The Weddell Sea Polynya is a seasonal opening within the sea ice cover of the Weddell Sea sector, typically found over the Maud Rise and inside the Weddell Gyre. It has been a rare occurrence in the satellite period, appearing in austral spring between 1973 and 1976 and again in 2016/17. The polynya formation has been shown to be complex, requiring a combination of ocean and atmospheric mechanisms to develop. The region is often poorly resolved in global climate models, with little agreement in ocean or sea ice dynamics. When Weddell Sea polynyas have occurred in models without forcing, it is not understood how they occur within the model, or why some models produce frequent polynyas and others produce none. Some studies have shown that increasing horizontal resolution improves the dynamics of the Southern Ocean, allowing for better parameterisation of small-scale features. Here, we use multi-model data of different resolutions, from the PRIMAVERA HighResMIP experiments, to determine how models of different atmospheric and ocean resolution resolve Weddell Sea polynyas. We assess the frequency, size, and location of polynyas in different resolutions, in addition to studying the ocean and atmospheric processes associated with the polynya in these models. Initial results of models that resolve frequent polynya show preconditioning in both the ocean and atmosphere, in addition to a small response to the polynya in the months following.

How to cite: Ayres, H. and Ferreira, D.: Atmosphere and ocean climate model resolution in resolving Weddell Sea polynyas and their ocean-atmosphere interactions., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2674, https://doi.org/10.5194/egusphere-egu23-2674, 2023.

Global Earth System Models (ESMs) seek to simulate physical, chemical and biological processes that are relevant for the evolution of global climate. One key feature of an ESM is the ability to simulate fluxes of greenhouse gases and aerosols between the atmosphere and ocean, keep track of the inventories in the respective model components and allow for feedback on the climate system. These fluxes are usually calculated based on bulk formulations derived from open water measurements, and are restricted by the sea ice fraction in regions covered by sea ice.

The air-sea gas exchange is determined by the difference in concentration across the air-sea interface, and a gas transfer velocity that is specific for the gas in question. Using CO₂ as example, the air-sea gas exchange is
          F_CO2 = (1 - βC_sea-ice ) ⋅ k_w(CO2) ⋅ ( [CO2]_sea - α[CO2]_air)   (1)
where C_sea-ice is the sea ice concentration, k_w(CO2) is the gas transfer velocity, and α is the Ostwald solubility coefficient. Traditional formulas use β = 1 (complete barrier), but in order to account for cracks and leads in the sea ice, Steiner et al. (2013) proposed a modified formula with β ∈ [0, 1], allowing the sea ice to act as a partial barrier (0 < β < 1) or allowing free exchange in sea ice covered regions (β = 0).

We implement the modified gas exchange formula (eq. 1) in the Norwegian Earth System Model NorESM2, for all model tracers exchanged over the air-sea interface (CO₂, O₂, N₂, N₂O, DMS). Experimenting with different β values, we find that small increases (β ∈ [0.01, 0.02]) may result in either increased or decreased gas fluxes in high latitude regions. This can be attributed to the internal variability of the sea ice area, in particular for the summer minimum, which responds to changes in greenhouse gases and aerosols in the atmosphere. For β ∈ [0.1, 0.2] we find an increase in CO₂ flux of 16% — 22% north of 68°N, and 5% — 8% south of 60°S. Observational datasets based on eddy covariance data for CO₂ in the atmospheric boundary layer will be used in
future work in order to determine a realistic range for β.

REFERENCES:

N. S. Steiner, W. G. Lee, and J. R. Christian. Enhanced gas fluxes in small sea ice leads and cracks: Effects on CO2 exchange and ocean acidification. Journal of Geophysical Research: Oceans, 118(3):1195–1205, 2013. doi: 10.1002/jgrc.20100.

How to cite: Torsvik, T.: Modeling influence of sea ice on gas exchanges between atmosphere and  ocean in a global Earth System Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9684, https://doi.org/10.5194/egusphere-egu23-9684, 2023.

The Southern Ocean is a hotspot of the climate-relevant organic sulphur compound dimethyl sulphide (DMS). Spatial and temporal variability in DMS concentration is higher than in any other oceanic region, especially in the marginal ice zone (MIZ). The MIZ is also an area of rich microalgal communities, including algal species that are renown for the production of dimethyl sulphoniopropionate (DMSP), the precursor of DMS. The link between DMS and microalgae has been studied closely over a five-year period (2012 to 2017) near Rothera Station in Ryder Bay (Western Antarctic Peninsula). Algal community structure and spatial heterogeneity of DMS and DMSP was studied and linked with environmental conditions, including sea ice melt. Concentrations of sulphur compounds, particulate organic carbon (POC) and chlorophyll a in the surface waters varied by orders of magnitude in time and space. Highest concentrations of DMS(P) were recorded in spring, associated with the dominance of autotrophic flagellates, including haptophytes and chlorophytes. These microalgae most likely originated from sea-ice communities, stressing the role of sea ice as a seeding vector for the spring bloom and as a potential source of DMS. The strong sea-ice signal in the distribution of haptophyte algal species and DMS(P) implies that DMS(P) production is likely to decrease with ongoing reductions in sea ice cover along the Western Antarctic Peninsula. This has implications for feedback processes on the region’s climate system.

How to cite: van Leeuwe, M., Stefels, J., Meredith, M., and Webb, A.: Impact of sea-ice melt on DMS(P) inventories associated with algal community dynamics in Antarctic surface waters., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13035, https://doi.org/10.5194/egusphere-egu23-13035, 2023.

Arctic changes are at the centre of climate concerns. Notably, recent Arctic warming drives rapid sea ice decline making the Arctic increasingly vulnerable. To better anticipate the consequences of this strong Arctic warming, it is crucial to better understand the driving processes responsible for large uncertainties in future climate projections. Interactions at the atmosphere-ocean-sea ice interface require particular attention. In this context, the PolarRES project aims at developing the coupled system MAR (atmosphere) - NEMO (ocean-sea ice) over the Arctic region at high spatial resolution (25 km). Such coupling will enable the climate community to access precise data at large scale. Since this coupling has never been applied to the Arctic, a proper model evaluation is required. Here standalone model simulations are compared against a newly compiled dataset including land station and buoys data. We find high correlations between the modelled and observed data. Our evaluation marks an important step in in the ongoing development of coupled models.

How to cite: Lambin, C., Kittel, C., and Fettweis, X.: Towards a high-resolution MAR-NEMO coupling to explore atmosphere-ocean-ice interactions in the Arctic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15723, https://doi.org/10.5194/egusphere-egu23-15723, 2023.

The atmospheric boundary layer in the Arctic winter is characterised by strong and long-lived low level stability which arises from long-wave radiative cooling of the surface during the polar night. This atmospheric temperature inversion is a necessary condition for the positive lapse rate feedback, which is a major contributor to Arctic Amplification. In this study, we assess the low-level stability of the winter-time Arctic boundary layer using ground-based and radiosonde observations collected during the MOSAiC (2019-2020) and SHEBA (1997-1998) expeditions, and from Soviet drifting stations (1955-1991). We compare these observations with the representation of Arctic boundary layer stability in models participating in the latest phase of the Coupled Model Intercomparison Project (CMIP6). The observations show a bimodal distribution of clear and cloudy states which has been reported previously. In the clear state, longwave radiative cooling from the surface leads to strong inversions and a stably stratified boundary layer. Whereas, in the cloudy state, inversions are weaker and not confined to the surface. Previous work has shown that many CMIP5-era climate models fail to realistically represent the cloudy state and often overestimate low-level stability. Here, we assess the extent to which the CMIP6 models also show such biases and examine the representation of surface net longwave radiation and turbulent heat fluxes as potential sources of the biases. Finally, we show that across CMIP6 models, low level stability over sea-ice is correlated with inter-model variation in Arctic amplification.

How to cite: Duffey, A., Mallet, R., Steckling, J., Hermant, A., Dutch, V., Day, J., and Pithan, F.: Stability of the winter-time Arctic Ocean boundary layer in CMIP6 climate models evaluated against Soviet drifting stations, SHEBA and MOSAiC observations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16224, https://doi.org/10.5194/egusphere-egu23-16224, 2023.

How to cite: Maure, D., Kittel, C., Lambin, C., and Fettweis, X.: High resolution atmospheric and oceanic modelling over Antarctica: a coupling interface to study sea-ice processes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16638, https://doi.org/10.5194/egusphere-egu23-16638, 2023.

How to cite: Pithan, F., Athanase, M., Dahlke, S., Sánchez-Benítez, A., Shupe, M., Sledd, A., Streffing, J., Svensson, G., and Jung, T.: Evaluating nudged coupled climate models against MOSAiC observations reveals weaknesses in the representation of clouds, boundary-layer turbulence and snow pack, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17080, https://doi.org/10.5194/egusphere-egu23-17080, 2023.

Snow is a crucial component of the Arctic sea ice system. It dominates the exchanges of heat and light between the atmosphere and the ocean, with important physical and biological implications. To address the imperative need for more realistic representation of snow on sea ice, recent efforts have focused on reanalysis-based snow depth and density reconstructions. However, none of the recent snow products account for snow losses due to snow and sea ice interactions.

This study quantifies the snow loss in snow-ice formation, and its effect in SnowModel-LG snow depth and density product. We coupled SnowModel-LG, a snow modeling system adapted for snow depth and density reconstruction over sea ice, with HIGHTSI, a 1-D sea ice thermodynamic model, to simulate snow-ice and thermal ice growth: SnowModel-LG_HS. We assumed that all negative freeboard would result in snow-ice formation. Pan-Arctic model simulations were performed over the period 1 August 1980 through 31 July 2021, and they were guided by observations where available. In SnowModel-LG_HS, snow depth was lower (domain average: 18%), and snow density was higher (2.3%) compared to SnowModel-LG. The differences were much larger in the Atlantic sector. Our simulations suggest that when snow models do not account for snow and ice interactions, snow depth can be significantly overestimated. In this talk we will discuss the magnitude of this overestimation in relation to the sub-grid parameterization of sea ice dynamics and their effect in snow redistribution over the ice floes. Sea ice dynamics (e.g. deformed ice formation), are likely an additional important snow sink that is not yet accounted for in snow models.

Finally, we use our snow depth and density results from SnowModel-LG_HS to obtain sea ice thickness retrievals from CryoSat-2. A validation of these retrievals against Airborne Electromagnetic Measurements shows that SnowModel-LG_HS performed better when compared to SnowModel-LG and snow climatologies.

How to cite: Merkouriadi, I., Liston, G., and Salilla, H.: Quantifying the effect of snow and sea ice interactions on SnowModel-LG snow depth and density product, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12357, https://doi.org/10.5194/egusphere-egu23-12357, 2023.

This study analyses the association of Southern Ocean extratropical cyclones and atmospheric rivers (ARs) with extreme temperature and/or moisture atmospheric anomalies over Antarctic sea ice. The hypothesis we test is whether the circulations associated with cyclones and ARs may routinely lead to the presence of unusually warm, moist airmasses over ice-covered regions. The analysis is conducted over the extended Austral winter seasons (May – September) between May 1979 and September 2012, based on the European Centre for Medium-Range Weather Forecasts Interim reanalysis data. Approximately 27% of intense Southern Ocean cyclones and 20% of ARs occur in the vicinity of extreme temperature anomalies, while 12% of intense cyclones and 46% of ARs occur in the vicinity of extreme moisture anomalies. We summarize our results as follows: (1) extreme atmospheric anomalies over sea ice often occur in the absence of cyclones or ARs; (2) intense cyclones have a stronger association with extreme temperature  anomalies than ARs; (3) approximately half of the ARs are in the vicinity of extreme moisture anomalies, while the latter’s link with cyclones is weak; (4) if an AR is in the vicinity of an extreme temperature anomaly, there will likely be a concurrent extreme moisture anomaly. This points to a strong association between ARs and moisture extremes, and a nuanced link between Southern Ocean polar cyclones and atmospheric anomalies over Antarctic sea ice.

How to cite: Hepworth, E., Vichi, M., and Messori, G.: Association between extreme atmospheric anomalies over Antarctic sea ice, Southern Ocean polar cyclones and atmospheric rivers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13518, https://doi.org/10.5194/egusphere-egu23-13518, 2023.

Satellite-derived sea-ice concentration measurements have traditionally been used to evaluate the impact of climate change on polar regions. However, concentration-based measurements of sea-ice variability do not allow the discrimination of the relative contributions made by thermodynamic and dynamic processes. This prompts the need to use sea-ice drift and type products and develop new methods to quantify changes in sea-ice properties that would indicate trends in the ice characteristics. A component of the sea-ice variability is driven by local weather events, and in some cases is the dominant driver of variability over larger-scale atmospheric features. Previous work by de Jager & Vichi (2022) has suggested that sea-ice vorticity (derived from low resolution sea-ice displacement vectors) may be a useful metric for quantifying dynamical features in Antarctic sea ice; specifically shorter term changes in the ice-interior driven by atmospheric storms. However, this study hypothesised that much of the rotational drift in the underlying sea-ice field was blurred as a result of the relatively large 48-hr temporal resolution of the drift product, therefore highlighting the necessity of measuring sea-ice properties at higher temporal frequencies. This study will therefore assess the usefulness of an overlapping swath-based method of sea-ice displacement retrieval recently made available by the EUMETSAT OSI-SAF. This swath-based method of retrieval allows for analysis of sea-ice variability at sub-daily timescales, which may be more suitable for measuring the effect of weather events on the sea-ice landscape than using daily averages of merged swaths. In situ data of sea-ice conditions were collected on board the SA Agulhas II research vessel in the Atlantic Sector in July, 2022, which will be compared to swath-based satellite estimates. Furthermore, the newly released 24-hr OSI-SAF drift product will also be compared. To complement these drift estimates, a modified swath-based ice-type retrieval method will be presented to add further context to any potential thermodynamic changes affecting the optical properties of the sea-ice surface.

How to cite: de Jager, W. and Vichi, M.: Sub-daily Antarctic sea-ice variability estimates using swath-based retrieval methods, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4860, https://doi.org/10.5194/egusphere-egu23-4860, 2023.

The study of Antarctic first-year ice as a biogeochemical habitat has been limited by samples mostly collected in pack ice during summer. Fewer winter data are available, and due to the harsh conditions, data from the marginal ice zone (MIZ) are even more difficult to obtain. The MIZ is broad and circumpolar in the Southern Ocean; it is found at different latitudes during the year with sufficient light and nutrients to sustain primary production and affect ecosystem functioning. We present the first dataset of biogeochemical properties of first-year ice collected in the Atlantic sector of the Southern Ocean during winter 2019, obtained from young pancake ice and consolidated first-year ice. Temperature, salinity, crystal structure, δ18O, chl-a and bulk macronutrient data were used to investigate the winter habitat and explain the transition from young ice to first year ice through exchanges with the ocean biogeochemistry. Data suggests that the sea ice sampled at the consolidated station was a result of thermodynamic processes combined with possibly multiple cycles of breaking and rafting induced by waves and dynamics, which ultimately enhanced the biogeochemical activity beyond what expected for first-year ice. A numerical model was used to support the hypothesis that winter first-year ice buffers biogeochemical components differently from the upper ocean winter concentrations, and this may determine the conditions for the biogeochemical development later in spring.

How to cite: Audh, R. R., Fawcett, S. E., Johnson, S., Rampai, T., and Vichi, M.: Enhanced winter biogeochemical activity in Antarctic first-year sea ice, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5458, https://doi.org/10.5194/egusphere-egu23-5458, 2023.