Modelling the seasonal variability of the Antarctic Slope Current

One of the main features of the oceanic circulation along Antarctica is the Antarctic Slope Current (ASC). This circumpolar current flows westwards and contributes to communication between the three major oceanic basins around Antarctica. The ASC is not very well known due to remote location and the presence of sea ice during several months, allowing in situ studies only during summertime. Moreover, only few modelling studies of this current have been carried out. Here, we investigate the sensitivity of this simulated current to four different resolutions in a coupled ocean-sea ice model and to two different atmospheric forcing sets. Two series of simulations are conducted. For the first series, global model configurations are run at coarse (2 degrees) to eddy-permitting (0.25 degrees) resolutions with the same atmospheric forcing. For the second series, simulations with two different atmospheric forcings are performed using a regional circumpolar configuration (south of 30 degrees S) at 0.5 degrees resolution. The first atmospheric forcing is based on a global atmospheric reanalysis and satellite data, while the second is based on a downscaling of the global atmospheric reanalysis by a regional atmospheric model calibrated to Antarctic meteorological conditions. Sensitivity experiments to resolution indicate that a minimum model resolution of 0.5 degrees is needed to capture the dynamics of the ASC in terms of water mass transport and re-circulation. Sensitivity experiments to atmospheric forcing fields shows that the wind speed along the Antarctic coast strongly controls the water mass transport and the seasonal cycle of the ASC. An increase in annual mean of easterlies by about 30% leads to an increase in the mean ASC transport by about 40%. Similar effects are obtained on the seasonal cycle: using a wind forcing field with a larger seasonal cycle (+30%) increases by more than 30% the amplitude of the seasonal cycle of the ASC. To confirm the importance of wind seasonal cycle, a simulation without wind speed seasonal cycle is carried out. This simulation shows a decrease by more than 50% of the amplitude of the ASC transport seasonal cycle without changing the mean value of ASC transport.