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

Since the early surveys carried out by the Eastern Tropical Pacific (EASTROPIC) and Scripps Tuna Oceanographic Research (STOP) projects in the tropical Pacific off Mexico, the northerly winds which blow over the Gulf of Tehuantepec were described as an important factor controlling the dynamics of this coastal ocean. In January-February 1989 an international team carried out the experiment Tehuano, designed to study the response of the gulf to these wind pulses, The complete evolution of the coastal ocean after an event of moderate intensity was observed. The forcing is characterized by a mostly symmetric, fan-shaped, offshore wind jet, which in turn produces a remarkably asymmetric upper ocean 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 cooling under the wind jet is caused by entrainment of subsurface water into the upper layer. The thermocline in the west deepens with the development of the eddy, which is formed initially by the advection of warm surface 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 same time, shoaling and compression of the deeper isotherms by curl-induced upwelling (Ekman pumping) strengthen the thermocline.