Displays

Net primary production (NPP) is indicative of the energy available to an ecosystem, which is central to ecological functioning and biological carbon cycling. The Southern Ocean’s Weddell Sea (WS) represents a point of origin where water masses form and exchange with the atmosphere, thereby setting the physical and chemical conditions of much of the global ocean. The WS is particularly understudied near Larsen C Ice Shelf (LCIS) where harsh sea-ice conditions persist year-round. We measured size-fractionated rates of NPP, nitrogen (N; as nitrate, ammonium, and urea) uptake, and nitrification, and characterized the phytoplankton community at 19 stations in summer 2018/2019, mainly near LCIS, with a few stations in the open Weddell Gyre (WG) and at Fimbul Ice Shelf (FIS). Throughout the study region, NPP and N uptake were dominated by nanophytoplankton (3-20 μm), with microphytoplankton (>20 μm) becoming more abundant later in the season, particularly at FIS. Here, we observed high phytoplankton biomass and diversity, and the community was dominated by diatoms known to enhance carbon export (e.g., Thalassiosira spp.). At LCIS, by contrast, the community comprised mainly Phaeocystis Antarctica. In the open WG, a population of small and weakly-silicified diatoms of the genus Corethron dominated the phytoplankton community. Here, euphotic zone-integrated uptake rates were similar to those at LCIS even though the depth-specific rates were lower. Mixed-layer nitrification was below detection at all stations such that nitrate uptake can be used as a proxy for carbon export potential sensu the new production paradigm – this was highest near FIS in late summer. Our observations can be explained by melting sea ice near the ice shelves that supplies iron and enhances water column stratification, thus alleviating iron and/or light limitation of phytoplankton and allowing them to consume the abundant surface macronutrients. That the sea ice melted completely at FIS but not LCIS may explain why late-summer productivity and carbon export potential were highest near FIS, more than double the rates measured in early summer and near LCIS. The early-to-late summer progression near the ice shelves contrasts that of the open Southern Ocean where iron is depleted by late summer, driving a shift towards smaller phytoplankton that facilitate less carbon export.

How to cite: Flynn, R., Burger, J., Smith, S., Spence, K., Bornman, T., and Fawcett, S.: Productivity and carbon export potential in the Weddell Sea, with a focus on the waters near Larsen C Ice Shelf, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21107, https://doi.org/10.5194/egusphere-egu2020-21107, 2020.

Sea ice growth into seawater proceeds by congelation of almost pure ice crystals, with most sea salts being rejected into a solute-enriched boundary layer. This solute enrichment does locally change the freezing point and thereby alter crystal growth due to a process termed "Constitutional Supercooling" (heat diffusing much faster than solute). It also sets up solute gradients that drive convection in a boundary layer of thickness D/V, where D is the coefficient of solute diffusion and V the ice growth velocity. The crystal pattern arising from such interaction of solute and temperature gradients, and respective solute and heat fluxes at the freezing interface, has been observed for other systems. It may be described by the theory of "Morphological Instability" formulated half a century ago by Mullins and Sekerka (1964). In principal this theory predicts the cell or dendrite spacing at a freezing interface of a saline solution. However, application of the theory to the case of sea ice growing from seawater has been incomplete so far due to three aspects: (i) detailed observations of the sea ice - ocean interface are difficult to obtain, as this high-porosity regime is rather fragile and often lost during sampling, and/or altered during cooling when the sample is removed from the water. However, from thin sections further up in the ice the interface is known to be lamellar, consisting of vertical oriented plates with a distance of 0.3 to 1 mm found for natural growth conditions; (ii) observations of the solutal boundary layer are even sparser and limited to a few laboratory studies, where this layer was heavily convecting; (iii) from a theoretical point it turns out that the classical Mullins-Sekerka theory needs to be modified due to such a convecting boundary layer. In the present talk I review the existing observations, and present novel 3-d observations of the microstructure near the interface of growing sea-ice. I propose an application of morphological stability theory to predict the plate spacing of sea ice and the salt fluxes through a convectively unstable solutal boundary layer. The predictions are consistent with observed plate spacings over a wide range of ice growth velocities, ranging from fast (100 cm/day) laboratory ice growth to slow (0.01 cm/day) accretion at the bottom of marine ice shelves. These predictions are not only of importance to predict ice properties near the interface, yet indicate the potential to trace sea ice and ice shelf growth rates through microstructure observations. Regarding the solutal boundary layer, the theory is found to be consistent with published observations of salt fluxes from growing sea ice.

W. W. Mullins and R. F. Sekerka, J. Appl. Phys., 1964, 35, 444.

How to cite: Maus, S.: Microstructure and solutal boundary layer at the sea ice - ocean interface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6039, https://doi.org/10.5194/egusphere-egu2020-6039, 2020.

The Bellingshausen Sea, located between the West Antarctic Peninsula and the Amundsen Sea, is poorly observed, compared with its neighbours. The Antarctic Slope Front (ASF), that rings the continental slope of Antarctica, supports a westward current (the Antarctic Slope Current). The structure and variability of this current affect exchange processes close to Antarctica such as the transport of warm Circumpolar Deep Water onto the Antarctic continental shelf. This water mass is responsible for the transport of heat across the shelf and therefore the basal melting of ice shelves. Due to the lack of observations, it is still unclear if the ASF even exists in the Bellingshausen Sea or if there are other processes moderating the transport of warm water onto the shelf.

We present ship-based and glider-based CTD data collected in 2007 and 2019, which in total provide 7 cross-slope sections in the Bellingshausen Sea. Geostrophic velocities are referenced to lowered ADCP data, shipboard ADCP data and the Dive Average Current. Cumulative transports show remarkable differences between the years 2007 and 2019. The sections of 2007 provide cumulative transports of up to 3.5 Sv eastward. In contrast, the sections in 2019 have cumulative transports up to 2 Sv westward. The sections from 2007 and 2019 are in very similar locations, indicating a temporal change rather than a spatial change.

We compare the cross-slope sections from the observations with sections from the NEMO 1/12 ° model output. A time series of cumulative transports from the model, covering the years from 2000 to 2010, allows us to identify seasonality and interannual variability in this current system.

How to cite: Oelerich, R., Heywood, K. J., Damerell, G. M., and Thompson, A. F.: Cross-Slope Observations in the Bellingshausen Sea, Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-463, https://doi.org/10.5194/egusphere-egu2020-463, 2020.

Millions of small, high-altitude lakes freeze their surface waters each winter season. Observations, however, reveal that their water temperature is increasing and the ice-on period is declining. Altering the thermal regime of ice-covered lakes have multiple impacts, including the loss of ecosystem services to increments in greenhouse gas emissions. These pervasive changes are affected by the heating rate of under-ice waters, which in turn regulates the water-to-ice heat flux, and therefore the rate of ice melting. In such aquatic systems, solar radiation warms the water beneath a diffusive boundary layer, thereby increasing its density and providing energy for convection in a diurnally-active mixing layer. Shallow regions are differentially heated to warmer temperatures, driving downslope buoyancy-driven currents that transport warm water to the interior basin. We characterize the energetics of these processes, focusing on the rate at which solar radiation supplies energy that is available to drive fluid motion. Using numerical simulations, we show that advective fluxes due to differential heating contribute to the evolution of the mixed-layer in waterbodies with significant shallow areas. We use a heat balance to assess the relative importance of differential heating to the one-dimensional effects of radiative heating and diffusive cooling at the ice-water interface in waterbodies of varying morphologies.

How to cite: Ulloa, H. N., Winters, K. B., Wüest, A., and Bouffard, D.: Radiative heating of ice-covered waterbodies of varying morphologies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20332, https://doi.org/10.5194/egusphere-egu2020-20332, 2020.

Passive microwave remote sensing observations and atmospheric data are used to characterize the impact of the Mertz Glacier Tongue (MGT) calving in February 2010 on the sea ice conditions in the D’Urville Trough, East Antarctic shelf (139°E-141°E). The main objective is to determine if conditions for dense shelf water production in this area were possibly influenced by the calving. In particular, we look for the existence of winter polynyas capable of sustaining significant sea ice production, a prerequisite for the formation of dense, saline waters. We show that polynyas in the D'Urville area are part of a complex icescape made of fast ice and drifting pack ice. The seasonal evolution of this icescape has been profoundly modified with the calving of the MGT and opening of new polynyas. Pre-calving and post-calving sea ice concentrations are analyzed to identify major patterns of variability. Examination of the fast ice distribution and atmospheric forcing helps to develop a scenario for the formation of low sea ice concentration regions and their relation to the sea ice fluxes, supporting the conclusion that the role of the Adelie Bank as a barrier to the drift ice may have strengthened after the calving.

How to cite: Ricard, L., Houssais, M.-N., Herbaut, C., Fraser, A., Massom, R., and Blaizot, A.-C.: Impact of the Mertz Glacier Tongue calving on the emergence of polynyas in the d'Urville Trough, East Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17820, https://doi.org/10.5194/egusphere-egu2020-17820, 2020.

Arctic sea ice is one of the most sensitive components of the Earth’s climate system. The underlying ocean plays an important role in the evolution of the ice cover through its heat flux at the ice-ocean interface which moderates ice growth and melt. Despite its importance, the spatio-temporal variations of this heat flux are not well understood. In this work, we combine direct numerical simulations of turbulent convection over fractal surfaces and analysis of time-series data from the Surface Heat Budget of the Arctic Ocean (SHEBA) program using Multifractal Detrended Fluctuation Analysis (MFDFA) to understand the nature of fluctuations in this heat flux. We identify key physical processes associated with the observed Hurst exponents calculated by the MFDFA, and how these evolve over time. We also discuss ongoing work on constructing simple stochastic models of the ocean heat flux to the ice, and potential use as a parameterisation.

How to cite: Toppaladoddi, S. and Wells, A.: From red to white: the time-varying nature of ocean heat flux to Arctic sea ice, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13781, https://doi.org/10.5194/egusphere-egu2020-13781, 2020.

A novel regional ocean-sea-ice model configuration is designed to investigate the mechanisms of ocean–ice-shelf-melt interactions in the Weddell Sea. It features explicit resolution of the cavities of eastern Weddell, Larsen and Filchner-Ronne ice-shelves (FRIS, at 1.5-2.5 km horizontal resolution), as well as of the adjacent continental shelves (~2.5 km) and deep open-ocean gyre (at 2.5-4 km), in presence of interannually-varying atmospheric and ocean boundary forcing as well as explicit ocean tides. Simulated circulation, water mass and ice-shelf melt properties compare overall well with available open-ocean and cavity observations, and simulated Weddell ice-shelf melting reveals large variability on tidal, seasonal and year-to-year timescales. The presence of ocean tides, investigated explicitly, is revealed to result in a systematic time-average enhancement of both the production of ice-shelf meltwater as well as its refreezing on ascending branches of especially the FRIS cavity circulation. This tide-driven enhancement of the melt-induced FRIS cavity circulation acts to increase net ice-shelf melting (by 50%, ~50 Gt/yr) and the meltwater export by the FRIS outflow, and modulates their seasonal and lower frequency variability. The tidal impact on ice-shelf melting is consistent with being primarily driven mechanically through enhanced kinetic energy of the time-varying flow in contact with the ice drafts. The dynamically-driven tide-induced melting is thereby to almost 90% compensated by cooling through meltwater produced, but not quickly exported from regions of melting in the Weddell cold-cavity regime. Ocean boundary layer thermal adjustment underneath ice drafts, minimizing departures from the in-situ freezing point, thus substantially dampens the impact of tides on Weddell ocean–ice-shelf interactions. Simulations furthermore suggest attendant changes on the open-ocean continental shelves, characterized by overall freshening and modest year-round sea-ice thickening, as well as a marked freshening of newly-formed bottom waters in the southwestern Weddell Sea.

How to cite: Hausmann, U., Sallée, J.-B., Jourdain, N., Mathiot, P., Rousset, C., Madec, G., Deshayes, J., and Hattermann, T.: The role of tides in ocean--ice-shelf interactions in the southwestern Weddell Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22464, https://doi.org/10.5194/egusphere-egu2020-22464, 2020.

The physical oceanographic environment, water mass mixing and transformation in the area adjacent to Larsen C Ice Shelf (LCIS) are investigated using hydrographic data collected during the Weddell Sea Expedition 2019. The results shed light on the ocean conditions adjacent to a thinning LCIS, on a continental shelf that is a source region for the globally important water mass, Weddell Sea Deep Water (WSDW). Modified Weddell Deep Water (MWDW), a comparatively warmer water mass of circumpolar origin, is identified on the continental shelf and is observed to mix with local shelf waters, such as Ice Shelf Water (ISW), which is a precursor of WSDW. Oxygen measurements enable the use of a linear mixing model to quantify contributions from source waters revealing high levels of mixing in the area, with much spatial and temporal variability. Heat content anomalies indicate an introduction of heat, presumed to be associated with MWDW, into the area via Jason Trough. Furthermore, candidate parent sources for ISW are identified in the region, indicating the potential for the circulation of continental shelf waters into the ice shelf cavity. This highlights the possibility that offshore climate signals are conveyed under LCIS. ISW is observed within Jason Trough, likely exiting the sub-ice shelf cavity en route to the Slope Current. This onshore-offshore flux of water masses links the region of the Weddell Sea adjacent to northern LCIS to global ocean circulation and Bottom Water characteristics via its contribution to ISW and hence WSDW properties. 

What remains to be clarified is whether MWDW found in Jason Trough has a direct impact on basal melting and thus thinning of LCIS. More observations are required to investigate this, in particular direct observations of ocean circulation in Jason Trough and underneath LCIS. Modelling experiments could also shed light on this, and so preliminary results based on NEMO global simulations explicitly representing the circulation in under-ice shelf seas, will be presented. 

How to cite: Hutchinson, K., Deshayes, J., Sallee, J.-B., Dowdeswell, J., de Lavergne, C., Ansorge, I., Luyt, H., Henry, T., and Fawcett, S.: Water Mass Characteristics and Distribution Adjacent to Larsen C Ice Shelf, Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-112, https://doi.org/10.5194/egusphere-egu2020-112, 2020.

Ice shelf dynamics play a key role in the climate. Melt-rates along the ice shelf-ocean interface are an important aspect in determining the character of global sea level rise. A representation of ice shelf melt is currently implemented in various z-level General Circulation Models (GCMs) by employing parameterisations of the small scale boundary layer dynamics. However, these parameterisations are strongly dependent on the near boundary flow and at the spatial scales for which GCMs are intended the boundary layer is not well resolved. We investigate the ability of a GCM in representing these small scale boundary effects. This is done using MITgcm in an idealised setting with a sloping ice-ocean interface.

How to cite: Patmore, R., Holland, P., and Vreugdenhil, C.: Assessing z-level modelling of the ice shelf - ocean boundary layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10388, https://doi.org/10.5194/egusphere-egu2020-10388, 2020.

We report on mooring observations of tidal currents in Prydz Bay, East Antarctica. Tides in Prydz Bay are mixed diurnal-semidiurnal and much weaker than that in the Ross Sea and the Weddell Sea, with the spatial and temporal averaged value of 2.58 cm s^-1 for all the current meter observations over the continental shelf. The major axes of the tidal ellipses are generally aligned south-north, probably steered by the topography. The tidal phases are modulated by both the baroclinic and barotropic tidal components. The averaged tidal kinetic energy can account for a fraction of ~13% with respect to the total kinetic energy at the Amery Ice Shelf calving front during the observing period. The long-term average tidal heat flux across the Amery Ice Shelf calving front is negligible, but the ratio of the tidal heat flux standard deviation to the residual heat flux standard deviation can be up to 41%. We also report on borehole observations of tide-like pulsing of potential temperature and salinity, indicating the indispensable tidal influences in the ice-ocean boundary layer. These mooring and borehole data support that the tidal processes should be highlighted in the investigations of the interaction between the Amery Ice Shelf and ocean.

How to cite: Liu, C., Wang, Z., Cheng, C., Liang, X., Wu, Y., Li, X., Liu, Y., and Yuan, X.: On the modified Circumpolar Deep Water upwelling over the Four Ladies Bank in Prydz Bay, East Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2319, https://doi.org/10.5194/egusphere-egu2020-2319, 2020.

Antarctic ice shelves restrain the flow of grounded ice into the ocean, and are thus an important control on Antarctica’s contribution to global sea level rise. West Antarctica represents the largest source of uncertainty in future sea level projections, and Thwaites Glacier has the potential to influence sea level more than any other outlet glacier in this region. The future behaviour of Thwaites Glacier is particular sensitive to basal melting in the grounding zone region. Basal melting is controlled by the turbulent transfer of heat through the ice shelf-ocean boundary layer. The physics of this boundary layer is poorly understood, however, and its inadequate representation in numerical models is hampering our ability to predict the future evolution of the Antarctic ice sheet. Using a hot-water drilled access hole, a turbulence instrument cluster was deployed in the grounding zone region of Thwaites Glacier in January 2020. By observing the momentum and scalar fluxes, these observations provide a unique opportunity to explore the important turbulent processes responsible for modulating the basal melt rate in this region. Ultimately, this observational effort will allow us to better constrain our parameterisations of the grounding zone region in large-scale numerical models, facilitating more accurate simulations of the Antarctic ice sheet in the changing climate.

How to cite: Davis, P., Nicholls, K., and Holland, D.: Turbulence Observations in the Grounding Zone Region of Thwaites Glacier, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-50, https://doi.org/10.5194/egusphere-egu2020-50, 2020.

The isotopic composition of seawater represents an important fingerprint of water masses, containing information about conditions during their formation and evolution. Following the spatial and temporal variability of either δD or δ¹⁸O of water in the ocean will provide a direct link to the freshwater cycle, allowing to discriminate between different water masses, such as the one coming from glacier and sea ice melting, freshwater rivers and precipitations.

Information from physical parameters (e.g. temperature and salinity) are not always enough for identifying the undergoing processes, and current knowledge of water isotopes or noble gases in the ocean remains very poor due to scarcity of measurements obtained from discrete sampling followed by laboratory analysis.

Here we present a novel in-situ Membrane Inlet Laser Spectroscopy (MILS) sensor which is currently under development. The sensor will provide simultaneous and continuous measurements of water isotopes (both δD and δ¹⁸O, expected precision of ~0.05‰) and will be adapted for deployment from vessels, through boreholes into the ice shelves, and to be integrated in autonomous underwater vehicles (AUVs). The instrument will run on batteries, with an autonomy of ~12h.

From 2021, the MILS sensor will be ready for field deployments, particular in the Southern Ocean, where high resolution water isotope data inside ice shelf cavities could be coupled with modelling approaches for better understanding the processes at work in ocean - ice shelf interactions, and better constraint the ice melting processes in Antarctica, which remains today a major challenge.

How to cite: Grilli, R. and Blouzon, C.: Subsea Water Isotope Sensors: A novel tool for continuous and in-situ analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2984, https://doi.org/10.5194/egusphere-egu2020-2984, 2020.

How to cite: Mack, S., Dinniman, M., Klinck, J., McGillicuddy, Jr., D., and Padman, L.: Modeling ocean eddies and their effects on ice shelf basal melt rate in the Ross Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20463, https://doi.org/10.5194/egusphere-egu2020-20463, 2020.

Predictions of ice shelf melting depend on dynamical insights into ocean boundary layers below ice shelves. Fundamental questions regarding the nature of stratified turbulence below the sloped and ablating ice shelf base remain. Laboratory experiments, direct numerical simulations, and observations have yielded important insights, but have yet to produce a robust relationship between ice shelf melt rates and shear- and buoyancy-driven mixing. This relationship is the target of our Large-Eddy Simulations (LES) of the ice-shelf ocean boundary layer. Several new developments were applied to the LES code PALM to produce dynamic melting as well as tides. In this presentation, we demonstrate these new model capabilities. We contrast profiles of vertical turbulent fluxes of heat, salt and momentum across different simulated ice shelf settings: cold, shear-dominated settings vs. warm, buoyancy-dominated settings. We also discuss our recent work toward a new ice-shelf melt parameterization for use in large-scale ocean models on the basis of these simulations. A new melt parameterization is a critical component of ongoing ice-ocean coupling efforts, both to place melt rate predictions on a more physical footing and to achieve convergence with vertical ocean model resolution, on which current parameterizations fail.

How to cite: Begeman, C. B., Asay-Davis, X., and Van Roekel, L.: Toward a new ice-shelf melt rate parameterization with large-eddy simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10848, https://doi.org/10.5194/egusphere-egu2020-10848, 2020.

The Totten ice shelf drains over 570 000 km² of East Antarctica. Most of the ice sheet that drains through the Totten ice-shelf is from Aurora Subglacial Basin and is marine based making the region potentially vulnerable to rapid ice sheet colapse.
Understanding how the changes in ocean circulation and properties are causing increased basal melt of Antarctic ice shelves is crucial for predicting future sea level rise.
In the context of the The PARAMOUR project (decadal predictability and variability of polar climate: the role of atmosphere-ocean-cryosphere multiscale interaction), we use a high resolution NEMO-LIM 3.6 regional model to investigate the variability and the predictability of the coupled climate system over the Totten area in East Antarctica.
In this poster, we will present our on-going work about the impact of landfast ice over the variability of the system. Landfast ice is sea ice that is fastened to the coastline, to the sea floor along shoals or to grouded icebergs. Current sea ice models are unable to represent very crudely the formation, maintenance and decay of coastal landfast ice. We applyed several parameterization for modeling landfast ice over the Totten ice shelf area.

How to cite: Van Achter, G., Pelletier, C., and Fichefet, T.: Investigating the climate variability in the Totten area using NEMO-LIM regional model., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8075, https://doi.org/10.5194/egusphere-egu2020-8075, 2020.

In this talk I will present preliminary results of direct numerical simulations of ice melting in a turbulent stratified shear flow. The model solves the evolution of the turbulent fluid phase and of the diffusive solid ice phase, due to melting and freezing, in a fully coupled way. This is done by combining a Direct Numerical Simulation (DNS) code with a novel formulation of the equations for the solid and liquid phases of water based on the phase-field method. DNS enables turbulent motions to be simulated without approximation, i.e. solving Navier Stokes equations, while the phase-field method allows the ice-ocean interface to be rough and evolve in response to melting. I will present results on the turbulent boundary layer and on the self-generated basal topography at the ice-water interface. The ultimate goal of this work is to propose a new DNS-based parameterization of the melting process at rough ice-ocean boundaries that takes into account the effects of temperature and salt stratification, and flow velocities.

How to cite: Couston, L.-A., Hester, E., Favier, B., Jenkins, A., and Holland, P.: Ice melting in a turbulent stratified shear flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19054, https://doi.org/10.5194/egusphere-egu2020-19054, 2020.

The ocean surface boundary layer in the Southern Ocean plays a critical role in heat and carbon exchange with the atmosphere. Submesoscale flows have been found to be important in setting mixed layer variability in the Antarctic Circumpolar Current (ACC). However, sparsity in observations, particularly south of the ACC in the Antarctic Seasonal Ice Zone (SIZ) where the horizontal density structure of the mixed layer is influenced by sea ice melt/formation and mesoscale stirring, brings into question the ability of climate models to correctly resolve mixed layer variability. We present novel fine-scale observations of the activity of submesoscale variability in the ice-free Antarctic SIZ using three deployments of underwater gliders over an annual cycle. Salinity-dominated density fronts of O(1)km associated with strong horizontal buoyancy gradients are observed during all deployments. There is evidence that stratifying ageostrophic eddies, energised by salinity driven submesoscale fronts are active across seasons, with intermittent equivalent heat fluxes of the same order to, or greater than local atmospheric forcing. This study highlights the need to consider future changes of Antarctic sea-ice in respect to feedback mechanisms associated with salinity (sea-ice) driven submesoscale flows.

How to cite: Giddy, I., Nicholson, S., Du Plessis, M., Thompson, A., and Swart, S.: The Seasonality of submesoscale variability in the Antarctic Seasonal Ice Zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9934, https://doi.org/10.5194/egusphere-egu2020-9934, 2020.

Boundary layer mixing at the ice-ocean thermodynamic interface is represented by turbulent transfer coefficients, Γ_T and Γ_S. Commonly used expressions for these are based on observations at the sea ice-ocean and ice shelf-ocean boundaries, and result in values ranging over 5 orders of magnitude (10^-7< Γ_T< 10^-2). To demonstrate the potential effect of the choice of turbulent transfer parameterisation we applied all of the available transfer coefficient values (12) to an idealised ice shelf-ocean cavity model experiment using the ISOMIP domain with ROMS. The mean ablation rate in warm cavity scenarios varies between 2.1 and 4.7 m/year, and in cold cavity scenarios between 0.03 and 0.17 m/year.

 Γ_T and Γ_S not only directly determine the ablation rate, but have effects on fresh water distribution in the ocean boundary layer. High Γ values develop deep mixed layers, while low Γ values stratify the top ocean grid cells. Thus the ocean boundary layer structure directly depends on vertical resolution in the ocean model and how well the mixing scheme can handle the stratification effects. The experiment results we are presenting here include comprehensively tested and quantified effects of tidal forcing, mixing schemes, vertical flux distribution and ocean model resolution on the ablation rates and the ocean boundary layer structure.

How to cite: Malyarenko, A., Jendersie, S., Williams, M., Robinson, N., and Langhorne, P.: Effects of model resolution on ice shelf-ocean boundary layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12570, https://doi.org/10.5194/egusphere-egu2020-12570, 2020.

Coastal polynyas of the Southern Ocean, such as the Mertz Glacier Polynya, are paramount features of the polar climate. They allow for exchanges of heat, momentum and moisture between the atmosphere and ocean where sea ice usually prevents such interactions. Polynyas are believed to have a profound impact on polar and global climate, thanks to their sustained sea ice production and the associated formation of Dense Shelf Waters. Less is known, however, about the impact of polynyas on the atmosphere. Changes in air properties and winds induced by heat and moisture flux could for instance affect precipitation regime over the ice sheet or sea ice. As the formation and evolution of coastal polynyas are tied to the state of the atmosphere, such changes can also induce important feedbacks to polynyas dynamics. Such processes have almost never been studied, whether on the field or with the help of coupled models. Here, we propose to describe the behavior of a coastal polynya and its relationship with the ocean and atmosphere. To do so, we developed a regional coupled model of the ocean, sea ice and atmosphere (including interactive basal melt of ice shelves) and applied it to the Adélie Land area, in East Antarctica. The dynamics of the Mertz Glacier Polynya is described, together with its impact on the atmosphere, sea ice growth, dense water production and ice shelf melt. To assess the importance of potential feedbacks, we compare the dynamics of the polynya from the coupled model to a forced ocean-sea ice model. We then use the regional coupled model to investigate the implications of the Mertz ice tongue calving in early 2010 which led to a drastic decrease of the Mertz Glacier Polynya extent. This experiment aims at investigating the sensitivity of the atmosphere to the activity of the polynya and to evaluate the impact of the calving on regional climate. This work improves the understanding of the Mertz Glacier Polynya dynamics, and of the impact of coastal polynyas on polar climate. It also constitutes an additional step in the modelling of the polar regions in Earth System Models.

How to cite: Huot, P.-V., Fichefet, T., Kittel, C., Jourdain, N., and Fettweis, X.: Investigating the dynamics of an Antarctic coastal polynya using a regional climate model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19677, https://doi.org/10.5194/egusphere-egu2020-19677, 2020.

The interaction between Ice-Shelves and the Ocean is an important component of the response of ice sheets to future warming oceans. Observational data in the ocean boundary layer beneath ice shelves is limited and the turbulent flow in the boundary layer is not well characterised. Our work uses small scale (9m depth) direct numerical simulations (DNS) of the Ice-Shelf-Ocean Boundary Layer, inspired by field observations made beneath the George VI Ice Shelf. Here, warm water has been observed directly beneath the ice shelf, and yet the observed melt rates are modest. To study this scenario, we simulate a forced turbulent flow underlying an ice shelf where the ice base is represented by a dynamic melting boundary condition. As the ice melts, a pool of relatively cold, fresh water develops below the ice base. Thermal diffusion causes the underlying water to cool and can drive turbulent convection. At the same time, the salinity gradient in the halocline is stabilising, but develops over a longer time scale. As a result, two flow regimes exist: one with active turbulent convection driven by double-diffusion of heat and salt, and the other with stratified turbulence leading to mixing of the halocline. By varying control parameters, we identify the transition between the flow regimes in terms of the temperature contrast (thermal driving) and the level of turbulence in the far field. We consider the behaviour of the diapycnal buoyancy flux near the ice base, and it provides insight into the drivers of both the double-diffusive convection and its modification by ambient turbulence. Finally, we discuss how the double-diffusive process we have described applies to real-world ice-shelf ocean boundary layers, and how it may be quantified within observations.

How to cite: Middleton, L., Vreugdenhil, C., Holland, P., and Taylor, J.: Onset of Double-Diffusive Convection in the Ice Shelf/Ocean Boundary Layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9112, https://doi.org/10.5194/egusphere-egu2020-9112, 2020.

Much of the Southern Ocean surface south of 55° S cooled and freshened between at least the early 1980s and the early 2010s. Many processes have been proposed to explain the unexpected cooling, including increased winds or increased surface freshwater fluxes from either the atmosphere or glacial meltwater. However, these mechanisms so far failed to fully explain the surface trends and the concurrently observed warming of the subsurface (100 to 500 m). Here, we argue that these trends are predominantly caused by an increased wind-driven northward transport of sea ice, enhancing the extraction of freshwater near Antarctica and releasing it in the open ocean. This conclusion is based on factorial experiments with a regional ocean model. In all experiments with an enhanced northward transport of sea ice, the open-ocean surface between the Subantarctic Front and the sea-ice edge is cooled by strengthening the salinity dominated oceanic stratification. The strengthened stratification reduces the downward mixing of cold surface water and the upward heat loss of the warmer waters below, thus warming the subsurface. This sea-ice induced subsurface warming mostly occurs around West Antarctica, where it likely enhances ice-shelf melting. Moreover, it could account for about 8±2% of the global ocean heat content increase between 1982 and 2011. Antarctic sea-ice changes thereby may have contributed to the slowdown of global surface warming over this period. The important role of sea-ice in driving changes in the high-latitude Southern Ocean are robust across all considered sensitivity cases, although their magnitude is sensitive to the forcing and the role of salinity in controlling the vertical stratification in the mean state. It remains yet unclear whether these sea-ice induced changes are associated with natural variability or a response to anthropogenic forcing.

How to cite: Haumann, F. A., Gruber, N., and Münnich, M.: Sea-ice Induced Southern Ocean Subsurface Warming and Surface Cooling in a Warming Climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22008, https://doi.org/10.5194/egusphere-egu2020-22008, 2020.