Coastal circulation in the absence of wind in the Gulf of Tehuantepec, Mexico: High-frequency radar observations

Abstract

Using high-frequency radars, ocean surface currents were mapped every hour over an area of ≈5000 km² in the inner Gulf of Tehuantepec (Mexico). The coastal circulation patterns (≈100 km offshore) were studied during spring, summer, and autumn 2006. The spring circulation was similar to the typical winter circulation, when the circulation is forced by outbursts of northerly winds (>8 m s^–1) known locally as Tehuanos. Although Tehuano events are less common in spring than in winter, they are perfectly capable of modifying the sea surface by triggering cyclonic and anticyclonic eddies (≈50–200 km in diameter). Under moderate wind conditions, the ocean circulation showed a quasipermanent westward coastal current (≈50 cm s^–1). Though the Tehuano winds were absent in summer, cyclonic eddies were observed and likely linked to the westward coastal current. Autumn was influenced by steady northerly winds with speeds of ≈12 m s^–1 that remained over the region for almost 15 days. These conditions allowed us to study the competition between the wind-induced circulation and the more intense (≈100 cm s^–1) westward coastal current during this period. The origin of this coastal current could be related to a warm coastal-trapped flow, composed of tropical low-salinity waters. The northwestward excursion of the observed coastal current is discussed, and the three-dimensional implications of surface current fields are studied by the Ekman theory and vorticity conservation.