High‐frequency response of the ocean to mountain gap winds in the northeastern tropical Pacific

Abstract

The wind jets generated by the three mountain gaps of Central America have a substantial impact on the mean state of the northeastern tropical Pacific. Here, we study the mean oceanic impact of individual high‐frequency wind events by synthesizing satellite and reanalysis products. Using daily sea level wind and sea level pressure (SLP) for the period from 1999 through 2007, we identified a total of 103 short‐term wind events for the Gulf of Tehuantepec, 59 events for the Gulf of Papagayo, and 36 events for the Gulf of Panama, most of them occurring between November and May. On the day of maximum wind, the composite peak wind speed reaches about 15 cm/s across nearly the entire Gulf of Tehuantepec, while the winds in the other two gulfs are somewhat weaker. Downwind of the gaps, the composite mean sea surface temperature (SST) drops substantially within one day in response to these wind events, but the spatial extent of the changes remains limited to the region of high wind speed. The high correlation with wind speed suggests that the cooling is mainly a response to wind‐driven turbulent mixing rather than wind stress curl induced upwelling. The boundary layer turbulence is strongly shear‐driven and weakly convectively driven in the Gulf of Tehuantepec and purely shear driven in the other two gulfs. Mean SST recovers quickly after the event, with the recovery largely being controlled by the anomalous surface heat flux induced by the SST disturbance. Each wind event is followed by a phytoplankton bloom, as evidenced by strong increases in satellite detected chlorophyll‐a (Chl‐a) and inferred net primary production (NPP). The initial response of chlorophyll and NPP is delayed relative to SST by only one day, likely owing to upward mixing of chlorophyll from the deep chlorophyll maximum. Chlorophyll remains elevated for several days after the SST has already recovered suggesting that the upward mixing of new nutrients has sustained the bloom. This is supported by the observation that the chlorophyll response remains also limited to the region of high wind speed. These wind events contribute substantially to the overall NPP in the northeastern tropical Pacific. In contrast to SST and Chl‐a, the response of sea surface height (SSH) to these wind events appears to involve both local coastal recirculations and regional mesoscale activities.