Modelling Indian Ocean responses to low frequency fluctuations in observed wind patterns

Abstract

Analysis of Comprehensive Ocean-Atmosphere Data Set (COADS) and United Kingdom Meteorological Office (UKMO) Global Sea-Ice and Sea Surface Temperature (GISST) data for 1900–1983 in terms of 21 year epochs has shown that during the austral summer the Indian Ocean region is characterised by substantial multidecadal variability in sea surface temperature (SST), wind stress, mean sea level pressure (MSLP), atmospheric vertical motion and cloudiness. The most pronounced changes appear to be a coherent waxing and waning of the semipermanent southern Indian Ocean anticyclone and SST in the midlatitudes of the southern Indian Ocean. An ocean general circulation model has been used to investigate possible links between the observed SST variability and epoch wind anomalies over the Indian Ocean region. Modulations to the southern Indian Ocean gyre and associated vertical motions largely control the model SST response when the epoch winds are imposed and lead to greatest SST anomalies in the Agulhas retroflection and outflow zones. Ocean transport fluctuations reach values up to 2.25 Sv, while SST anomalies, of the order of 0.05–0.1° C, are in the correct sense but smaller in magnitude and areal extent than is observed.