6EquatorialCurrents:Observationsand TheoryAnts LeetmaaJulian P. McCreary, Jr.Dennis W. Moore6.1 IntroductionHistorically, our knowledge of the circulation patternsin the tropics was derived from compilations of ship-drift data and so was restricted to a description of thesurface currents. Although information of this type iscrude, a picture of the spatial and temporal structureof the surface flow field was deduced over the years,and it has been little improved upon in the modern eraof instrumentation. By contrast, almost all of the in-formation about subsurface equatorial flows has beenacquired recently. It is remarkable that one of the majorocean currents, the Pacific Equatorial Undercurrent,was not discovered until 1952.For many reasons, progress in understanding equa-torial circulations has been slow. The equatorial re-gions are vast and remote. The swift currents and highvertical shears put special demands on instrumenta-tion. The geostrophic approximation, which is so use-ful at mid-latitudes, breaks down close to the equatorand cannot be relied on to give accurate informationabout the currents. Finally, the flows seem much moretime dependent than at mid-latitudes. Hence, on thebasis of individual cruises, haphazardly taken in timeand space, it is difficult to develop a consistent pictureof the circulation patterns.The variability of equatorial circulations has onlyrecently been appreciated. This is partly because a greatdeal of the information about equatorial circulationscomes from the central Pacific, where historically themean appears to dominate the transient circulation;recent NORPAX (North Pacific Experiment) observa-tions (Wyrtki, McLain, and Patzert, 1977; Patzert, Bar-nett, Sessions, and Kilonsky, 1978), however, suggestthat the variability can be quite large even there. Inother regions such as the western or eastern Pacific orthe Indian Ocean, the fluctuating components are aslarge as or larger than the means.The goal of this chapter is to give a short overviewof the outstanding features of the equatorial ocean cir-culation patterns, the dominant spatial and temporalstructures of the Pacific equatorial wind field (as anexample of the kinds of driving mechanisms that needto be considered), and a summary of some of the the-oretical ideas that have been developed to explain theocean circulation and its relation to the wind field. Noattempt is made to be comprehensive because in recentyears there have been numerous excellent reviews ofequatorial phenomena and theories for them. Theseinclude articles by Knauss (1963), Tsuchiya (1970), Rot-schi (1970), Philander (1973), Gill (1975a), and Mooreand Philander (1977). A collection of papers discussingvarious topics of equatorial oceanography is containedin the proceedings of the FINE (1978) workshop, heldi84A. Leetmaa, J. P. McCreary, Jr., and D. W. Mooreat Scripps Institution of Oceanography during the sum-mer of 1977. A comprehensive discussion of analytictechniques for studying forced baroclinic ocean mo-tions in the equatorial regions is presented in a three-part paper by Cane and Sarachik (1976, 1977, 1979). Indiscussing the theories, we shall stress the importantphysical ideas of each model (avoiding whenever pos-sible the use of mathematics), put them in historicalperspective, and relate them to the observations. Theobjective here is to identify the observations for whichwe have physical theories, and thereby indicate wherefurther work is needed.The importance of knowing the detailed time andspatial structure of the wind field is emphasizedthroughout this chapter. The reason is that in the trop-ics, the characteristic response times for baroclinicoceanic processes are much shorter than they are atmid-latitude and are much closer to the time scalescharacterizing the wind variations. Therefore the baro-clinic response to atmospheric forcing is expected tobe much stronger than at mid-latitude. The implicationis that in order to arrive at a satisfactory explanationof the oceanic features, the temporal and spatial struc-ture of the atmospheric forcing must be known accu-rately.6.2 Observations6.2.1 The OceanThe surface currents are characterized by zonal bandsin which the flow is alternately eastward or westward(Knauss, 1963). The eastward flows are referred to ascountercurrents because they flow counter to the di-rection of the easterly trade winds. The westward flowsare referred to as North and South Equatorial Currents.In the Atlantic and the Pacific, the North EquatorialCountercurrent (NECC) is approximately located be-tween 5 and 10°N with westward flow to the north ofthis region in the North Equatorial Current (NEC) andwestward flow to the south of it in the South EquatorialCurrent (SEC). There is also evidence for a South Equa-torial Countercurrent (SECC) in both oceans between5 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 thevarious currents vary seasonally (Knauss, 1963; Merle,1977). The SEC and NECC are strongest during Julyand August. In the northern winter and spring the SECgenerally vanishes and the NECC is weak; in the east-ern Pacific there is some evidence that during this timethe NECC is discontinuous at some longitudes or isentirely absent (Tsuchiya, 1974). In the northern sum-mer the Pacific NECC assumes its northernmost po-sition, whereas in the northern winter the current liesclosest to the equator. The data base is insufficient toshow an analogous migration of the Atlantic NECC.The structure of the surface currents in the IndianOcean differs markedly from those in the other twooceans (African Pilot, 1967). In the Indian Ocean theSEC usually lies totally south of 4S. The predomi-nantly eastward flow in the Indian Ocean is almosttotally 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 Oceanis no doubt related to the nature of the wind field. Inthe Atlantic and the Pacific, the southeast and thenortheast trades are well developed over most of theocean throughout the year. The
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