6 Equatorial Currents Observations and Theory Ants Leetmaa Julian P McCreary Jr Dennis W Moore 6 1 Introduction 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 i84 A Leetmaa J P McCreary Jr and D W Moore at Scripps Institution of Oceanography during the summer of 1977 A comprehensive discussion of analytic techniques for studying forced baroclinic ocean motions in the equatorial regions is presented in a threepart paper by Cane and Sarachik 1976 1977 1979 In discussing the theories we shall stress the important physical ideas of each model avoiding whenever possible the use of mathematics put them in historical perspective and relate them to the observations The objective here is to identify the observations for which we have physical theories and thereby indicate where further work is needed The importance of knowing the detailed time and spatial structure of the wind field is emphasized throughout this chapter The reason is that in the tropics the characteristic response times for baroclinic oceanic processes are much shorter than they are at mid latitude and are much closer to the time scales characterizing the wind variations Therefore the baroclinic response to atmospheric forcing is expected to be much stronger than at mid latitude The implication is that in order to arrive at a satisfactory explanation of the oceanic features the temporal and spatial structure of the atmospheric forcing must be known accurately 6 2 Observations 6 2 1 The Ocean The surface currents are characterized by zonal bands in which the flow is alternately eastward or westward Knauss 1963 The eastward flows are referred to as countercurrents because they flow counter to the direction of the easterly trade winds The westward flows are referredto as North and South EquatorialCurrents In the Atlantic and the Pacific the North Equatorial Countercurrent NECC is approximately located between 5 and 10 N with westward flow to the north of this region in the North Equatorial Current NEC and westward flow to the south of it in the South Equatorial Current SEC There is also evidence for a South Equatorial Countercurrent SECC in both oceans between 5 and 10 S Reid 1964b Tsuchiya 1970 Merle 1977 these flows are not as well developed however as the NECC Both the intensity and location of the various currents vary seasonally Knauss 1963 Merle 1977 The SEC and NECC are strongest during July and August In the northern winter and spring the SEC generally vanishes and the NECC is weak in the eastern Pacific there is some evidence that during this time the NECC is discontinuous at some longitudes or is entirely absent Tsuchiya 1974 In the northern summer the Pacific NECC assumes its northernmost position whereas in the northern winter the current lies closest to the equator The data base is insufficient to show an analogous migration of the Atlantic NECC The structure of the surface currents in the Indian Ocean differs markedly from those in the other two oceans African Pilot 1967 In the Indian Ocean the SEC usually lies totally south of 4S The predominantly eastward flow in the Indian Ocean is almost totally confined between the equator and the SEC North of the equator the flow direction varies season ally During the northeast monsoon it is to the west The different circulation pattern in the Indian Ocean is no doubt related to the nature of the wind field In the Atlantic and the Pacific the southeast and the northeast trades are well developed over most of the ocean throughout the year The mean stress is generally greater than the annual or semiannual components In the Indian Ocean south of about 100S the southeast trades are reasonably steady North of 10 S the mean winds are weak and the stress field is dominated by the strong regular forcing of the southwest and northeast monsoons The earliest measurements that indicated that the currents at depth
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