ETA Model Simulations and AMSR Images to Study an Event of Polynya at Terra Nova Bay, Antarctica

In the Terra Nova Bay (TNB) region, the near-surface winds are persistently strong (in particular during the winter season) and blow offshore with a high degree of directional constancy. This region is also known as a preferential zone of coastal polynyas. Polynyas are recurring areas of open waters/thin ice surrounded by an ice covered sea. Coastal polynyas form along ice bound coasts; they are believed to be due to strong and persistent offshore winds and/or ocean currents which drive the sea ice away. As the ice is removed from the region of origin, open water is exposed, refrozen and the new ice is pushed away, so that coastal polynyas provide a source of new ice production. The horizontal surface temperature differences among the land ice, water and sea ice are strong, because the open water is close to the freezing point (-1.8 °C for typical salt water). The energy exchange between the ocean and the atmosphere in the Antarctic marginal sea ice zone is influenced by the extent of sea-ice cover. While the sea ice acts as insulation, a direct contact is established in areas with open water and intense energy exchanges occur, due to the large difference of temperature between the water and the air above it. This implies that the polynyas are areas where the ocean exchanges energy with the atmosphere and as a result they have an effect on the polar meteorology/climate. In Antarctica, the atmospheric numerical models which provide good results at the mid latitude, are put to the test, because the observations are scarce, the initial and boundary conditions are sometimes inadequate, complex terrain, sea ice and polynyas are present. Here, the numerical simulation of a real event of coastal polynya at TNB is shown, using a recent version of the Eta Model. The horizontal resolution is approximately 20 km and 50 level are considered in the vertical up to 25 hPa. Initial and boundary conditions are obtained from ECMWF analyses. The event, occurred from 12 to 18 July 2006, was selected by the analysis of the sequence of daily AMSR- derived Sea-Ice Concentration (SIC) maps. The computation of the sea ice free area provided information on the temporal development of the polynya which reached its maximum extent of about 4000 km2 on 16 July. Therefore, a realistic polynya size was included in the initial conditions for the simulation of the period 15 – 17 July. The Eta Model reproduces very well the evolution of upper and mid-level conditions in agreement with AVHRR observations. Also, the evolution of the simulated 10m wind is strictly correlated to the observed extent of the polynya. In order to isolate the effect of the presence of the open water area on the structure of the atmospheric boundary layer and on the atmospheric circulation, a further simulation was performed without the presence of the polynya, i.e. with its extent covered with sea ice. The numerical simulations show that the polynya acts to increase the speed of the air above it and strong heat fluxes warm the air. As a result, the polynya modifies the atmosphere over a long distance from its location and till to a height of several hundred meters.