An anomalous late-season change in the regional sea ice regime in the vicinity of the Mertz Glacier Polynya, East Antarctica

This paper examines a major late-season shift in atmospheric circulation over the East Antarctic sea ice zone that occurred in October 1999. The mean wind direction at 65degreesS, 145degreesE changed from south-southeasterly to west-southwesterly, resulting in almost a complete reversal of the climatological East Wind Drift pattern in large-scale sea ice advection over the Southern Ocean sector 135degrees-170degreesE. By comparison with National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis time series from 1980 to 2000, this shift is shown to be unusual in terms of both its persistence and impact, although similar though less prolonged patterns occurred in 1984 and 1985. Lasting from October 1999 to February 2000, it had a profound impact on sea ice concentration and extent over a vast area. By coinciding with the period of maximum ice extent and persisting through to the springtime melt phase, it also contributed to the rapid annual meltback of sea ice. It also had a large and complex effect on the behavior of the globally important Mertz Glacier Polynya and on the areal extent of thick perennial sea ice to the east. While a midlatitude blocking high pattern provided the initial impetus for the shift'' in 1999, the persistence of west-southwesterly winds through February 2000 appears to relate to an unusually persistent southward migration of the Antarctic Circumpolar Trough (ACT). In addition to the prolonged 1999 episode, a strong seasonal signal is apparent throughout the mean wind speed and direction time series, related to the ACT migration. Although less persistent than in 1999, this annual shift has implications for the observed dramatic annual meltback of East Antarctic sea ice.