Recent velocity profile data for the western equatorial Indian Ocean are presented and discussed. These display a very low frequency variability of small vertical scales. Wunsch's (1977) theoretical model of a forced equatorial response at the annual period is shown to provide a rational explanation of the phenomenon.

Introduction and historical background

The oceanographic response of the western Indian Ocean to the atmospheric monsoon regime is a complicated one, both from the observational and theoretical points of view. Several phenomena have a clearly documented relationship to the monsoon forcing although many of the details are not clearly understood. The most dramatic of these monsoon phenomena, the Somali Current, has been discussed by Swallow in Chapter 29 of this book.

In the open, equatorial Indian Ocean two phenomena exist whose seasonal variability is clearly associated with the monsoon–the equatorial undercurrent and the equatorial surface jet. Recent observations suggest there may be other monsoon-related currents at greater depths.The equatorial undercurrent is a permanent feature of both the Atlantic and Pacific Oceans (Philander, 1973) and is generally believed to be associated with the persistent trade winds from the east. This westward stress at the surface, and corresponding difference in elevation of the sea surface, is balanced by a zonal pressure gradient between the boundaries. Indian Ocean such a zonal pressure gradient can only be established by the northeast monsoon. The observations (Taft and Knauss, 1967; Swallow, 1967; Knox, 1976; Kort et al., 1975) have shown an undercurrent only during the latter part of the northeast monsoon, with large variations in strength from year to year.