A Reconstruction of the Air–Sea Interaction Associated with the Weddell Polynya

Abstract

A recent major climatic event was the occurrence of approximately 350 000 square kilometers of open water in the normally ice covered Weddell Sea near Antarctica during the winters of 1974?76. Within this polynya there was vigorous air?sea interaction resulting in the densification of the surface waters, a convective overturning of the water column, and the formation of large amounts of Antarctic Bottom Water. In order to further our understanding of this important event, the NCEP?NCAR reanalysis dataset is used to reconstruct the air?sea interaction associated with this polynya. The reconstruction shows that the polynya had a profound impact on the surface meteorology of the region. Surface air temperatures over the polynya were on the order of 20°C warmer than climatology. Total cloud cover over the polynya was 50% higher than climatology. The magnitude of the monthly mean sensible and latent heat fluxes during the winter months were on the order of 150 and 50 W m?2, respectively, while precipitation was on the order of 1 mm day?1. Furthermore, the reconstructed air?sea fluxes are highly variable in time with instantaneous values 5?10 times larger than monthly mean values. A cross-correlation analysis suggests that much of this variability can be attributed to the passage of transient synoptic-scale weather systems. The reconstructed buoyancy flux within the polynya during winter was on average negative, indicating that the surface waters were becoming denser thereby driving oceanic convection and Antarctic Bottom Water formation. Nevertheless there were instances when the buoyancy flux was positive. During these events, the freshwater flux due to precipitation was larger than the effect of cooling, thus resulting in a reduction in the density of the surface waters of the polynya. The integrated buoyancy flux over the winter period exceeds a previous estimate by 30%?40%, suggesting that the oceanic convection that took place as a result of the existence of the polynya may have been significantly more vigorous than previously thought.