Throughflow from the Pacific to the Indian Ocean through South East Asian waters

Abstract

The South East Asian Waters form the only tropical latitude link between the world's ocean basins. The region has an important role in the circulation of the Pacific and Indian Oceans. In the Indian Ocean the area is the source of the South Equatorial Current and the polewards flowing Leeuwin Current off Western Australia. The throughflow from the Pacific to the Indian Ocean is investigated using a model driven by a forcing term representing the difference in sea level between the Pacific and Indian Oceans. Initially steady forcing is used in a cross equatorial channel model. The transient motions are controlled by the width of the channel. No evidence for westward propagating Rossby waves is found in a channel of width 560km. At steady state southward flow occurs throughout the channel in a western boundary layer where the required change in planetary vorticity is balanced by dissipation of vorticity. Meridional flow through the channel at steady state is found to be largely due (> 80%) to the geostrophic balance with the cross channel sea level gradient. Results are presented for linear and non liner baroclinic models and a linear two layer model. The effects of islands, sills and continental shelves are included in the channel models and lead to the development of a linear baroclinic 'realistic' model (20km grid) covering the region 110^oE-140^oE, 20^oS-10^oN. Forcing this model by the seasonal difference in sea level between the Pacific and Indian Oceans gives a mean throughflow of 7.5Sv with a seasonal oscillation of ±7.9Sv. Maximum throughflow occurred in August and minimum in January. Flow through the Lombok Strait accounted for 50% of the total transport into the Indian Ocean. The seasonal forcing produces a pulse of outflow through the Timor Sea confined to the North West Australian Shelf during March-July (in phase with the observed development of the Leeuwin Current). During the remainder of the year outflow turned westwards along 10^oS south of Java. Changes in the Pacific sea level reached the North West Australian Shelf by Kelvin waves. Changes in the Indian Ocean sea level south of Java occur only after Rossby waves have had time to propagate.

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