Direct observations of the upper layer circulation in the southern Gulf of Mexico

The upper layer circulation in the Bay of Campeche is analyzed with three years of data recorded by surface drifters, current meter moorings, and satellite altimetry. The measurements show that the mean cyclonic circulation observed by previous authors extends below 1000 m, and that its size and location are delimited by the particular topography of the region: a deep basin to the west, and a shallower and gentle sloping submarine fan to the east. An Empirical Orthogonal Function analysis and large correlations of the surface flow with the deeper currents suggest that the topographic constraint is the result of potential vorticity conservation for an equivalent barotropic flow. The variability of the surface currents in the western basin is mostly due to changes in the size, form, position and intensity of the cyclonic gyre due to its interaction with northern Gulf of Mexico eddies, particularly Loop Current Eddies traveling the southern route towards the western boundary. By contrast, the eastern basin is characterized by a weak northward drift, with the occasional generation of anticyclones in the southeastern boundary, the genesis of which remains to be understood. This suggests that the variability in the eastern basin is mostly driven by locally generated disturbances, rather than by an influx of northern Gulf of Mexico eddies. Strong northward flows in the central and eastern basins result from the flow convergence between locally generated anticyclones and the cyclonic gyre.