Variability in the Tropical Southwest Indian Ocean and Influence on Southern African Climate

Abstract

This study considers satellite era reanalysis products to characterize inter-annual variability in the SW Indian Ocean and influence on the climate of southern Africa. Decorrelation of temperature from a buoy area (8°S, 55°E) is found to vary by season; widespread in the first half of the year and almost zero in the second. This leads to an analysis of the annual cycle and inter-annual variability via wavelet filtered principal components. Contrasts between SODA and GODAS reanalysis products reveal the former exhibits higher amplitude annual to inter-annual variability. Good agreement is found for sub-surface temperature in the SW Indian Ocean thermocline ridge, with rhythmic fluctuations of 2.5-5 yr period. When an ocean Rossby wave crest arrives in the southwestern part of the basin, increased SST and local convective rainfall alters the large-scale circulation. The NW monsoon becomes active and moisture that would normally be exported to southern Africa is retained in the SW Indian Ocean. Daily data from a mooring at 8°S, 55°E reveals intra-seasonal pulses at 15-50 days in Nov.-Mar. season. Intercomparisons with satellite and reanalysis data reveal a cool bias in NCEP during the SE monsoon. Hovmoller analysis highlights how surges in NW monsoon winds relate to heat fluxes and rossby wave patterns. A case study is made to contrast Dec.-Mar. 2011 vs 2012, the former having negative sea surface height anomalies, the latter being positive. Regional wind responses to SW Indian Ocean heat anomalies contribute to a 15% change in seasonal rainfall and maize production in South Africa.