Coastal stratification and mixing on the western margin of the Gulf of Tehuantepec within a synoptic atmospheric and oceanographic environment

Abstract

We present, for the first time for this region, the evolution of stratification in a 24 m depth water column in the western margin of the Gulf of Tehuantepec, northeastern tropical Pacific, during the dry season: December 2011 to March 2012. Stratification, defined here as the temperature difference observed between 5- and 24-m depth, although never equal to zero, decreased as a function of wind, but also as a function of the ambient thermal structure created by the transit of mesoscale eddies. A common wind scenario is made of the occurrence of northerlies over the central gulf, and westerlies over the western margin. As northerly winds decay, westerly winds may become dominant over the entire gulf. Stratification weakens under relatively strong northerlies over the gulf or strong westerlies over the western margin. In contrast, stratification strengths as northerly winds relax and the westerly wind on the western margin is weak or incipient. The degree of stratification is also dependent on the sign of the mesoscale eddies transiting the area; it can vanish under cyclonic circulation but become strong under anticyclonic circulation. We hypothesize that the increase of stratification on the western margin of the Gulf of Tehuantepec is the result of a two-layer circulation that develops between the gulf and its western margin during the northerly wind relaxation. Relatively warm water flows, near the surface, from the western margin towards the central gulf, and mixed relatively cold-water, flows in opposite direction below. This finding represents a novel result for this region.