OBSERVATIONS OF GEOSTROPHIC TRANSPORT VARIABILITY IN THE WESTERN TROPICAL INDIAN-OCEAN

Dynamic height was calculated from XBTs collected along several shippi ng routes in the western Indian Ocean. In the northern hemisphere, low dynamic heights prevail during the NE monsoon and high dynamic height s during the SW monsoon inducing an alternating Somali Current. In the southern hemisphere, at 7 degrees-8 degrees 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 calculate d for the XBT routes show spatially coherent patterns with strong seas onal variations. In the Arabian Sea, an anticlockwise gyre develops du ring the NE monsoon alternating with a clockwise gyre during the SW mo nsoon. Between 5 degrees N and 5 degrees S, the transport variations s how eastward flow twice each year during the monsoon transitions, exce pt on the westernmost route where the annual variations of the gyre in the Arabian Sea extend to the equator. The mean transport of the Sout h Equatorial Current increases regularly with longitude from east to w est in all months. It has small annual variations, and the phase of th e annual maximum progresses consistently westward.