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Near-Surface Shear Flow in the Tropical Pacific Cold Tongue Front

M.F. Cronin and W.S. Kessler

NOAA, Pacific Marine Environmental Laboratory, Seattle, Washington, 98115

Journal of Physical Oceanography, 39, 1200–1215
Published by the American Meteorological Society. Further electronic distribution is not allowed.

1. Introduction

Away from the equator, the earth's rotation causes the net wind-forced response to be to the right (left) of the wind in the Northern (Southern) Hemisphere. Consequently, trade wind forcing in the tropics tends to drive a divergent "Ekman" meridional transport that causes upwelling on the equator. Although there have been several studies that estimate the poleward Ekman and equatorward geostrophic transports of the meridional overturning cell (Wyrkti 1981; Bryden and Brady 1985; Johnson et al. 2001; Meinen et al. 2001; Kug et al. 2003), there have been few direct measurements of their structure. In particular, most of these measurements have relied either upon shipboard (e.g., Johnson et al. 2001; Wijffels et al. 1994) or moored (e.g., Halpern and Freitag 1987; Weisberg and Qiao 2000) acoustic Doppler current profilers (ADCPs), neither of which observe the velocity profile within the top 25 m, precisely where the wind-driven poleward flow is expected to be largest. In this poorly defined situation, some analyses of the meridional overturning cell have assumed a constant shear within the surface layer (e.g., Johnson et al. 2001; Weisberg and Qiao 2000), while others have assumed a slab layer (e.g., Wijffels et al. 1994). Because the zero crossing of meridional velocity is relatively shallow, these differing assumptions have a profound effect upon the estimated net transport. Thus, in this paper, we ask the question: Is there shear within the off-equatorial near-surface mean poleward flow? To address this, from May 2004 through February 2005, a test mooring near the 2°N, 140°W TAO mooring was instrumented with five current meters in the top 25 m.

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