Supersquirt: Dynamics of the Gulf of Tehuantepec, Mexico

T H E LAST TWO DECADES have seen a continuing effort to observe and model the response to wind forcing in the coastal ocean along eastern boundaries of ocean basins. The emphasis of research has developed through several stages including: studies of the large-scale steady state, the transient responses in localized areas, the alongshore propagation of wind-induced disturbances, and most recently the evolution of cold filaments extending offshore from the coastal upwelling zone into the open ocean. Frequently associated with such filaments is a strong jet-like current referred to as a 'squirt (Ramp et al., 1991). One interesting area that has been neglected by sea-going oceanographers with modern instrumentation is the Gulf of Tehuantepec. There, large negative temperature anomalies extend several hundred kilometers seaward as a result of offshore, rather than alongshore, winds (Fig. 1 ). By comparison with the size of the typical upwelling-filament temperature and current structure, they might be termed Supersquirts.' Significant chlorophyll anomalies seen in Coastal Zone Color Scanner (CZCS) imagery indicate enhanced primary production in the same region. The Gulf, which is situated offthe Pacific coast of southern Mexico, is subjected to intermittent forcing by an intense, offshore wind jet during the winter months (first documented by Hurd, 1929). The jet is generated when atmospheric pressure rises to high levels over the Gul f of Mexico as a result of cold outbreaks from the North American continent. At the same time, pressure over the Pacific remains low (Fig. la). The mountains of