Summer Mesoscale circulation in the Mexican Tropical Pacific

The main components of the large-scale circulation of the Eastern Tropical Pacific were identified in the mid 20th century, but the details of the circulation at length scales of 102 km or less, the mesoscale field, are less known particularly during summer. The winter circulation is characterized by large mesoscale eddies generated by intense cross-shore wind pulses. These eddies propagate offshore to provide an important source of mesoscale variability for the Eastern tropical Pacific. The summer circulation has not commanded similar attention, the main reason being that the frequent generation of hurricanes in the area renders ‘in situ ’ observations difficult. Before the experiment presented here, the large-scale summer circulation of the Gulf of Tehuantepec was thought to be dominated by a poleward flow: the Costa Rica Coastal Current. A drifter-deployment experiment carried out in June 2000, supported by satellite altimetry and wind data, was designed to characterize it. We present a detailed comparison between altimetry-estimated geostrophic and ‘in situ ’ currents estimated from drifters. Our results, however, show no evidence of the existence of a poleward coastal flow. During the 10-week period of observations we documented a recurrent pattern of circulation within 500 km of shore, forced by a combination of local winds and the regional-scale flow. Instead of the Costa Rica Coastal Current we found a summer eddy field capable of influencing large areas of the Eastern tropical Pacific. Even in summer the cross isthmus wind jet is capable of inducing eddy formation