The Sverdrup Relation in the Indian Ocean, and the Effect of Pacific-Indian Ocean Throughflow on Indian Ocean Circulation and on the East Australian Current

The observed distribution of the depth-integrated steric height (mass transport function) in the Indian Ocean compares favorably with the distribution computed from the Sverdrup relation and annual average wind stresses, provided the integration starts from observed values of mass transport function near the eastern boundary. These eastern boundary values increase substantially from 9 to 115°S, presumably because the Pacific-Indian through flow is geostrophically balanced. This flow is estimated to have an order of magnitude of 10 × 106 m3 s−1—rather stronger than previous estimates. The flow may [as suggested by Cox (1975)] contribute to the observed anomalous weakness of the East Australian Current, though southwards flow along the South American coast may also contribute to its weakness. If the Pacific-Indian throughflow did not occur it is shown that the Sverdrup circulation pattern in the Indian ocean south of the equator would be similar to that in the other tropical oceans, in that the flow would be generally northwestwards over most of the ocean. It also is quite likely that blocking the Pacific-Indian flow would lower water temperatures off Western Australia by an amount of order 3°C in the upper 300 m; thus the throughflow may be responsible for the observed anomalous lack of upwelling (or at least for the lack of cold, nutrient-rich water) along the Western Australian coast. It is suggested that internal Kelvin waves may propagate south from the equatorial Pacific along the West Australian coast, resulting (with internal Rossby wave propagation) in high values of mass transport function throughout the Indian Ocean from 15 to 35°S. The fact that internal Rossby waves propagate westward may explain why the Pacific-Indian throuhflow continues into the Indian Ocean as an essentially zonal jet.