Characteristics of Near surface circulation patterns in the eastern equatorial pacific

Abstract

Since June 1981 several CTD and velocity profiler sections have been made across the equator at 85°W as part of the Equatorial Pacific Ocean Climate Studies (EPOCS). Two of these sections, those in June 1981 and December 1982, are discussed in order to characterize the circulation patterns that occur when the ocean is forced by meridional wind stress. In this area the winds are predominantly southerly. Use of an Ametek-Straza acoustic doppler backscatter profiler during the 1982 cruise gave velocity data to a depth of 270 m every 5.5 km along the shiptrack. This allowed the shear and velocity fields to be examined in greater detail than had been possible before. Large areas had shears greater than 0.01 s −1 and values as high as 0.03 s −1 were observed. Greatest shears tended to coincide with regions of large vertical density gradients. Comparisons with geostrophic computations suggest that a large fraction of the shear was geostrophic. Richardson number computations indicated that over 40% of the area sampled by the CTD casts had values of less than one and about 5% had values less than one quarter. Values of less than one quarter were concentrated in the mixed layer but were not confined to it. Below the mixed layer, regions with low Richardson number were associated with regions where the vertical density gradient was quasi-linear. Since such regions were not confined to the vicinity of the equator these observations suggest shear mixing as an important mechanism over large areas of the upper ocean. Within the top 40 m of the water column there was a general tendency for motions in the mixed layer to be the left of the wind in the southern hemisphere and to the right in the northern hemisphere. However, no clear examples were found of Ekman spirals. Most of the veering was confined to the mixed layer which was of relatively constant depth over the whole survey area. South of the equator a large fraction of the shear across the base of the mixed layer appeared to be geostrophic, whereas north of the equator, these shears were ageostrophic. Estimates of the various terms in the momentum equation in the upper ocean indicated that near the equator non-linear effects, such as upwelling and northward advection, were as important as Coriolis forces, pressure gradients and wind stress. The pressure gradient, integrated over the depth of the mixed layer, was larger than the stress in the vicinity of the equator and the relative importance of other terms varied from place to place. The horizontal scale of variation was as small as 10 km. Near surface upwelling occurred in shallow cells with horizontal dimensions of 100 km or less.