The mixed layer over the Antarctic continental shelf

The mixed layer over the Antarctic continental shelf

Alex Petty (PHD student), Daniel Feltham, Paul Holland (CASE supervisor)

The temperature of waters above the shelf seabed in the Amundsen and Bellingshausen (AB) seas, Antarctica, is seen to be several degrees warmer than in the Weddell and Ross (WR) shelf seas, Figure 1. In the AB seas, unmodified Circumpolar Deep Water from the Antarctic Circumpolar Current floods the continental shelf all year round, while in the WR seas the cold wintertime mixed layer reaches the seabed.

Reanalysis data show the Weddell atmosphere to be colder, drier, and less cloudy than the Amundsen. We want to test the hypothesis that the difference in shelf water properties is due to differences in the atmospheric forcing between the two regions.

To do this we have developed a simple sea ice-mixed layer model to study the sensitivity of the wintertime mixed layer depth to a variation in surface fluxes. The study is also looking at the impact of initial water column profiles to see if a change in surface fluxes can result in a switch from one mode to the other, e.g. Figure 2. We shall later incorporate this mixed layer model into CICE to more accurately represent the buoyancy fluxes to the mixed layer from sea ice growth and melt during the annual sea ice cycle.

Figure 1: Temperature at the continental shelf sea bed from an observational study by Clarke et al., Journal of Geophysical Research [2009].

Figure 2: Ten year output of a coupled sea ice-mixed layer model using Amundsen Sea forcing (from NCEP CFSv1 re-analysis) and model initialisation. Top: the calculated sea ice area fraction and thickness. Middle: water column temperature. Bottom: water column salinity. The white line shows the mixed layer depth as calculated by the model.