Abstract
Historical XBT data are used to construct a mean climatology of the three-dimensional geostrophic circulation in the northeast tropical Pacific (southwest of Mexico and Central America) and are diagnosed based on linear dynamics forced with satellite scatterometer winds. Unlike the familiar central tropical Pacific, where the zonal scales are very large and the wind forcing nearly a function of latitude alone, the North Pacific east of about 120°W is strongly influenced by wind jets blowing through gaps in the Central American cordillera. The curl imposed by these wind jets imprints on the ocean, producing a distinctive pattern of thermocline topography and geostrophic currents that are consistent with the Sverdrup balance. Notably, the weakening of the North Equatorial Countercurrent near 110°W is due to the wind forcing. Given the observed stratification and wind stress curl, planetary vorticity conservation also determines the distribution of vertical velocity in the region, with about 3.5 Sv (Sv ≡ 106 m3 s−1) of upwelling through the base of the thermocline under the Costa Rica Dome. This upwelling is associated with stretching of the water column under the dome, which thereby causes the northern “Subsurface Counter Current” (SSCC or Tsuchiya Jet) to turn away from the equator; about half the transport of the SSCC upwells through the thermocline via this mechanism. This may be part of the process by which intermediate-depth water, flowing into the Pacific from the south, is brought to the surface and into the Northern Hemisphere.
Highlights
The northeastern tropical Pacific is close to a well-populated coast, is crossed by often-traveled shipping lanes, and is the long-term site of major fisheries, there has been relatively little attention paid to the dynamics that control the mean circulation in the region since the pioneering investigations done at the Scripps Institution of Oceanography more than 30 years ago (Cromwell 1958; Wyrtki 1964, 1966, 1967)
The east Pacific warm pool lies at the east end of the band of high SST that extends across the basin from the west Pacific warm pool, under the ITCZ and along the North Equatorial Countercurrent (NECC)
The mean geostrophic circulation is consistent with the Sverdrup balance (Fig. 6), driven principally by the three wind jets blowing through the gaps in the Central American cordillera
Summary
The northeastern tropical Pacific is close to a well-populated coast, is crossed by often-traveled shipping lanes, and is the long-term site of major fisheries, there has been relatively little attention paid to the dynamics that control the mean circulation in the region since the pioneering investigations done at the Scripps Institution of Oceanography more than 30 years ago (Cromwell 1958; Wyrtki 1964, 1966, 1967). Most of these differences arise because the drifters sample the total near-surface flow field, unlike Fig. 3 which shows only the geostrophic part that can be estimated from the XBT data.
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