ALBERT

Description: McMurdo Sound sea ice can generally be partitioned into two regimes: (1) a stable fast-ice cover, forming south of approximately 77.6 ∘S around March – April and then breaking out the following January – February, and (2) a more dynamic region north of 77.6 ∘S that the McMurdo Sound and Ross Sea polynyas regularly impact. In 2019, a stable fast-ice cover formed unusually late due to repeated break-out events. Here we analyse the 2019 sea-ice conditions and relate them to a modified storm index (MSI), a proxy for southerly wind events. We determined there is a strong correlation between the timing of break-out events and several unusually large MSI events and our key finding is that an increase in the frequency of intense winter storms in 2019 resulted in a delayed formation of a stable fast-ice cover. Further, recent observations (post 2019) demonstrate that fast-ice conditions in 2019 were not unique and suggest that the fate of fast ice in the sound may be a symptom of some larger change. Winter fast-ice dynamics in the sound appear to be largely driven by synoptic events as there are no identifiable trends in monthly-averages of atmospheric drivers (e.g. air temperature, mean sea level pressure and wind speed and direction) of fast-ice breakout in the period 1985 – 2022. This study offers new insights into the mechanisms behind individual break-out events and is one of a few case studies that investigate the stability of a fast-ice cover in winter.

Description: Snow cover affects the variability of the physical properties of sea ice. The snow’s unique thermal and optical properties govern the mass and energy fluxes in the sea ice system. They are important for sea ice evolution, energy exchanges between the ocean and the atmosphere, and light availability for ecosystems below the sea ice. Furthermore, snow significantly impacts remote sensing retrievals, especially for sea ice thickness. Yet, data on the physical properties of snow and its effects on sea ice are extremely limited, especially in Antarctica. This leads to large uncertainties in the coupling of climate feedback and results in significant biases in model representations of the sea ice cover. During our field campaign from October-December 2022 in McMurdo Sound, we quantitatively investigated the physical properties of snow on Antarctic sea ice, following the same protocols used during the MOSAiC expedition. The season’s unique sea ice conditions provided the ideal laboratory to study a range of snow conditions and to differentiate between sea ice and snow drivers for the atmosphere-sea ice-ocean system. Our set of snow measurements on sea ice, unprecedented in Antarctica, includes ground snow/ice measurements, automatic weather and radiation stations, and drone-based measurements. These extensive measurements made it possible to capture the physical properties of snow and their spatial variability and simultaneously measure the different components of the energy balance at varying spatial scales. We will use this dataset to improve our understanding of snow's role in the Antarctic sea ice system.

Description: Potential future impacts on sea ice and other climate variables from changing freshwater outflow from the Antarctic ice sheets and ice shelves were examined in this study. Fully-coupled model experiments were run using ramped freshwater, including latent heat effects, under historical and RCP8.5 greenhouse gas forcings. Specifically, three climate model scenarios in CCSM4.0 for 1980-2100 (FWIncr, FWConst, FWOff) and one idealized experiment for 1850-1950 (FWIncrHist) were run and compared with a control. We chose to run from base years of 1850 for pre-industrial conditions and 1980 for a year we assumed the Antarctic ice sheet was in mass balance. We then increased freshwater fluxes to give the approximate equivalent of 3 m of sea level rise over a 150 year period. For FWIncrHist, sea ice area continued to increase over the 100 year period. For FWIncr, sea ice area increased for approximately 78 years, when it began to decrease. Two shorter branched runs (FWConst, and FWOff) were carried out to test the persistence of the effects, one where the additional fresh water and latent heat effects were switched off and the other where the freshwater was held constant at the point where sea ice area behaviour “turned around”. This was to separate out the effects of greenhouse gas forcing on the ramped freshwater and latent heat effects over long time periods. The persistence of the effects on sea ice area were relatively short, with a return to control conditions in less than a decade when freshwater was switched off.

Description: The increasing amount of fresh water entering the Southern Ocean due to mass loss from the Antarctic ice sheet and ice shelves loss has been proposed as a mechanism responsible for the lack of decline in Antarctic sea ice area, in contrast to the sea-ice loss seen in the Arctic. Though increased Antarctic ice-mass loss is expected to impact climate it is absent from almost all current coupled climate models, which typically enforce that the continent remain in perpetual mass balance. Further, previous model experiments suggest that the climate response to Antarctic mass loss depends on the model used, and that the reasons for this model dependence are not clear. We use the HadGEM3-GC3.1 model to contribute model experiments to the Southern Ocean Freshwater release model experiments Initiative (SOFIA), an international model intercomparison, in which freshwater is added to the ocean surrounding Antarctica to simulate the otherwise missing ice-sheet mass loss. This unique suite of models will allow us to evaluate HadGEM3-GC3.1, identify reasons for model discrepancies, and quantify the potential impact of the absence of increasing Antarctic ice-mass loss on Antarctic sea ice and climate. We will give an overview of the SOFIA project and present preliminary results from the “antwater” experiment outlined in the SOFIA protocol in which a constant freshwater input of 0.1 Sv is distributed evenly around Antarctica at the ocean surface under pre-industrial forcing. We will show the response of Antarctic sea ice and the local and global climate to this freshwater forcing.