Structure and variability of surface atmospheric circulation anomalies over the tropical south-west Indian Ocean in the austral summer

Abstract

The structure and the variability of surface circulation anomalies in the tropical south-west Indian Ocean are investigated. A review of analyses of empirical orthogonal functions (EOFs) is presented, with emphasis on the southern summer optimum (mid January). EOFs 1 and 3 provide insight to the north-west monsoon outflow and its interaction with the easterly trade winds of the southern hemisphere. Inter-annual variability of the summer surface circulation is highlighted by the spatial structure of wind-stress anomalies and by time-series of wind-stress components, vorticity and divergence averaged over three areas: the Mascarene High, and the northern and the southern Moçambique Channel. The circulation patterns are associated with changes in convective intensity and cyclogenesis in the region surrounding Madagascar. This is brought out by correlation of standardized departures of rainfall and marine winds around southern Africa. Correlations in the zonal component are highest in the equatorial south-west Indian Ocean (r = −0,82, easterly) and near the southern tip of Madagascar (r = +0,93, westerly), suggestive of an anomaly in anticyclonic circulation during wet summers encompassing the entire eastern seaboard of Africa and centred on Madagascar. Correlations in the meridional component are highest in the equatorial south-west Indian Ocean (r = +0,92, southerly), in the 20°S latitude band (r = −0,66, northerly) and south of Africa (r = +0,83). The meridional correlation structure indicates a weakening of the monsoon outflow together with convergence between 25 and 30°S during wet summers.