The temperature of the Arctic Ocean and adjacent areas has increased over recent decades and is expected to continue to increase through this century. Discrepancies between previous studies based on different periods, domains, and datasets suggest that the warming of the Arctic Ocean is nonlinear and characterized by large temporal and spatial variability. The behavior of this warming signal has numerous local effects on aspects from sea ice cover and surface fluxes to ecosystems, all of which react differently to linear warming compared to episodic warming events. The spatiotemporal warming pattern will also play a role in defining the product of dense-water formation in the region, which feeds into the lower limb of the Atlantic Meridional Overturning Circulation. Utilizing the ORAS5 reanalysis, the Arctic Subpolar gyre sTate Estimates, as well as in-situ observations, we present an overview of the warming of the Arctic Ocean and the Nordic Seas between 1958 and 2023. We shed light on the variability of trends and seasonal signals across the Arctic Ocean, from surface to abyss. This analysis provides a solid baseline for detecting regional changes in the mean state and variability of the Arctic Ocean with global warming and exploring the physical mechanisms causing the warming trend. As such, it will be key for grounding investigations of future changes in heat budgets for the Arctic Ocean and the Nordic Seas. 

How to cite: Rinde, B., He, S., and Li, C.: Multi-scale temperature variability in the Arctic Mediterranean between 1958 to 2023 – from surface to abyss , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20262, https://doi.org/10.5194/egusphere-egu24-20262, 2024.

How to cite: Franzke, C. and Harnik, N.: Long-Term Trends of the Atmospheric Circulation and Moist Static Energy Budget in the JRA-55 Reanalysis , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3002, https://doi.org/10.5194/egusphere-egu24-3002, 2024.

It has previously been shown that trends in sensible heat from climate models have had a substantial contribution to global precipitation changes. We illustrate that this is the case also in the most recent Coupled Model Intercomparison Project Phase 6 (CMIP6). However, we find that over the period since 1980 reanalysis do not support the reduction in sensible heat from the CMIP6 models and rather estimate a global increase in sensible heat which would contribute to a precipitation reduction. Satellite data over a period of 2 decades over global ocean similarly to reanalysis show an opposite sign of the sensible heat trend to the CMIP6 models.

How to cite: Myhre, G. and Jouan, C.: Strong contribution from sensible heat to global precipitation increase by climate models is not supported by observational based data. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3902, https://doi.org/10.5194/egusphere-egu24-3902, 2024.

Global mean precipitation is anticipated to increase by 2-4% per degree Kelvin, with intense events scaling at 7%, driven by boundary layer humidity. The understanding of the change in daily-to-annual precipitation probability density function remains rather incomplete. To address this knowledge gap, we employ Gini index to evaluate spatial unevenness and temporal inequality of precipitation under global warming in CMIP6 models. We observe heightened spatial unevenness of daily precipitation in tropics and extratropics over land and ocean. While the tropics maintain this unevenness over time, indicating large-scale convection aggregation, extratropical precipitation evens out with increasing timescales. This disparity suggests distinct processes governing daily and annual mean precipitation, underscoring the intensification of stronger storms over weaker events.

Globally, temporal inequality is on the rise, with more pronounced intensification in regions where projected precipitation deviates significantly from Clausius–Clapeyron scaling. Our hypothesis posits that the shift in precipitation distribution under warming projections stems from an increase in no-rain days coupled with rainfall events scaled by a constant. To assess this proposition, we construct a toy model predicting projected temporal inequality based on local hydroclimate conditions pre-warming, the projected mean precipitation, and a theorem-derived stretching parameter. The toy model demonstrates robust performance overall, except in regions notably influenced by the Hadley cell. Additionally, the model suggests that local precipitation events are scaled by a constant of approximately 1.07. Our analysis establishes meaningful connections among changes in mean precipitation, precipitation distribution, and dry-day number, offering comprehensive insights into hydroclimate transformations under global warming.

How to cite: Hsu, H. and Fueglistaler, S.: Robust predictions of changes in evenness of global precipitation under global warming, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4138, https://doi.org/10.5194/egusphere-egu24-4138, 2024.

The ocean temperature response to tropical cyclones (TCs) is important for TC development, local air–sea interactions, and the global air–sea heat budget and transport. As TCs and ocean temperature structures are changing in the recent decades, it is worthy to study their contribution on ocean heat uptake. The modulation of the upper ocean temperature structure after TCs were studied at the observation stations in the northern South China Sea. The upper ocean temperature and heat response to the TCs mainly depend on the combined effect of mixing and vertical advection. Mixing cooled the sea surface and warmed the subsurface, while upwelling (downwelling) reduced (increased) the subsurface warm anomaly and cooled (warmed) the deeper ocean. An ideal parameterization that depends on only the nondimensional mixing depth (HE), non-dimensional transition layer thickness (HT), and nondimensional upwelling depth (HU) was able to roughly reproduce sea surface temperature (SST) and upper ocean heat change. After TCs, the subsurface heat anomalies moved into the deeper ocean. The air–sea surface heat flux contributed little to the upper ocean temperature anomaly during the TC forcing stage and did not recover the surface ocean back to pre-TC conditions more than one and a half months after the TC. This work shows how upper ocean temperature and heat content varies by a TC, indicating that TC-induced mixing modulates the warm surface water into the subsurface, and TC-induced advection further modulates the warm water into the deeper ocean and influences the local and global ocean heat budget.

How to cite: Zhang, H.: Modulation of Ocean Temperature Structure and Heat Content by Tropical Cyclones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6938, https://doi.org/10.5194/egusphere-egu24-6938, 2024.

The amount of shortwave radiation absorbed by atmospheric water vapor is highly model dependent. This study examines how differences in the atmospheric water vapor shortwave radiation absorption affect the CO₂-induced climate response pattern. We control the atmospheric water vapor shortwave radiation absorption in Community Earth System Model 1.2.2 (CESM1-CAM4-POP2) by altering the water vapor shortwave absorptivity parameter k by 60% to 120% of the default value. The pre-industrial control simulations with different k values are integrated for 150 years and additional 150 years are integrated after abruptly quadrupling CO₂ concentrations. Regardless of the k value, the Atlantic meridional overturning circulation (AMOC) weakens in response to the quadrupling of CO₂. However, the simulation with a higher k value exhibits a faster AMOC recovery approximately 30 years after the quadrupled CO₂, with the lowest k simulation exhibiting a persistent AMOC weakening with no sign of recovery for the entire 300-year integration period. The faster AMOC restoration with a larger k value is attributed to the climatologically colder and saltier subpolar North Atlantic sea surface condition arising from the larger Arctic sea ice fraction due to colder temperature associated with stronger atmospheric shortwave absorption. The colder and more saline subpolar North Atlantic sea surface facilitates a more rapid destratification of surface density, establishing a favorable condition for the AMOC restoration. The faster restoration of the AMOC with the higher k value leads to a larger inter-hemispheric energy asymmetry followed by a more northward ITCZ shift as well as a stronger equilibrium climate sensitivity. This study demonstrates the complex interaction among different elements within the Earth system, encompassing radiation, sea ice, AMOC, and large-scale atmospheric circulation, suggesting a way to reduce uncertainties in future climate projections by improving the parameterization of shortwave radiation absorption by atmospheric water vapor.

How to cite: Lee, D., Kim, H., and Kang, S.: Future AMOC recovery modulated by atmospheric water vapor shortwave absorption, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8399, https://doi.org/10.5194/egusphere-egu24-8399, 2024.

The Northeast U.S. continental shelf is a highly productive and economically important region that has experienced robust changes in upper-ocean properties in recent decades. Warming rates exceed the global and North Atlantic average and in particular several episodes of anomalously warm temperatures, so called marine heatwaves, have had devastating impacts on regional fisheries over the past decade. There are also indicators of a salinification of the region, which might be linked to large-scale changes in the North Atlantic circulation as well as changes in regional processes, such as the number of Warm Core Rings shedding of the Gulf Stream, driving an increased salinity flux into the continental slope and shelf region. With now more than a decade of remote-sensing sea surface salinity data, we revisit seasonal to interannual salinity variability and assess the role of salinity for modulating stratification on the continental shelf. We provide important regional context for the interpretation of data from the OOI Coastal Pioneer array, a local shelf-break observatory. We find that the local seasonal cycle is an interplay of seasonal freshwater input via local river discharge, driving decreasing salinities in spring and summer not just on the shelf but also in the Slope Sea. An observed salinification in the fall is likely linked to offshore forcing over the slope associated with the presence of Warm Core Rings. A coherent low-frequency salinity variability is found over the slope and shelf region in the Mid-Atlantic Bight (MAB) and Gulf of Maine, highlighting that shelf conditions in particular in the MAB are not solely dominated by upstream shelf conditions but are significantly impacted by local offshore variability. Furthermore, we synthesise hydrographic data from the NOAA ECOsystem MONitoring (ECOMON) program to construct mean cross-shelf sections along the MAB to investigate the relative contributions of thermal and haline components to the seasonal stratification. Overall, salinity serves as a valuable tracer, in addition to temperature, of these multi-variate processes and with now more than a decade of satellite surface salinity can shed new light on the spatio-temporal variability on the Northeast U.S. continental shelf. 

How to cite: Ryan, S., Ummenhofer, C. C., and Gawarkiewicz, G. G.: New insights into seasonal to interannual salinity variability on the Northeast U.S. continental shelf and slope, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10237, https://doi.org/10.5194/egusphere-egu24-10237, 2024.

Observational evidence, supported by high resolution numerical model simulations, indicate that meso- and submesoscale dynamics exists in the deep ocean (>2000m). However, over most parts, observing the deep ocean is restricted to address either spatial but not temporal (ship surveys) or temporal but not spatial (moored sensors) scales of variability. The advent of a growing number of offshore geodesy experiments, conducted with networks of distributed sensor arrays, aiming to evaluate tectonic deformation through strain measurements can potentially provide new ways to observe deep sea hydrographic variability. Despite the different observing objectives of offshore geodetic and oceanographic experiments, a great overlap in the measured parameter space exists, which has motivated analyses exploring possible cross-benefits. Here we present the evaluation of temperature, pressure, and sound speed observations from a 2.5-year offshore geodesy experiment centered along the northern Chilean subduction zone (~21.5°S and ~71.5°W to ~70.5°W). Our analysis confirms multi-year warming trends that previous studies have reported for the deep ocean but shows an additional regionalization of warming trends. Superimposed onto the multi-year warming trend are temperature fluctuations that show multi-hourly to multi-weekly periods and amplitudes that show both spatial and depth/regional dependencies. Aside from a general decrease in energy levels of the fluctuations with depth, we see evidence of ocean-topography interactions through barotropic topography waves. Taken together, the observations reveal de-coupled dynamical regimes seaward and landward of the deep-sea trench that mark the extent of the abyssal part of the eastern boundary current off Chile and demonstrate the potential of time series from offshore geodetic surveys for hydrographic analyses.

How to cite: Jegen, A., Lange, D., Karstensen, J., Pizarro, O., and Kopp, H.: Deep ocean hydrographic heterogeneity inferred from offshore geodetic experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11362, https://doi.org/10.5194/egusphere-egu24-11362, 2024.

The temporal variability of temperature and salinity in the oceans is strongly impacted by the ocean stratification, which tends to constrain lateral advection and mixing to preferentially take place along approximately neutral surfaces. As a result, it is natural to seek a decomposition of thermohaline variability into heave and spice components, which splits temperature and salinity into a component contributing to density and one that is density-compensated. In this talk, I will outline the theoretical foundations for such an approach, based on a recent redefinition of spiciness, and illustrate its usefulness for understanding the variability of the ocean heat and salt contents in the EN4 dataset over the past century.

How to cite: Tailleux, R.: A new spice/heave decomposition of thermohaline variability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14534, https://doi.org/10.5194/egusphere-egu24-14534, 2024.

Multidecadal changes in the background state of the Indian Ocean, such as variations in ocean circulation patterns, sea level and heat storage, can act as a carrier wave for the climate change and other variabilities. The long-term (~60 years since 1958) analysis of subsurface ocean heat content (sub-OHC) in the Indian Ocean exhibits the presence of a dominant multidecadal meridional dipole mode in the region. The analysis shows that until the late 1980s, a basin-wide meridional dipole mode is present, followed by the mode embedded in uniform basin-wide patterns. The trends of thermocline depth and sea surface height also exhibit the similar patterns. It is found that two distinct mechanisms are account for the observed patters in the Indian Ocean. Firstly, Local wind forcing is responsible for the meridional dipole patterns. In the off-equatorial southern Indian Ocean region, wind stress and Ekman pumping velocity trends favor downwelling (upwelling), resulting in thermocline depth deepening (shallowing) during 1958-1975 and 1976-1987, respectively. Secondly, the observed basin-wide warming and cooling trends during 1988-2000 and 2001-2014 are explained by the combined effect of local wind forcing and heat transport from the western Pacific through the Indonesian throughflow.

How to cite: Amere, A. B., Dash, M. K., and Senapati, B.: Multidecadal meridional dipole mode in the Indian Ocean subsurface ocean heat content, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14622, https://doi.org/10.5194/egusphere-egu24-14622, 2024.

A set of 1000-year long abrupt stabilization simulations have been performed with the EC-Earth3 climate model. Each simulation follows a sudden stabilization of the external forcing, starting at different years of the CMIP6 historical and SSP5-8.5 scenario. The final global mean temperature increases range between 1.4 and 9.6 K with respect to the pre-industrial baseline.

We first explore here the evolution of the climate response at multi-centennial timescales and its dependence on the level of forcing, with regards to the climate feedback parameter and to patterns of surface warming. We then focus on the rate of heat storage in the global ocean, which is the main driver of the climate response at multi-centennial timescales. We find that the rate of warming of the deep ocean is almost independent from the amplitude of the forcing, so that most of the additional heat remains in the upper layers at high forcing. We hypothesize that this is due - at least partly - to a decreased ventilation of the deep ocean, caused by a general reorganization of the Meridional Overturning Circulation (MOC).

How to cite: Fabiano, F., Davini, P., Meccia, V. L., Zappa, G., Bellucci, A., Lembo, V., Bellomo, K., and Corti, S.: Multi-centennial evolution of the climate response and deep ocean heat uptake in a set of abrupt stabilization scenarios, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15419, https://doi.org/10.5194/egusphere-egu24-15419, 2024.

The Atlantic Meridional Overturning Circulation (AMOC) plays a pivotal role in the meridional transport of heat, freshwater and major dissolved gases such as carbon, and oxygen, making it a crucial component of earth’s climate system and the biosphere. On millennial timescales, the AMOC is believed to act as a conveyor belt of ocean currents wherein the flow varies coherently across latitudes. Past studies have drawn on this conveyor-belt idea to establish links between the AMOC and the Earth's climate tipping points. However, recent research and observations suggest that, on shorter timescales (days to decades), the AMOC may not operate as coherent flow of water. Understanding AMOC variability and coherence on such timescales and how these might respond to anthropogenic influences is crucial to predicting the climate of the next decades. This is, however, challenging due to the sparseness of the observational data in both time and space. Here, we present a Bayesian Hierarchical modelling framework that combines observations from altimetry, gravimetry, and Argo floats to estimate meridional heat transport across the Atlantic. Our approach considers error structures jointly and accounts for spatiotemporal dependencies between processes (thermosteric, halosteric and ocean mass), providing a coherent way to propagate uncertainty and overcoming the limitations of hydrography-only based analyses. Our estimate of heat transport is in very good agreement (correlation of ~0.8 for 3-month means) with that from RAPID observations at 26°N. A meticulous comparison of mean and variance further underscores the precision of our estimates compared to those derived from heat budgets. Our method can be extended to gain further insights into the dynamics and meridional coherence of AMOC at shorter timescales.

How to cite: Vallivattathillam, P. and Calafat, F. M.: Estimating Atlantic meridional heat transport through Bayesian modelling of altimetry, Argo and GRACE data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16423, https://doi.org/10.5194/egusphere-egu24-16423, 2024.

This study examines how the geographic location of sea surface temperature (SST) biases influences global atmospheric responses. Utilizing an intermediate-complexity atmospheric model, 106 century-long simulations with idealized SST perturbations—emulating biases in coupled climate models—were performed. The intensity of the global atmospheric response to SST biases is evaluated by quantifying changes in global wave energy and interannual variance. The findings underscore the response's dependency on local background SST. Notably, with an imposed SST bias of +1.5 K, a significant global response is triggered once background SST surpasses approximately 25°C. This geographic dependency is related to the critical SST threshold for intense convection. Consequently, these results highlight the need for heightened focus on tropical oceans, especially the Indo-West Pacific, where SST biases can significantly impact the accuracy of global climate simulations.

How to cite: Zhao, Y.-B., Zagar, N., and Lunkeit, F.: On critical dependence of atmospheric circulation response to regional SST biases on background SST, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16722, https://doi.org/10.5194/egusphere-egu24-16722, 2024.

Absorbing aerosol species, such as Black (BC) and Brown (BrC) Carbon, are able to warm the atmosphere. The role of aerosols is one of the least clear aspects in the so called “Arctic Amplification” (AA) and up to now this was mostly modelled [1,2]. For this reason, we took part in four scientific cruises (AREX, Arctic-Expedition, summer 2018, 2019, 2021 and EUREC4A, 2020) in the North Atlantic, eastward and south-eastward of Barbados, aiming at the determination of the aerosol chemical composition and properties from the Tropics to the North Pole.

The Heating Rate (HR) was experimentally determined at 1 minute time-resolution along different latitudes by means of an innovative methodology [3], obtained by cumulatively taking into account the aerosol optical properties, i.e. the absorption coefficients (measured by AE33 Aethalometer) and incident radiation (direct, diffuse and reflected) across the entire solar spectrum. The HR computed along AREX and in Milan (in the same period) were used to determine the energy gradient, due to the LAA induced heat storage at mid-latitudes, which contributes to AA through the atmospheric heat transport northward.

Moreover, aerosol chemical composition was achieved by means of sampling via high volume sampler (ECHO-PUF Tecora) and analysis via ion chromatography, TCA08 for Total Carbon content, Aethalometer AE33 (for BC), ICP-OES for elements.

A clear latitudinal behaviour in Black Carbon concentrations, with the highest values at low latitudes (e.g. average BC concentration in Gdansk up to 1507±75 ng/m³) and a progressive decrease moving northwards and away from the big Arctic settlements (Black Carbon concentrations within the 81st parallel: 5±1 ng/m³).

According to the latitudinal behaviour of BC concentrations and solar radiation (decreases towards the north while the diffuse component increases), HR decreases noticeably towards the Arctic: e.g. higher in the harbor of Gdansk (0.290±0.010 K/day) followed by the Baltic Sea (0.04±0.01 K/day), the Norvegian Sea (0.010±0.010 K/day) and finally with the lowest values in the pure Arctic Ocean (0.003±0.001 K/day). Accordingly, the energy density added to the system by the aerosol, a positive forcing that differs by 2 orders of magnitude between mid-latitudes and North Pole was found: 347.3 ± 11.8 J/m³ (Milan), 244.8 ± 12.2 J/m³ (Gdansk) and 2.6 ± 0.2 J/m³ (80°N). These results highlight the presence of a great energy gradient between mid-latitudes and Arctic that can trigger a heat transport towards the Arctic. Moreover this was strengthen by the HR value for EUREC4A in Barbados that was 0.175±0.003 K/day. Finally, preliminary results from Antarctica collected onboard the Italian RV Laura Bassi cruising the Southern Ocean and the Ross Sea will be shown.

Acknoledgements: GEMMA Center, Project TECLA MIUR – Dipartimenti di Eccellenza 2023–2027. JPI EUREC4A-OA project. CAIAC (oCean Atmosphere Interactions in the Antarctic regions and Convergence latitude) PNRA project

References

[1] Navarro, J. C. A. et al. (2016) Nat. Geosci. 9, 277–281.

[2] Shindell, D. and Faluvegi, G. (2009) Nat. Geosci. 2, 294–300.

[3] Ferrero, L. et al. (2018) Environ. Sci. Technol. 52, 3546 3555.

How to cite: Ferrero, L., Losi, N., Rigler, M., Gregorič, A., Močnik, G., Markuszewski, P., Makuch, P., Zielinski, T., Pakszys, P., Rinaldi, M., Paglione, M., Lupi, A., and Bolzacchini, E.: Heating rate and energy gradient from the tropics to the North Pole, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17118, https://doi.org/10.5194/egusphere-egu24-17118, 2024.

Numerical models have become an important tool for investigating the oceans heat and salt contents. The large-scale thermohaline overturning circulation in the world ocean is directly linked to the transformation of water masses caused by small-scale diapycnal mixing, which is parameterized in models. However, in addition to this physically justified "physical mixing", numerical transport schemes rely on additional "numerical mixing" for stability reasons. Thus, the simulated overturning circulation in ocean models is strongly affected by this spurious mixing.
Diagnostics of spurious mixing in terms of local tracer variance decay offer a detailed analysis of water mass transformations (WMT). Vice versa, analysis methods for WMT can be used to deduce information about the effects of mixing. In contrast to direct mixing diagnostics based on discrete variance decay (DVD) in geographical space, the WMT analysis framework is based on a mapping to tracer space, where diatracer fluxes that quantify the WMT can directly be diagnosed. Recently, a new local framework was derived, which combines the classical WMT framework with the local DVD analysis (Klingbeil & Henell, 2023). The derived analytical relations between dia-surface fluxes and mixing were demonstrated in an isohaline framework by Henell et al. (2023) [see corresponding submission to this session].
We will present how this methodology can be transferred to the world ocean in order to diagnose local diapycnal mixing and to quantify the spurious contribution to the simulated thermohaline overturning circulation in ocean models. In particular, the extension to density space requires the consistent quantification of density DVD, which is challenging in numerical models with prognostic equations for salinity and temperature and a non-linear equation of state.

Henell, E., H. Burchard, U. Gräwe, K. Klingbeil (2023) Spatial composition of the diahaline overturning circulation in a fjord-type, non-tidal estuarine system. Journal of Geophysical Research (Oceans). https://dx.doi.org/10.1029/2023JC019862.

Klingbeil, K. and E. Henell (2023) A Rigorous Derivation of the Water Mass Transformation Framework, the Relation between Mixing and Diasurface Exchange Flow, and Links to Recent Theories in Estuarine Research. Journal of Physical Oceanography. https://doi.org/10.1175/JPO-D-23-0130.1.

How to cite: Klingbeil, K., Henell, E., Banerjee, T., Burchard, H., and Danilov, S.: Analysis of water mass transformations and the spurious thermohaline overturning circulation in numerical ocean models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19893, https://doi.org/10.5194/egusphere-egu24-19893, 2024.

We apply a local water mass transformation framework to quantify and decompose the exchange flow associated with diahaline mixing. As a realistic example we analyze two years of numerical model results for the Baltic Sea, which serves as a natural laboratory for processes relevant on the global scale. Despite this regional focus, the diagnostic methods of this study are applicable to diverse regions, as well as for other tracers than salinity, e.g. temperature. We verify relations between local diahaline volume and diffusive salt fluxes, and local diahaline mixing, and present them as maps on chosen isohaline surfaces. In this way, hot spots for mixing and the diahaline circulation are visualized. Two dominant types of diahaline exchange flow are analyzed. First of all there is a large scale overturning circulation with inflow at places where the isohaline surface is close to the bottom and with outflow at places where the isohaline is surfacing. Secondly, there is the well-known small-scale overturning circulation localized inside the bottom boundary layer over sloping bathymetry, driven by boundary mixing. One major result is that about 50% of the simulated diahaline exchange flow is generated by numerical mixing caused by the truncation error of the advection scheme, despite the fact that an anti-diffusive advection scheme and vertically-adaptive coordinates are used. We also demonstrate how model ensembles can be used to study short-term episodic and local events.

How to cite: Henell, E., Burchard, H., Gräwe, U., and Klingbeil, K.: A Detailed Analysis of the Diahaline Overturning Circulation in a Marginal Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1830, https://doi.org/10.5194/egusphere-egu24-1830, 2024.

Equatorial superrotation is a striking feature in planetary circulations, also found in atmospheric circulation models. Geological evidence shows that Earth was in a state of super-rotation during the Eocene and Pliocene. On Earth, such a time period of super-rotation is sometimes referred to as permanent El Niño. While it is well established that a tropical wave source is needed for superrotation, the mechanism that provides this wave source, and what conditions allow it to be maintained are still not understood, and vary between different models. Specifically, in shallow water models with Earth like parameters, superrotation has only been found when relatively strong thermal damping was added. In this study we examine the spontaneous evolution of super-rotation in fully developed isotropically forced two-dimensional moist shallow-water turbulence, and examine the role of moisture by varying the strength of moisture coupling, and performing large ensembles of simulations. 

We find that while the dry runs exhibit both superrotation and sub-rotation, with spontaneous transitions between the two states, moisture results in all runs eventually reaching a stable superrotating state. We further find that a stable superrotation develops in the dry runs when we strengthen the thermal damping. We find that a meridional mass flux from the equator to the subtropics, develops in the runs with stable superrotation, and examine the role of this mass flux, which is enabled by the latent heating and the thermal damping, for the maintenance of the stable superrotation. 

How to cite: Harnik, N., Schröttle, J., Suhas, D., and Sukhatme, J.: Spontaneous equatorial flow reversals at the equator in  moist shallow water turbulence, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11925, https://doi.org/10.5194/egusphere-egu24-11925, 2024.