Observations of geostrophic transport variability in the western tropical Indian Ocean

Dynamic height was calculated from XBTs collected along several shipping routes in the western Indian Ocean. In the northern hemisphere, low dynamic heights prevail during the NE monsoon and high dynamic heights during the SW monsoon inducing an alternating Somali Current. In the southern hemisphere, at 7°–8° S a trough of low dynamic height occurs during the whole year. EOF analysis is used to document the variation of these features. The geostrophic transports calculated for the XBT routes show spatially coherent patterns with strong seasonal variations. In the Arabian Sea, an anticlockwise gyre develops during the NE monsoon alternating with a clockwise gyre during the SW monsoon. Between 5°N and 5°S, the transport variations show eastward flow twice each year during the monsoon transitions, except on the westernmost route where the annual variations of the gyre in the Arabian Sea extend to the equator. The mean transport of the South Equatorial Current increases regularly with longitude from east to west in all months. It has small annual variations, and the phase of the annual maximum progresses consistently westward.