Interannual Variability in the Indian Ocean Wind-Driven Circulation 1977-1992

A numerical model of wind-driven Indian Ocean circulation and the wind data used to drive it are analyzed to study the patterns of interannual variability in both data sets and their relationship. The variability is quantified by an EOF analysis. The first EOF of the model circulation accounts for 48.2% of the variation in upper layer thickness and reveals a basin-scale redistribution of mass north and south of 10��S. The second and third EOFs of the model circulation account for 14.6% and 9.7% of the variation in upper layer thickness, respectively, and reveal an east/west redistribution of mass south of 5��S between 70��E and 90��E. Together, the first three EOFs account for 71.4% of the total variance in upper layer thickness. Reconstructing the data from these first three EOFs, a propagating pattern emerges, that connects the equatorial wave guide with higher latitudes in both hemispheres. The perturbation propagates cyclonically in both hemispheres with a time scale of approximately 10 years. The wind EOFs and the average zonal winds reveal an increase in the trade winds and equatorial westerlies from 1983-1985.

We hypothesize that the basin-scale pattern seen in the model output is triggered by the increase in the trade winds and equatorial westerlies from 1983-1985. This increase will strengthen the East African Coastal Current and the Somali Current and generate variations in a superposition of equatorial and extra-tropical planetary waves. These waves will reduce the north/south slope in upper layer thickness at the Indonesian throughflow region, thus weakening the throughflow. This effect is magnified because the equatorial Indian Ocean is approximately resonant at the semiannual period. The effects propagate from the equatorial wave guide to higher latitudes in both hemispheres through reflection at the eastern boundary as westward-propagating Rossby waves and poleward propagating coastal Kelvin waves. Interaction of these reflected waves with the open boundary representing the Indonesian passages at 10��S to 15��S may lead to marked difference in the oceanic response to the north and south of 10��SS.

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