Monsoonal response in the western Indian Ocean

Abstract

Infrared observations from spacecraft are used to investigate the response of the Somali Current to the onset of the southwest monsoon. Selected satellite observations from three years were available for this study (Nimbus series 1966, 1969, and 1970). The time-dependent development of horizontal temperature gradients at the sea surface serves as an indicator for the formation of the baroclinic structure of the Somali Current. A comparison is made with the simultaneous development of the southwest component of the monsoon wind. The investigation reveals that the temperature gradients during the early formation stage in all years are directly proportional to the wind speed. The phase lag between the development of wind and temperature gradient during the buildup of the boundary current has a mean value of twelve days. During the decay period in late summer and fall, the lag increases continuously up to forty days. The observations suggest that two phenomena of different spatial scales play an important role during the formation of the Somali Current; in the early stage (May, June), local wind-induced upwelling seems to be the more important source of baroclinicity; in the later stage of the buildup (July), large-scale geostrophic effects seem to be dominating.