Acoustic Doppler current profilers have been used with great success to monitor vertical shear in oceanic currents. However, whether mounted on ships or moored at depth, these profiles have a "blind spot" near the surface, precisely where we expect the strongest response to wind forcing. The Johnson et al. (2001) compilation suggests that more than 50% of the meridional overturning cell's poleward transport across 2°N lies above 15 m. For downward-looking shipboard ADCPs, the depth of the first measurement bin is determined by the depth of the ship's hull and the blanking depth of the sensor; for upward-looking moored ADCPs, the top bin is determined by the depth of the ADCP and the angle of its beams (Gordon 1996). For most applications such profilers have been set up to monitor the current structure between 250 and 25 m, and the current profiles must generally be extrapolated to the surface for calculations of, for example, mixed layer velocity, integrated transport, transport divergence, and vertical velocity. Extrapolations in these cases are typically based upon some assumption of the shear within the top 25 m, either that it is zero (i.e., a slab layer) or that it is uniform and equivalent to the shear at ∼25–30-m depth.
Using a set of five current meters mounted at 5-m intervals between 5-m and 25-m depth on a test mooring in the cold tongue front at 2°N, 140°W, we show that the near-surface shear is highly sensitive to the vertical and horizontal temperature (i.e., density) distribution. In particular, the horizontal density gradient gives rise to a geostrophic (thermal wind) shear that affects the dynamics in (at least) two ways: through the surface boundary condition (2b), where it is part of the total shear and acts as a component of the effective stress forcing the ageostrophic shear, and as part of the interior shear that is added to that ageostrophic shear forced by the effective surface stress. At 2°N, 140°W, the SST front is oriented southwestward, with cool water to the south and east, while the trade winds are to the northwest. Thus, the mean zonal component of the thermal wind shear is westward, tending to balance the westward wind stress and, as a result, the ageostrophic shear required to balance the zonal wind stress is small. On the other hand, the meridional component of thermal wind shear is southward. A large northward ageostrophic surface shear is thus required to balance the northward wind stress. The net effect is that the observed Ekman-like ageostrophic spiral is shifted roughly 60° to the right of the expected classical Ekman spiral. At 2°N, 140°W this dynamic, combined with the southwestward thermal wind shear, was manifest in total (measured) near-surface shears that were oriented to the left of the wind stress. The near-surface shear is not a linear combination of the geostrophic thermal wind shear and shear associated with the classical Ekman spiral. We show, instead, that realistic ageostrophic currents with an Ekman-like spiral were reproduced when the Ekman equation was forced by the portion of the wind stress that is out of balance with the surface geostrophic shear. We refer to this as the "frontal Ekman" model. Alternatively, the shear equation can be written in terms of stress, with the requirement that at the depth of no stress the viscosity is zero. The resulting "generalized Ekman" model is valid both in frontal regions and on the equator. Both the frontal and generalized Ekman models produced near-surface currents relative to 25 m that were in qualitative agreement with those observed at 2°N, 140°W.
Considering that the Ekman (1905) model is over a hundred years old, it is somewhat surprising to the au-thors that its failure in frontal regions has not been thoroughly studied. This may be because the geostrophic shear depends upon 1/f and therefore decreases with latitude. That is, the surface thermal wind stress magnitude may be comparable to the wind stress only in frontal regions in the tropics. Another reason may be that near-surface shear measurements are very rare. The present study is based upon five current meters with thermistors in the top 25 m of a surface mooring at 2°N, 140°W. Although a single mooring in the frontal region at 2°N, 140°W was sufficient to see the influence of the front on the Ekman spiral, a more comprehensive observing strategy would be necessary to resolve the influence of the front on the vertical motion. Our results suggest that the front might have a narrow meridional-vertical (secondary) circulation cell that splits the tropical Pacific meridional overturning cell into two or three cells. This, however, cannot be resolved with the present data.
The authors thank the TAO project office and the PMEL Engineering Development Division for assistance with this project. In particular, the authors wish to thank Paul Freitag, Patricia Plimpton, Sonya Noor, Curren Fey, and David Zimmerman for their help processing the data and preparing the sensors. The authors also thank three anonymous reviewers and LuAnne Thompson, Lief Thomas, Eric D'Asaro, RenChieh Lien, Fabrice Bonjean, Renellys Perez, ChuanLi Jiang, and Andy Chiodi for illuminating discussions of this work. Support for this work was provided by the NOAA/Climate Programs Office and Office of Oceanic and Atmospheric Research.
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