Eta model simulations and AMSR images to study an event of polynya at Terra Nova Bay, Antarctica

Abstract

In the Terra Nova Bay (TNB) region, 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 water/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 and refrozen and the new ice is pushed away, so that coastal polynyas provide a source of new ice production. As the ice forms, much of the salt content in the freezing water is rejected, forming dense salty water, which tends to sink, eventually contributing to the deep ocean circulation. 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 strongly influenced by the extent of sea-ice cover. While the sea ice acts as insulation, a direct contact between water and air is established in areas of open water, and intense energy exchanges occur due to the large difference of temperature between the water and the air above it. As a result, the polynya areas have an important impact on polar meteorology/climate. In Antarctica, atmospheric numerical models which provide good results at mid-latitudes are put to test because the observations are scarce, the initial and boundary conditions are sometimes inadequate, and complex terrain, sea ice, and polynyas are present. In the present contribution, numerical simulation of a real event of a coastal polynya at TNB is shown, using a recent version of the Eta model. The horizontal resolution used is approximately 20 km, with 50 layers in the vertical, and the model atmosphere top at 25 hPa. Initial and boundary conditions are obtained from ECMWF analyses. An event which occurred from 12 to 18 July 2006 was selected by inspection of the sequence of daily AMSR-derived sea-ice concentration (SIC) maps. Evaluation of the area with sea-ice concentration values below a predetermined threshold provided information on the temporal development of the polynya, which reached its maximum extent of about 4,000 km2 on 16 July. Thus, an open water polynya of realistic size was included within the initial conditions of the simulation, done for the period 15–17 July. The Eta model reproduced the evolution of upper and mid-atmospheric states in what we find a good agreement with AVHRR observations. The evolution of the simulated 10 m winds is well correlated with the observed extent of the polynya. In order to identify the effect of the presence of the open water area on the structure of the atmospheric boundary layer and atmospheric circulation, an additional simulation was performed without the presence of the polynya. Comparison of these two numerical simulations shows that the polynya acted to increase the air speed above it and had induced intense heat fluxes, warming the air. This polynya modification impacted the atmosphere over a rather long distance and up to a height of several hundred meters.