The three-dimensional structure of the tropical circulation cell in the central equatorial Pacific Ocean

Perez, R.C. and W. S. Kessler

Journal of Physical Oceanography, 39(1), 27-49.

Abstract The surface limb of the tropical circulation cell in the central equatorial Pacific ocean is characterized by strong equatorial upwelling, near-surface poleward flow, and downwelling near 4° latitude. Meridional and vertical velocity fluctuations associated with tropical instability waves (TIWs) are much larger than those associated with the cell and can modify the background circulation through rectification. OGCM experiments are used to simulate the spin-up of the cell along 140°W in response to perturbed Trade winds during various phases of the annual cycle. Equatorially-modified versions of geostrophy and Ekman theory and zonal filtering isolate the large zonal scale wind-driven response. Weakening the Trades in any season rapidly weakens the cell, decreases the shear of the zonal currents, and reduces the amplitude and propagation speed of the TIWs. In boreal fall and winter when the background winds and TIWs are seasonally strong, the spin-down of the cell is equatorially asymmetric and there is clear evidence of rectification by the modified TIWs. The ageostrophic cell response is dominated by an equatorially-modified Ekman response and is essentially linear within a given season, with relatively large interseasonal differences due to nonlinear interactions with the background cell and TIWs. Cross-equatorial velocity measurements with fine meridional and temporal sampling are required to quantify the relative contributions of these two processes to the equatorial asymmetry of the cell in boreal fall and winter.
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