Introduction

Synoptic-scale weather systems, comprising the extra-tropical cyclones (depressions) and anticyclones (highs), are the main atmospheric systems found in the Antarctic coastal region and over the Southern Ocean. They typically have a horizontal length scale in the range 1000–6000 km and a lifetime of between one day and a week. Within the spectrum of atmospheric disturbances they lie between the mesoscale (less than 1000 km diameter) phenomena, such as cloud clusters, mesocyclones and squall lines, and the planetary-scale long waves that have wavelengths of many thousands of kilometres. Satellite imagery of the high southern latitudes, such as that reproduced in Figure 5.1, shows clearly the locations of the major synoptic-scale low-pressure systems through their frontal cloud bands that spiral into the centre of the systems. Such imagery is an important tool in the production of operational analyses over the Southern Hemisphere and allows the depressions to be tracked in areas sparse in data. The infra-red image shown here is of the Bellingshausen Sea on 28 April 1993 when the area was dominated by an active depression. The high, cold cloud of the front and the shower clouds in the cold air to the west of the system are the most significant features of the image.

Case studies and climatological investigations have shown that depressions form mainly on horizontal temperature gradients (baroclinic zones) in the troposphere and grow through baroclinic instability. In the Southern Hemisphere the major meridional temperature change in the troposphere occurs at the polar front separating the temperate mid-latitude air to the north and the cold polar air masses to the south.