Relationships between Antarctic sea-ice and South African winter rainfall

Abstract

Evidence is presented of a statistical relationship between anomalies in winter sea-ice over the South Atlantic sector of the Antarctic and winter rainfall over western South Africa. A positive (negative) correlation exists between sea-ice concentration over the Weddell Sea/Drake Passage region (east of the Weddell Sea from 0 to 30° E) and rainfall between May and September. When broken down into early, mid and late winter, the relationships appear stronger for early (May to July) and mid (June to August) winter than for late (July to September) winter and the month of July. In all cases, the relationships occur at 1 to 2 mo lead, suggesting that some predictability of winter rainfall may exist based on sea-ice concentration found earlier in the season. Analysis of circulation patterns associated with anomalously wet and dry winters from 1982 to 2004 indicates that the former are characterised by a cyclonic anomaly over southern South Africa that stretches southwest over the mid-latitude South Atlantic. In addition, there are increases in low level westerly moisture flux, and enhanced uplift, relative cyclonic vorticity and convergence over and upstream of the region. These patterns, combined with evidence of a northward shifted and more intense subtropical jet, indicate that the mid-latitude storm track is located anomalously far north during wet winters and that the cold fronts, which climatologically bring most of the annual rainfall, are likely to intensify just upstream of southwestern South Africa.