Poleward Flows off Mexico’s Pacific Coast

Abstract

Poleward flows are predominant off the west coast of Mexico. Off the peninsula of Baja California, the northern third of the Mexican Pacific coastline, one finds the southern extension of the California Current System. Poleward flows in this region occupy a broad domain of about 200 km width, adjacent to the coast, and include an intensified countercurrent that can reach 50 cm s ⁻¹ , often within 20 km of the shelf and slope. From late winter to early summer, the upwelling favorable northwesterly winds oppose the poleward flow at the surface and the current proceeds as an undercurrent that hugs the continental slope. This poleward flow advects Subtropical Subsurface Water from the tip of Baja California into the Southern California Bight and farther north. The coast of mainland Mexico is exposed, on the contrary, to a broad extension of the equatorial return flow that feeds the North Equatorial Current from the Equatorial Countercurrent. This flow is known as the Costa Rica Coastal Current; it extends around the Costa Rica Dome, and northwestwards to the mouth of the Gulf of California, where the Eastern Pacific Transition Zone usually abutts the continent. This poleward flow is most prominent during the fall and may reverse directions in the spring, appearing as an intensified California Current extension. The strength of the large scale drifts, the surface expression of the counterflows, and the latitude at which the currents meet, all vary seasonally and depend on the intensity of the dominant winds. Direct observations of the poleward flows off Mexico are few and restricted to northern Baja California. They confirm a counterflow is probably present throughout the year, mostly over the shelf and slope regions, but with its surface expression obliterated by the upwelling favorable northwesterly winds that blow strongly in the late winter and spring. There are no direct measurements to provide a detailed description of the Costa Rica Coastal Current and, in particular, to know whether it possesses a counterflow.