Studying the monsoon circulation in the Indian Ocean using altimeter data

The oceanic variability in response to the monsoon system in the northern Indian Ocean was for the first time studied using the three altimeters Geosat, ERS-1 and Topex/Poseidon. The root mean square, computed from the sea surface height anomalies indicates values of 15-17 cm in highly energetic areas such as the Somali Current, and 13-15 cm in the western part of the Bay of Bengal. As the most prominent variations occur, and because most in situ observations are from these areas the investigation was focused on these two areas. Complex principal component analysis was applied to the gridded SSH anomalies and the results indicate that the annual signal is strongest when the monsoon period is at its peak, and the semi-annual time series seems to be related to the onset and decay phase of the SW and NE monsoon. CPCA analysis of the Topex/Poseidon data indicates characteristic periods of 40-50 days which are probably related to earlier observations in the study area of mesoscale variability generated by the horizontal shear in the velocity field. Westward propagating waves are identified from the annual and semi-annual modes, and they are observed to have a westward propagating speed of 13 km/d at the equator, with decreasing speed away from the equator. Mesoscale eddies with diameters between 250-1000 km and maximum amplitudes up to 45 cm are observed during the monsoon periods. The propagation and life time of eddies in the study area are in good agreement with those reported in previous investigations using in situ, data and numerical models.