6 Equatorial Currents : Observations and Theory Ants

Historically, our knowledge of the circulation patterns in the tropics was derived from compilations of shipdrift data and so was restricted to a description of the surface currents. Although information of this type is crude, a picture of the spatial and temporal structure of the surface flow field was deduced over the years, and it has been little improved upon in the modern era of instrumentation. By contrast, almost all of the information about subsurface equatorial flows has been acquired recently. It is remarkable that one of the major ocean currents, the Pacific Equatorial Undercurrent, was not discovered until 1952. For many reasons, progress in understanding equatorial circulations has been slow. The equatorial regions are vast and remote. The swift currents and high vertical shears put special demands on instrumentation. The geostrophic approximation, which is so useful at mid-latitudes, breaks down close to the equator and cannot be relied on to give accurate information about the currents. Finally, the flows seem much more time dependent than at mid-latitudes. Hence, on the basis of individual cruises, haphazardly taken in time and space, it is difficult to develop a consistent picture of the circulation patterns. The variability of equatorial circulations has only recently been appreciated. This is partly because a great deal of the information about equatorial circulations comes from the central Pacific, where historically the mean appears to dominate the transient circulation; recent NORPAX (North Pacific Experiment) observations (Wyrtki, McLain, and Patzert, 1977; Patzert, Barnett, Sessions, and Kilonsky, 1978), however, suggest that the variability can be quite large even there. In other regions such as the western or eastern Pacific or the Indian Ocean, the fluctuating components are as large as or larger than the means. The goal of this chapter is to give a short overview of the outstanding features of the equatorial ocean circulation patterns, the dominant spatial and temporal structures of the Pacific equatorial wind field (as an example of the kinds of driving mechanisms that need to be considered), and a summary of some of the theoretical ideas that have been developed to explain the ocean circulation and its relation to the wind field. No attempt is made to be comprehensive because in recent years there have been numerous excellent reviews of equatorial phenomena and theories for them. These include articles by Knauss (1963), Tsuchiya (1970), Rotschi (1970), Philander (1973), Gill (1975a), and Moore and Philander (1977). A collection of papers discussing various topics of equatorial oceanography is contained in the proceedings of the FINE (1978) workshop, held