Modeling the oceanic response to westerly wind bursts in the western equatorial Pacific

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A 3D tropical upper-ocean circulation model is employed to study the generation mechanism for subsurface reversing currents forced by strong westerly wind bursts in the western equatorial Pacific. The westerly wind bursts last from a few days to a few weeks and reverse the surface current from westward to eastward, setting up a zonal pressure gradient that generates a westward subsurface current. Although less affected by the local wind, the eastward Equatorial Undercurrent (EUC) decelerates. The authors verified the hypothesis that the subsurface reversal is a local response to westerly wind bursts. However, the model response is very sensitive to the wind fetch. The observed reversal of the subsurface current can only be reproduced by wind bursts with a zonal extent of less than 700 km. This is because the zonal extent determines the timescale on which the pressure gradient is set up by equatorial waves. This timescale must be shorter than the timescales of vertical mixing and downwelling in order for the pressure-driven subsurface westward acceleration to overtake the eastward acceleration due to downward transport of momentum. The results emphasize the importance of resolving spatial variations in simulating the upper ocean response to atmospheric forcing. The influence of off-equator winds on equatorial currents is also investigated. It is found that the remote effect of an off-equator wind can be larger on subsurface currents than on surface currents. An off-equator westerly (easterly) wind decelerates (accelerates) the EUC but has little effect on the surface equatorial current. When a cyclone with a westerly wind at the equator and an easterly off-equator wind is present, the local response near the surface is dominated by the westerly wind. But the remote effect of the off-equator wind significantly modifies the local wind effect at the depth of the EUC.

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Journal of Physical Oceanography