Seasonal Variability of the Alongshore Flow over the Continental Slope and Shelf of the Western Gulf of Mexico

Georgina Rojo de Anda, Center for Scientific Research and Higher Education at Ensenada, Ensenada, BJ, Mexico, Paula Perez-Brunius, CICESE, Physical Oceanography, Ensenada, BJ, Mexico, Sharon Z Herzka, Center for Scientific Research and Higher Education at Ensenada, Biological Oceanography, Ensenada, BJ, Mexico, Manuel Lopez, CICESE, San Diego, CA, United States and Enric Pallas Sanz, Center for Scientific Research and Higher Education at Ensenada, Physical Oceanography, Ensenada, BJ, Mexico
In the western Gulf of Mexico, from the northern part of Veracruz to the southern part of Texas, persistent flows are found, both, over the continental shelf, and slope. Some studies suggest that their most important forcing is the wind. In the continental shelf, numerical models show that this flow follows Ekman dynamics due to the local wind, furthermore, these models suggest that this flow varies in direction seasonally, going north in summer and south in winter. On the other hand, studies using ship drift data show that the flow in the continental slope is always northward, being more intense in June and less intense in January, following the seasonal variability of the offshore wind-stress curl. In this work the seasonal variability of the surface velocity of these flows is obtained by using observations of the velocity field with twenty years of surface drifter data, and eight years of current-meter mooring data located in two transects over the slope. Over the continental shelf, the drifter data suggest a northward flow in both seasons north of Veracruz, while in the northern Tamaulipas’s shelf there is a seasonal reversal of the flow, directed southwards in the winter. In the continental slope, both drifter and mooring data show a northward flow throughout the year, being more intense in summer than in the winter, in agreement with the ship drift data study. The horizontal structure of the flow changes seasonally, being narrower in the summer with the maximum speed near the continental shelf break, making the flow in the shelf indistinguishable from the one over the continental slope. In winter the velocity maximum is displaced offshore, allowing these two flows to be distinguished from each other. The role that the local wind-stress and offshore wind-stress curl play in the variability of this flow is currently being explored and will also be discussed.