

How to cite: Jeong, H., Lee, S.-S., Park, H.-S., and Stewart, A.: Future changes in Antarctic coastal polynyas and bottom water formation simulated by a high-resolution coupled model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4760, https://doi.org/10.5194/egusphere-egu23-4760, 2023.

The Dotson Ice Shelf (DIS) shows high rates of basal melting in recent decades. Relatively warm ocean currents access the sub-ice shelf cavity and interact with the base of the ice shelf providing heat for its melting. The water mass transformation associated with the mixture of warm water and meltwater creates buoyant plumes that shallow as they flow out from the cavity. Here, we show that high turbulent kinetic energy (TKE) dissipation rates (up to order 10⁻⁷ W kg⁻¹) and diapycnal eddy diffusivities (up to order 10⁻² m² s⁻¹) are associated with the outflow current from DIS. Four high-resolution Vertical Microstructure Profile (VMP) and ship-based Acoustic Doppler Current Profiler (SADCP) sections were conducted in January and February 2022 at the western side of DIS spanning the outflow as it hugs the steep topographic slope. Near-bed TKE dissipations rates are elevated by up to 3 orders of magnitude and elevated mixing rates are also observed mid-water column around the edges of the outflow. These elevated TKE are associated with friction near the bed and current shear at the outflow boundary. In this presentation, we explore the consequences for dissipation of physical and biogeochemical properties.

How to cite: Dotto, T., Hall, R., Sheehan, P., Damerell, G., Zheng, Y., Boehme, L., Stammerjohn, S., and Heywood, K.: Mixing Processes in the Dotson Ice Shelf Outflow, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15466, https://doi.org/10.5194/egusphere-egu23-15466, 2023.

The rates and mechanisms of ocean mixing are important controls on how the oceans function; yet, our understanding of mixing in the ocean is significantly limited by complex variability in mixing rates and processes and by a scarcity of direct observations. In the Arctic Ocean, the challenges involved in understanding mixing space-time geography and its implications are significant: mixing measurements are especially sparse, and latitude, ice, and stratification make the mixing environment unique. In this talk, I’ll discuss various ways we are mapping Arctic Ocean mixing rates and deriving insights into what sets their variability in space and in time using pan-Arctic measurements from a variety of autonomous instrument platforms and the archived data record. I’ll also show results from our experiments with realistic ocean models to argue that this map matters both to our understanding of Arctic Ocean functioning and our ability to make robust predictions of climate change.

How to cite: Waterman, S.: Filling in the Map: Arctic Ocean mixing space-time geography & its implications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13961, https://doi.org/10.5194/egusphere-egu23-13961, 2023.

I will present an analytical relation that directly shows how mixing locally drives the overturning circulation. The theory is based on the application of the well-known Water Mass Transformation framework to each local water column. The budgets for volume and tracer content mapped to tracer space are supplemented by a budget for the squared tracer. Mixing is defined by the destruction term of squared tracer, which is equivalent to the decay of tracer variance. I will present maps of the simulated diahaline mixing and the associated diahaline exchange velocity in the Baltic Sea. In addition, our numerical model offers to separately diagnose the mixing due to turbulence parameterizations and the spurious mixing due to discrete transport schemes. This enables us to also quantify the amount of spuriously induced overturning circulation. The planned application to diapycnal exchange and the global overturing circulation will be outlined.

How to cite: Klingbeil, K., Henell, E., Gräwe, U., and Burchard, H.: Breaking down the overturning circulation to local mixing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13931, https://doi.org/10.5194/egusphere-egu23-13931, 2023.

Mesoscale ocean turbulence is the best-known expression of Chaotic Intrinsic Variability (CIV), which spontaneously emerges from the unstable ocean circulation regardless of the atmospheric variability. Substantial amounts of CIV are also found up to the scale of basins and decades, potentially produced by large-scale baroclinic instability or resulting from spatiotemporal inverse cascade processes.

A 56-year atmospherically-forced 50-member 1/4° large ensemble simulation of the global eddying ocean/sea-ice system has been performed to explore these phenomena using the NEMO model. We first show that the low-frequency large-scale (LFLS) CIV has climate-relevant imprints over most of the globe, is largest in western boundary currents and south of about 30°S, and competes with (and in certain zones exceeds) the atmospherically-forced ocean variability (AFV) in terms of amplitude.

However, the separability of AFV and CIV is questionable in certain cases. Concepts from dynamical system and information theories are leveraged to avoid this separation, and to probabilistically describe the ocean variability as an atmospherically-modulated oceanic "chaos". The partly random character of multi-scale ocean fluctuations in the eddying regime questions the attribution of observed signals to sole atmospheric drivers, the turbulent ocean predictability and its potential influence in high-resolution coupled simulations.

How to cite: Penduff, T.: Describing the ocean variability as an atmospherically-modulated oceanic "chaos", EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2013, https://doi.org/10.5194/egusphere-egu23-2013, 2023.

Ocean mesoscale eddies are ubiquituos in the global ocean. They are responsible of about 80% of the total eddy kinetic energy and are suggested to exert a significant impact on air-sea interactions, ocean large-scale circulation, weather and marine ecosystems. They have been qualified as "coherent" structures as they can leave for months if not years propagating in the ocean interior. As ocean observations are very sparse, they have been essentially characterized from satellite altimetry fields, which provides access to a limited number of surface characteristics of only those eddies having an imprint on sea surface height.  Observations of mesoscale eddies 3D structure, or even 2D vertical sections are rare.  On the other hand, accurate description of ocean eddies from high-resolution ocean numerical simuation are also limited. In general, they have been accoubted for via statistics, instead of individual descriptions as the latter is difficult as they move away from fixed positions. In this work we present a detailed study of ocean eddies (surface and subsurface intensified) sampled during 10 oceanographic cruises which have a sufficient horizontal spatial resolution of the vertical eddy sampling - 9 in the Atlantic Ocean (during experiments EUREC4A-OA, M124, MSM60, MSM74, M160, HM2016611, KB2017606, KB 2017618), and one in the Indian (during the Physindien 2011 experiment). Our study characterizes the eddy core and boundary in a generic way using diagnostics based on active (PV, oxygen) and passive (temperature, salinity) tracers. Despite the different resolutions of the eddy sampling in the 9 studied regions, we show that the 3D boundary of an eddy behaves like a frontal zone characterized by the Ertel PV where the water mass trapped in the eddy joins with the surrounding waters. Whatever the  origin and size of the eddy are, the core is homogeneous in properties with the anomaly maximum located at depth, which makes its altimetric characterization difficult. Moreover, these analyses provide a new metrix for defining the coherence of an ocean eddy, a concept that has been always ill-defined because of the elusive character and undersampling of these structures.

How to cite: Barabinot, Y. and Speich, S.: Characterizing the spatial coherence of mesoscale eddies using in-situ data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-558, https://doi.org/10.5194/egusphere-egu23-558, 2023.

The large spread in projections of ocean heat uptake found in CMIP simulations is known to be problematic, leading to large uncertainties in the projected future ocean heat storage. This study introduces a new diagnostic, super-residual transport (SRT), to trace ocean heat uptake processes consistently in different models. SRT is the contribution to ocean heat uptake associated with residual mean advection and isopycnal diffusion, and therefore explains large and mesoscale heat transports in terms of spatial scale regardless of model resolution. We compare two different resolutions (eddy-parameterising and eddy-present models) of the global coupled HadGEM3-GC3.1 models to investigate performance of ocean heat uptake simulation and suggest where focus should be applied in model development.

We find that high-latitude regions show substantial inter-resolution differences in SRT of the mean state. Due to strong along-isopycnal heat uptake poleward of 50°S, a large amount of heat is stored in the Southern Ocean with little sea surface warming. The ocean heat uptake in the mixed layer is stronger and deeper near Drake passage in the eddy-present model which has steeper isopycnal surfaces of the Southern Ocean. The deep ocean warming varies with model resolution due to different properties of deep water formation in Weddell Sea and North Atlantic, which provides different paths from the surface to the bottom of the ocean. We demonstrate that mesoscale eddy advection due to baroclinic instability, implemented by Gent-McWilliams parameterisation, is key to understanding the differences in warming Antarctic Bottom Water and North Atlantic Deep Water across resolutions.

In the context of CO₂-forced change, SRT shows much higher similarity across model resolutions than in the mean state. For both model resolutions, the mixed layer warming driven by SRT is much reduced in the high-latitude Southern Ocean. This results mainly from slumping of isopycnals, which brings excessive heat further northward of 50°S and then downward by enhanced Deacon cell. Consistent with our findings in the mean state, deep ocean warming penetrated to the bottom of the Southern Ocean is only observed in the eddy-present model. An important implication of this result is that better agreement across model resolutions in AMOC strength and North Atlantic warming is achieved in CO₂-induced SRT. This suggests that whether ocean mesoscale is explicitly resolved or parameterised becomes less influential with respect to the patterns of ocean warming as the climate warms, which results from abrupt changes in mean circulation and reduced effect of Gent-McWilliams parameterisation.

How to cite: Lee, J., Kuhlbrodt, T., Tailleux, R., and Storkey, D.: Using super-residual heat transport to elucidate ocean heat storage in a resolution hierarchy of models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11728, https://doi.org/10.5194/egusphere-egu23-11728, 2023.

The Earth energy imbalance (EEI) at the top of the atmosphere is responsible for the accumulation of energy in the climate system. While necessary to better understand the Earth’s warming climate, measuring the EEI is challenging as it is a globally integrated variable whose variations are small (0.5-1 W.m−2) compared to the amount of energy entering and leaving the climate system (~ 340 W.m-2). Accuracies better than 0.1 W.m−2 are needed to evaluate the temporal variations of the EEI at decadal and longer time-scales. The CERES experiment provides EEI time variations with a typical uncertainty of ± 0.1 W.m−2 and shows a trend in EEI of 0.50 +/- 0.47 W.m−2 per decade over the period 2005-2019.

The combination of space altimetry and space gravimetry measurements provides an estimate of the ocean heat content (OHC) change which is an accurate proxy of EEI (because >90% of the excess of energy stored by the planet in response to the EEI is accumulated in the ocean in the form of heat). 

In Marti et al. (2021), the global OHC was estimated at global scales based on the combination of space altimetry and space gravimetry measurements over 2002-2016. Changes in the EEI were then derived with realistic estimates of its uncertainty.

Here we present the improvements brought to the global OGC and EEI over an extended period (2002-2021), such as the calculation of the expansion efficiency of heat over the total water column, the improvement of ocean mass solution, the empirical correction of the wet tropospheric correction of Jason-3 altimeter measurements (Barnoud et al., 2022).

The space geodetic GOHC-EEI product based on space altimetry and space gravimetry is available on the AVSIO website at https://doi.org/10.24400/527896/a01-2020.003.

References:

Barnoud A., Picard B., Meyssignac B., Marti F., Ablain M., Roca R. Reducing the uncertainty in the satellite altimetry estimates of global mean sea level trends using highly stable water vapour climate data records. Submitted to JGR: Oceans.

Marti, F., Blazquez, A., Meyssignac, B., Ablain, M., Barnoud, A., Fraudeau, R., Jugier, R., Chenal, J., Larnicol, G., Pfeffer, J., Restano, M., and Benveniste, J.: Monitoring the ocean heat content change and the Earth energy imbalance from space altimetry and space gravimetry, Earth Syst. Sci. Data, 14, 229–249, https://doi.org/10.5194/essd-14-229-2022, 2022.

How to cite: Marti, F., Blazquez, A., Meyssignac, B., Ablain, M., Barnoud, A., Fraudeau, R., Rousseau, V., Chenal, J., Larnicol, G., Pfeffer, J., Restano, M., Benveniste, J., Dibarboure, G., and Bignalet-Cazalet, F.: New improvements for monitoring the Ocean Heat Content and the Earth Energy imbalance (MOHeaCAN)., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14931, https://doi.org/10.5194/egusphere-egu23-14931, 2023.

By regulating the supply of carbon, nutrients, and heat, ocean circulation at high latitudes plays a critical role in global climate.  During the last ice age, the Atlantic’s overturning circulation was repeatedly perturbed, associated with major changes in climate, but little is known of the response of biogeochemistry and circulation in the Pacific.  Here we present new high-resolution data that illuminate the coupled changes in circulation, CO₂ and nutrient supply, and biological productivity associated with rapid climate change events at northern high latitudes.  We show that abrupt stadial cold events are consistently associated with pulses of enhanced nutrient supply and diatom productivity at mid latitudes in the North Atlantic.  Abrupt changes are also seen in the North Pacific, but are anti-phased, with peaks of productivity and nutrient supply occurring during abrupt interstadial warming.  Using model simulations, we show that these productivity changes can be explained by abrupt switches in the mode of overturning circulation, with weakened overturning associated with accumulation of nutrients in the subsurface waters that supply the surface via winter mixing and upwelling, alongside a southward shift of nutrient-rich subpolar waters.  Our results demonstrate the persistent operation of an Atlantic-Pacific seesaw in overturning circulation and biogeochemistry on centennial to millennial timescales and provide a valuable test for simulation of interlinked changes in circulation, biogeochemistry, and climate.

How to cite: Rae, J., O'brien, C., Bradtmiller, L., Burke, A., Gebhardt, H., Gray, W., Littley, E., Wills, R., Zhang, X., Sarnthein, M., and Thornalley, D.: An Atlantic-Pacific Seesaw in Circulation and Biogeochemistry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9602, https://doi.org/10.5194/egusphere-egu23-9602, 2023.

It is now established that the increase in atmospheric CO2 is likely to cause a weakening, or perhaps a collapse of the Atlantic Meridional Overturning Circulation (AMOC). To investigate the mechanisms of this response in CMIP5 models, Levang and Schmitt (2020) have estimated offline the geostrophic streamfunction in these models, and decomposed the simulated changes into a contribution caused by the variations in temperature and salinity. They concluded that under a warming scenario, and for most models, the weakening of the AMOC is fundamentally driven by temperature anomalies while freshwater flux changes actually act to stabilize it.

However, given that both ocean temperature and salinity are expected to respond to a forcing at the ocean surface, it is unclear to what extent the diagnostic is informative about the nature of the forcing. To clarify this question, we used the Earth system Model of Intermediate Complexity (EMIC) cGENIE, which is equipped with the C-GOLDSTEIN friction-geostrophic model (Marsh et al. (2011)). First, we reproduced the experiments simulating the RCP8.5 warming scenario and observed that cGENIE behaves similarly to the majority of the CMIP5 models considered by Levang and Schmitt (2020), with the response dominated by the changes in the thermal structure of the ocean.

Next, we considered hysteresis experiments associated with (1) water hosing and (2) CO2 increase and decrease. In all experiments, changes in the ocean streamfunction appear to be primarily caused by the changes in the temperature distribution, with variations in the 3-D distribution of salinity compensating only partly for the temperature contribution. These experiments reveal also limited sensitivity to changes in the ocean's salinity inventory. That the diagnostics behave similarly in CO2 and freshwater forcing scenarios suggests that the output of the diagnostic proposed in Levang and Schmitt (2020) is mainly determined by the internal structure of the ocean circulation, rather than the forcing applied to it.

Our results illustrate the difficulty of inferring any information about the applied forcing from the thermal wind diagnostic and raise questions about the feasibility of designing a diagnostic or experiment that could identify which aspect of the forcing (thermal or haline) is driving the weakening of the AMOC.

Acknowledgements

This is a contribution to the WarmAnoxia project funded by the Belgian National Fund of Scientific Research.

References:

Levang, S. J. and Schmitt, R. W. (2020). What causes the amoc to weaken in cmip5? Journal of Climate, 33(4):1535–1545.

Marsh, R., Müller, S., Yool, A., and Edwards, N. (2011). Incorporation of the c-goldstein efficient climate model into the genie framework:" eb_go_gs" configurations of genie. Geoscientific Model Development, 4(4):957–992.

How to cite: Gérard, J. and Crucifix, M.: Diagnosing the AMOC slowdown in a coupled model: a cautionary tale, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7387, https://doi.org/10.5194/egusphere-egu23-7387, 2023.

A bi-stable mode of the Atlantic Meridional Overturning Circulation (AMOC) is found in a 10-member ensemble simulation of the SSP2-4.5 scenario using the NASA GISS-E2-1-G climate model. Local feedbacks in the subpolar North Atlantic region in conjunction with internal variability in sea-ice transport and melt play a critical role in causing the divergent behavior of the AMOC in the ensemble members. While other fully coupled models have demonstrated the important role of surface freshening in leading to AMOC shutdown, either through hosing experiments or increased precipitation and greenhouse gas warming at high latitudes, in the GISS simulations, there are no external freshwater perturbations. This is the first time that a CMIP-class model has shown such a bifurcation across an initial condition ensemble.

How to cite: Romanou, A., Rind, D., Jonas, J., Miller, R., Kelley, M., Russell, G., Orbe, C., Nazarenko, L., Latto, R., and Schmidt, G. A.: The role of internal variability and feedbacks controlling AMOC stability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3615, https://doi.org/10.5194/egusphere-egu23-3615, 2023.

The fate of glacial meltwater introduced into the ocean is an important problem both in paleoceanography and in modern oceanography. A long-standing question in paleoceanography concerns the evolution and consequences of the glacial meltwater delivered from the great ice sheets which covered a large fraction of North America and Europe during glacial periods. Although the associated rise in mean sea level of about 130 m has long been estimated, the pathways and impacts of the glacial meltwater for ocean circulation and climate remain poorly understood. Notably, the ocean components of climate models do not generally have a spatial resolution that is fine enough to properly simulate coastal phenomena which are known to contribute to the offshore export of shelf water in the modern ocean.

Here we apply a regional eddy-revolving numerical model of ocean circulation in order to explore the pathways of glacial meltwater emanating from the St. Lawrence Channel – a major ice stream of the Laurentide Ice Sheet of North America during the last glacial period. Emphasis is placed on the offshore entrainment of glacial water by the Gulf Stream (GS), which according to paleoceanographic observations detached from the continental slope near Cape Hatteras, as it does today. First, a simulation of the eddying circulation in the glacial western North Atlantic is obtained by integrating the regional model to statistical steady state under glacial atmospheric forcing. Second, a series of glacial water discharge experiments are conducted for various assumptions about the discharge, including its volume transport, its density, and its seasonal timing. Mechanisms of glacial water export away from the slope are identified, such as the eastward entrainment by (anticyclonic) warm core rings and the subsequent incorporation of the glacial water into the GS offshore. The implications of our results for the interpretation of sediment records from the Laurentian Fan and for the simulation of glacial water discharges in paleoclimate models are then clarified.

How to cite: Marchal, O. and Condron, A.: On the Entrainment of Glacial Meltwater by the Gulf Stream, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4006, https://doi.org/10.5194/egusphere-egu23-4006, 2023.

The Atlantic Meridional Overturning Circulation (AMOC) transports heat and salt between the tropical Atlantic and Arctic Oceans. The interior of the North Atlantic Subpolar Gyre (SPG) is responsible for the much of the water mass transformation in the AMOC, and the export of this water to intensified boundary currents is crucial for projecting air-sea interaction onto the strength of the AMOC. However, the magnitude and location of exchange between the SPG and the boundary remains unclear. We present a novel climatology of the SPG boundary using quality controlled CTD and Argo hydrography, defining the SPG interior as the oceanic region bounded by 47° N and the 1000m isobath.  From this hydrography we find geostrophic flow out of the SPG around much of the boundary with minimal seasonality.  The horizontal density gradient is reversed around West Greenland, where the geostrophic flow is into the SPG.  Surface Ekman forcing drives net flow out of the SPG in all seasons with pronounced seasonality, varying between 2.45 ± 0.73 Sv in the summer and 7.70 ± 2.90 Sv in the winter.  We estimate heat advected into the SPG to be between 0.14 ± 0.05 PW in the winter and 0.23 ± 0.05 PW in the spring, and freshwater advected out of the SPG to be between 0.07 ± 0.02 Sv in the summer and 0.15 ± 0.02 Sv in the autumn. These estimates approximately balance the surface heat and freshwater fluxes over the SPG domain. Overturning in the SPG varies seasonally, with a minimum of 6.20 ± 1.40 Sv in the autumn and a maximum of 10.17 ± 1.91 Sv in the spring, with surface Ekman the most likely primary driver of this variability.  The density of maximum overturning is at 27.30 kgm^-3, with a second, smaller maximum at 27.54 kgm^-3.  Upper waters (σ0 < 27.30 kgm^-3) are transformed in the interior then exported as either intermediate water (27.30-27.54 kgm^-3) in the North Atlantic Current (NAC) or as dense water (σ0 > 27.54 kgm^-3) exiting to the south.  Our results support the present consensus that the formation and pre-conditioning of subpolar Mode Water in the north-eastern Atlantic is a key determinant of AMOC strength.

How to cite: Jones, S., Fraser, N., Cunningham, S., Fox, A., and Inall, M.: Observation-based estimates of volume, heat and freshwater exchanges between the subpolar North Atlantic interior, its boundary currents and the atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15026, https://doi.org/10.5194/egusphere-egu23-15026, 2023.

Based on data collected from 14 air–sea buoys in the Gulf Stream, this study presents an examination of how the hourly air–sea turbulent heat fluxes vary on subdaily timescales under persistent marine atmospheric boundary layer (MABL) stability conditions. The annual mean magnitudes of the subdaily variations in the latent heat (LH) and sensible heat (SH) at all stations are determined to be 40 and 15 Wm^-2, respectively. Under near-neutral conditions, the hourly anomalies of the air–sea humidity and temperature differences are the major drivers of the subdaily variations in LH and SH, respectively, followed by nonlinear effects and wind anomalies. Wind anomalies play dominant roles in determining the subdaily variations in LH and SH when the MABL is stable. In contrast, the contributions of the hourly anomalies of the air–sea differences in humidity and temperature are secondary but also significant. For a convectively unstable MABL, the wind anomalies control the subdaily variations in LH, whereas the subdaily variations in SH are dominated by the air–sea temperature anomalies. Accordingly, the above mechanism also controls the subdaily magnitudes. Quantitative estimates of the above relations are given in this study. However, compared to the observations when using daily mean SST, the subdaily variations in the reanalysis are found to be underestimated on average by 17% and 5% for LH and SH, respectively. Resolving the subdaily variations contributes significantly to the mean LH/SH estimates. For near-neutral and unstable MABLs, the subdaily contributions are O(100) and O(20) Wm^-2 for LH and SH, respectively, while they are O(10) Wm^-2 for LH/SH under stable conditions.

How to cite: Song, X., Xie, X., Yan, Y., and Xie, S.-P.: Observed subdaily variations in air–sea turbulent heat fluxes under different marine atmospheric boundary layer stability conditions in the Gulf Steam, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4621, https://doi.org/10.5194/egusphere-egu23-4621, 2023.

The Kuroshio-Oyashio Extension (KOE) is the North Pacific oceanic frontal zone where air-sea heat and moisture exchanges allow strong communication between the ocean and atmosphere. Using satellite observations and reanalysis datasets, we show that the KOE surface heat flux variability constitutes an essential component of the seasonal and decadal Pacific ocean/atmosphere variability. We first show a strong covariability between the winter air-sea heat exchange and decadal fluctuations of the Kuroshio Extension (KE) sea surface height (SSH; the SSH reflects upper-ocean heat content anomalies). Interannual to decadal variations of ocean subsurface heat content become strongly connected to the surface during early winter (i.e., November-December-January, NDJ), where they influence the strong ocean-to-atmosphere heat transfer over the KOE. During the early winter (NDJ), the enhanced Aleutian-Low-like atmospheric circulation associated with KE SSH helps to induce a substantial sea-air temperature difference through northwesterly winds over the warm ocean surface. The analysis over an extended time period (i.e., 1959-2022) exhibits that the KOE upward latent and sensible heat flux anomalies have been significantly enhanced since the late 1980s mainly due to increasing variance of the oceanic variability (e.g., KOE sea surface temperature) rather than atmospheric forcing changes (e.g., Aleutian Low). Our findings suggest that winter KOE heat flux variations can be useful climate proxies (e.g., KE SSH) as a physical indicator that links the subsurface ocean and atmosphere.

How to cite: Joh, Y., Delworth, T., Wittenberg, A., Yang, X., Rosati, A., Johnson, N., and Jia, L.: Increasing role of Kuroshio-Oyashio Extension variations as a conveyor of decadal ocean oscillation to seasonal air-sea heat exchange since the late 1980s, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10855, https://doi.org/10.5194/egusphere-egu23-10855, 2023.

Climate change and climate variability play a relevant role on the occurrence of conflicts in several parts of the world, including the tropics, where events of flooding and droughts are dictated by El Niño Southern Oscillation (ENSO). In order to better analyze possible relations between conflicts hot-spots and ENSO impacts on society and security, a better understanding of the dynamics of the latter is needed. ENSO is the result of an ocean-atmosphere feedback, which produces an irregular oscillation between a warm (El Niño) and a cold (La Niña) phase, peaking in boreal winter and recurring every 2-5 years. During La Niña phase, intensified trade winds accumulate warm water in to the west of the Pacific, leading to droughts in the northern US and catastrophic floods in regions such as northern Australia. During El Niño warm phase, westerly winds advect warm waters eastward towards the coasts of America, generating dry conditions in northern US and Canada, and wetter periods in the US Gulf Coast areas. Most of the studies on ENSO focused on the coupling between changes in the depth of the main thermocline, heat content in the surface layer of the water column and oceanic feedback on the zonal wind pattern. According to these works, the subsurface memory of the ocean (i.e. the heat stored in the surface layer), depends on the depth of the thermocline and the zonal shape of the isothermal surfaces is sustained by the dynamical balance between the zonal pressure gradient and the trade winds. This process systematically transfers heat westward and “charges” the western Pacific, which is then “discharged” through the action of eastward propagating internal Kelvin Waves (KW). While westerly wind events are known to play an important role in the generation of KW associated with El Niño, much less is known on the role of easterly winds. Here we show that the encountering between Westerlies and Easterlies determines the convergence, providing the initial forcing exciting internal, downwelling Rossby and Kelvin waves. Only KW formed east of 175^oE  reach the eastern Pacific boundary and determine an El Niño events, that become the more intense the more the waves are formed eastward, indicating a “zonal position” triggering of El Niño. It is shown here that the zonal shifts of the Easterlies/Westerlies convergence region displaces zonally in phase with region of the deep atmospheric convection and with the Southern Oscillation Index, indicating that changes in the large scale pressure system, the zonal position of westerly wind events, the easterly wind variability, the position of the deep atmospheric convection and El Niño are all intimately related features of the whole tropical Pacific climate system.

Funding from the STO Office of Chief Scientist 907EUR30 is gratefully acknowledged.

How to cite: Carniel, S. and Eusebi Borzelli, G.: Easterlies/Westerlies convergence in the tropical Pacific triggering El Niño initiation?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13731, https://doi.org/10.5194/egusphere-egu23-13731, 2023.