Sea-ice growth, drift and deformation off Eastern Antarctica

Antarctic sea-ice thickness, concentration and extent all play a vital role in controlling the exchange processes between ocean and atmosphere in the polar regions. This study investigates how distributions of these characteristics are determined by sea-ice growth, drift and deformation. Drift and deformation of East Antarctic sea ice are investigated using in situ observations of sea-ice drift since the mid 1980s, and results from numerical models. From the drift data a climatology of the East Antarctic sea-ice velocity distribution is derived, and recurrent features in the drift pattern are identified. The average daily drift speed in the westward flow near the Antarctic coast is 0.22 m s -1(19.0 km d-1) with considerable spatial and temporal variability. In the eastward flow north of the Antarctic Divergence the average is 0.17 m s -1(14.7 km d-1). The average ice motion over wide parts of the overall East Antarctic sea-ice zone has a net northward component (0.04 m s -1or 3.5 km (1-1) transporting pack ice equatorward. A new method is used to derive the mean winter position of the Antarctic Divergence from ice-movement data. It is found that during winter the Antarctic Divergence is further north than the mean annual position derived from hydrographic measurements. Thermodynamic ice growth near the coast of East Antarctica is modelled using field data obtained since the late 1950s. The availability of atmospheric and some hydrographic observations makes it possible to estimate the heat flux from the oceanic mixed-layer to the underside of the sea ice. The magnitude of this oceanic-heat flux is highest in early spring, and lowest in autumn. Annual mean values of the oceanicheat flux vary between 5 and 12 W M-2 , with an average of 7.9 W M-2. On decadal time-scales a decrease in oceanic-heat flux is found from the 1950s to 1980s for the area studied. At sites close to the coast the annual maximum ice thicknesses are linked to the extent of the cooling of the oceanic mixed-layer occuring in upstream polynyas. Data from a regional experiment in the coastal current off Adelie Land yield new insight into sea-ice deformation and the variability of forcing processes. Although ice drift is dominated by long-term changes, sea-ice deformation exhibits significant short-term variability. Numerical simulations show that in the absence of tidal forcing, inertial motion can account for the formation of substantial open water within the pack. Regional modelling shows that such a motion has a considerable impact on the ice growth, and can roughly double the rate of ice production by allowing rapid growth in recurrent open-water patches. This also significantly increases the amount of salt rejected to the ocean.