OFFSHORE WIND FORCING IN THE GULF OF TEHUANTEPEC, MEXICO - THE ASYMMETRIC CIRCULATION

Since the early surveys carried out by the Eastern Tropical Pacific (E ASTROPIC) and Scripps Tuna Oceanographic Research (STOP) projects in t he tropical Pacific off Mexico, the northerly winds which blow over th e Gulf of Tehuantepec were described as an important factor controllin g the dynamics of this coastal ocean. In January-February 1989 an inte rnational team carried out the experiment Tehuano, designed to study t he response of the gulf to these wind pulses, The complete evolution o f the coastal ocean after an event of moderate intensity was observed. The forcing is characterized by a mostly symmetric, fan-shaped, offsh ore wind jet, which in turn produces a remarkably asymmetric upper oce an response. While analytical results based on Ekman theory forced by a symmetric offshore wind predict the formation of a symmetric dipolar circulation, the observed flow consists mainly of a large (200 km in diameter) anticyclonic warm-core eddy in the western gulf, with only a weak cyclonic counterpart in the eastern gulf. Intense surface coolin g under the wind jet is caused by entrainment of subsurface water into the upper layer. The thermocline in the west deepens with the develop ment of the eddy, which is formed initially by the advection of warm s urface waters from west of the gulf. East of the axis of the wind, the mixed layer deepens due to wind-induced entrainment, while, at the sa me time, shoaling and compression of the deeper isotherms by curl-indu ced upwelling (Ekman pumping) strengthen the thermocline.