Introduction

The Indian Ocean is of especial significance to oceanographers because of its unique feature of currents that switch direction on an annual cycle. This feature makes the Indian Ocean a natural ‘laboratory’ where oceanographers can seek to test their understanding of the dynamics of how ocean currents respond to changing wind patterns. In addition, Parts I and II of this book have suggested how the oceanography of the Indian Ocean may be of crucial importance to meteorologists. Charney and Shukla, in Chapter 6 of this book, indicated the potential ‘predictability’ of monsoon meteorology given a prediction of boundary values, including, especially, sea-surface temperatures. In this context, they emphasized that timescales of oceanic responses are all relatively slower than their atmospheric counterparts, so that longer-term forecasting of ocean boundary values (for use as the input to atmospheric models) might ultimately be attainable.

They are certainly right about timescales of response. Indeed, it has long been known for a midlatitude ocean that the timescale associated with the accumulation of the baroclinic part of its response to wind stress (including its western boundary current) may be as much as a decade (Veronis and Stommel, 1956). On the other hand, for a low-latitude system such as the Indian Ocean, the response is by no means as slow as that, and almost a decade ago Lighthill (1969) suggested that the corresponding timescale would be about a month rather than a decade.