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

A 3D tropical upper-ocean circulation model is employed to study the genera tion mechanism for subsurface reversing currents forced by strong westerly wind bursts in the western equatorial Pacific. The westerly wind bursts las t from a few days to a few weeks and reverse the surface current from westw ard to eastward, setting up a zonal pressure gradient that generates a west ward subsurface current, Although less affected by the local wind, the east ward Equatorial Undercurrent (EUC) decelerates. The authors verified the hy pothesis that the subsurface reversal is a local response to westerly wind bursts. However, the model response is very sensitive to the wind fetch. Th e observed reversal of the subsurface current can only be reproduced by win d 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 b y equatorial waves. This timescale must be shorter than the timescales of v ertical 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 s patial variations in simulating the upper ocean response to atmospheric for cing. The influence of off-equator winds on equatorial currents is also investiga ted. It is found that the remote effect of an off-equator wind can be large r 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 nea r the surface is dominated by the westerly wind. But the remote effect of t he off-equator wind significantly modifies the local wind effect at the dep th of the EUC.