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Polar Oceans PhD Student talks

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Speakers:

David BettThe effect of the Amundsen Sea freshwater balance on ocean melting of the West Antarctic Ice Sheet.

Rachael SandersDrivers of Subantarctic Mode Water variability.

Abstracts:

The effect of the Amundsen Sea freshwater balance on ocean melting of the West Antarctic Ice Sheet: The Amundsen Sea has some of the highest thinning rates of ice shelves in Antarctica, due to increased ocean melting. This increased input of freshwater into the region could affect the currents and mixing, due to density being strongly dictated by salinity in the polar regions. However first a clear understanding of the sources and sinks of freshwater in the region is lacking, which is needed before the effect of this excess freshwater can be understood. Therefore here we present the preliminary results and methods of investigating the distribution and effect of freshwater from different sources in the Amundsen Sea, using passive tracers in both idealised and realistic regional MITgcm models. The vertical distribution of ice shelf meltwater is investigated in an idealised model and is found to be dependent on the Circumpolar Deep Water layer thickness. Meanwhile the local freshwater feedback of ice shelf meltwater on melt rates is found to be small for the oceanographic ranges seen in the Amundsen Sea. Climatological freshwater distributions are also investigated in the realistic model, where we found that ice shelf and sea ice freshwater tracers climatologically dominate close to the coast, in the shelf sea regions of the model.

Drivers of Subantarctic Mode Water variability: Subantarctic Mode Water (SAMW) forms on the northern edge of the Antarctic Circumpolar Current in the Southern Ocean. The formation is critical to the subducting portion of the Southern Ocean overturning circulation, which is vital to the transfer of carbon and heat from the atmosphere into the deep ocean. A number of processes are involved in the formation, and freshwater fluxes due to sea ice have been shown to make a significant contribution. In recent decades, there has been an observed freshening of SAMW , linked to changes in the concentration and northward transport of sea ice, but variability in these mechanisms is not well understood. Here we present the methods and preliminary results of a study investigating the drivers of recent variability in SAMW properties. Using the output from the Southern Ocean State Estimate (SOSE), a data assimilating Southern Ocean model, we compute the mixed layer budgets in the SAMW formation region of the southeast Pacific to determine the processes responsible for changes in mixed layer temperature and salinity, which subsequently drive changes in the subducted SAMW . We find that calculating these budgets using the methods of Dong et al. (2007) leaves a significant residual between the rate of change of salinity/temperature and the sum of the processes driving these changes. We therefore focus on adjusting the calculation of each individual term to obtain budgets that are as near to closure as possible.

This talk is part of the British Antarctic Survey - Polar Oceans seminar series series.

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