U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

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.

4. Results

4.2. Time Series

In the analysis of the mean shear, it was shown that the geostrophic shear can strongly influence the ageostrophic response to the wind, and can cause the near-surface currents to rotate with depth either to the right or left of the wind stress. To illustrate the sensitivity of the shear to the orientation and strength of the front we focus on the near-surface shear response during the passage of TIWs. Although the trade winds were relatively steady at 2°N (Fig. 3), the orientation of the SST front, indicated by the direction of the thermal wind shear, varies substantially as tropical instability waves propagate through the region. Near 4°–5°N the front can form "cusp" patterns, but at 2°N the shape of the front typically has a more wavelike character, with the front's angle of orientation generally varying within a ∼90° range (i.e., from northwest to southwest).

figure 3

Fig. 3. Five-day-averaged time series of the 2°N, 140°W winds (blue vectors), temperature (color shading), and (a) currents (black vectors) and (b) currents relative to 20 m (black vectors) and surface geostrophic current relative to 20 m (green vectors). The vector scale for the currents, winds, and relative currents is shown at the bottom.

The time series of the 5-day-averaged measurements at 2°N, 140°W (Fig. 3a) show that, during the cool phase of the TIW, the upper ocean flow tends to be northward and, during the warm phase, the flow tends to be southward (Flament et al. 1996). The observed shears (Fig. 3b) also appear to be influenced by the passage of the fronts, although the pattern is not as distinct as for the current measurements. The sensitivity to wind and frontal variations, however, is clearly seen when the composite TIW is computed following the methodology described in section 3.

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