N11070300o3070110S-.6 -.4 -.2 0dyn/cm2Figure 6.3 Profiles of zonal wind stressillustrating the annual cycle. To emphstructure, the weak monthly mean valstress (the average of figures 6. i and 6. lthe monthly mean value at each latitud.... ,,,.,......_,· ........_..._,.,...___.the depth-integrated value of the meridional velocityusually does not vanish, the suggestion of Montgomeryand Palmen (1940) does not represent a proper expla-nation for the North Equatorial Countercurrent.Sverdrup (1947) suggested that the North EquatorialCountercurrent was not only a consequence of thezonal wind stress but also was related to the curl of thewind stress and continuity requirements. He assumedin his model that the currents were steady and van-ished at a deep level. The equations were integratedfrom this depth to the sea surface; hence the solutiongave no information about the vertical structure of themotion field. The apparent success of Sverdrup's theoryin the eastern Pacific is often cited as observationalendorsement of its widespread use in large-scale ocean-circulation theory.In light of the earlier discussion about the large var-iability in the position and amplitude of the NECC,and the considerable monthly variation in the windsin this region, it is surprising that this steady theoryshould be applicable there. In fact, Sverdrup (1947) andReid (1948), instead of using the mean wind stress in|1ovir ----L--^- rrterfein 1ev 3 LI~- hT-,ho ~ --rnoa r r .2 .4 .6 tneir cumputatluns, useu lilc uil;oier-lNUVllln val-ues. They also used October-November oceanographicdata to compute vertically integrated pressure. Implicitfrom 100 to 120°W, in their theory, then, was the assumption that thelasize the meridionaltasize the meridional oceanic response to the fluctuating winds was suffi-lue of the equatorialwas subtracted from ciently rapid that the ocean was almost always in equi-e. librium with the instantaneous winds (i.e., quasi-6.3 Theories6.3.1 Integrated TheoriesThe earliest theoretical attempts sought an explanationfor the North Equatorial Countercurrent. This currentdefies intuition since it flows opposite to the prevailingwinds. Montgomery and Palm6n (1940) suggested thatthe easterly wind stress in the equatorial zone is bal-anced by the vertically integrated zonal pressure gra-dient. They presented supporting observational evi-dence in the Atlantic. One interesting result was theirdemonstration that the baroclinic pressure gradientsgenerally were confined to the top few hundred metersof the water column. As an explanation for the NorthEquatorial Countercurrent, they hypothesized that inthe doldrums, i.e., in the vicinity of the ITCZ, wherethe magnitude of the zonal stress is greatly reduced,the pressure gradient would maintain the value that ithad on either side of this region and hence no longerbe balanced by the wind stress. As a result, an eastwardflow would develop, which they suggested would beretarded by lateral friction. Charts of dynamic topog-raphy (Tsuchiya, 1968) indicate that the zonal pressuregradient in this region is, in fact, less than it is fartherto the north and south. Furthermore, since 1°or 2°offthe equator Coriolis terms cannot be neglected andsteady).Therefore, it is of some interest to redo the compu-tations of Sverdrup and Reid using the values for themonthly mean stress field from Wyrtki and Meyers(1975) and to compare the results with oceanographicdata from different times of the year to see whetherSverdrup balance really is quasi-steady. The monthlyvalues of the zonal Sverdrup transport are shown infigure 6.4. The October and November curves lookquite similar to those of Sverdrup. The overall spatialand temporal evolution of the currents throughout theyear resembles the picture that has been derived forthe surface flow field from ship-drift observations. It ispossible to check to see whether the historical inte-grated geostrophic transports follow a similar patternof change.During 1967 and the first part of 1968, hydrographicsections at four different longitudes between 100 and120°W were occupied across the equator at seven dif-ferent times. The geostrophic velocity computationsrelative to 500 db for these sections are given by Love(1972). Using these figures, the geostrophic transportfor 2°bands of latitude from 2 to 12°N was computedby a planimetric integration. The minimum contourinterval for the velocities was 5 cms-1; this intervalintroduces an uncertainty of about 3 x 109 kgs-intoI88A. Leetmaa, J. P. McCreary, Jr., and D. W. MooreI10NO010'N0'10'N0'Figure 6.4 Upper panel depicts monthly Sverdrup transport.Lower two panels show geostrophic transports observed dur-each transport computation (2.5 cms-x 500 m x2°x 1 gcm-3). The results of these integrations for thesections at 119°W and 112°W are shown in figure 6.4.The tendency at both sections is for the transport inthe south equatorial current to be strongest during thesummer. The NECC lies close to the equator early inthe year and moves northward during the summer andsouthward again during late fall. The transport patternsat both sections during the first part of 1967 and 1968are quite similar. This agreement could be coincidenceor indicate that these fluctuations perhaps have a reg-ular annual cycle. The observed geostrophic transportpatterns are visually similar to the theoretical pattern;there clearly are quantitative differences, however.The important conclusion from this study is that itcannot be said whether the Sverdrup relation providesan accurate description of the currents in the easterntropical Pacific. Sverdrup and Reid were probably for-tunate in that their results seemed to agree so wellwith observations. The real problem with testing Sver-drup theory, in light of what we now know about bar-otropic and baroclinic adjustment, is associated withthe assumed level of no motion. For an ocean with afree-slip flat bottom, Sverdrup balance should hold forthe integrated flows and pressures, as long as the in-tegral goes all the way to the bottom and
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