Abstract

The winter advance of the sea ice edge in the Bellingshausen Sea is frequently interrupted by periods of rapid retreat lasting a few days. The frequency and duration of such events strongly controls the location of the late winter sea ice edge in this sector of the Antarctic. Since winter ice extent in the Bellingshausen Sea controls winter temperatures in the western Antarctic Peninsula – a region that has warmed rapidly in recent decades – it is important to understand the processes that control ice extent in this sector. In this talk I will examine the dynamics and thermodynamics of a retreat event that occurred in May 2001 using data from a drifting buoy array together with diagnostics from a kinematic/thermodynamic ice growth model and a high-resolution (11 km) regional coupled ocean-ice model. During the retreat event, the ice edge retreated by 250 km over 13 days in response to strong and persistent northerly winds associated with a quasi-stationary low pressure system. Ice motion in the outer part of the pack was convergent and correlated strongly with local wind forcing, indicating conditions close to free drift. By contrast, in the region closer to the coast, ice motion was less well correlated with wind forcing and internal ice stresses were significant. Model diagnostics indicate that ice thickening resulting from convergence in the outer pack was largely balanced by basal melting, which was driven by a large ocean-to-ice heat flux maintained by “stirring” of the ocean mixed layer by the ice-ocean stress. The ocean-ice model simulated the characteristics of the retreat event realistically, providing confidence in the ability of such models to reproduce ice conditions in this sector.