Ring-slope interactions and the formation of the western boundary current in the Gulf of Mexico

Hydrographic data from the Gulf of Mexico (gulf) provide evidence that a we stern boundary current was set up by the interaction of an anticyclonic Loo p Current (LC) ring with the continental margin of the western gulf during March-August 1985. The March 1985 geostrophic circulation reveals a remnant anticyclonic ring colliding with the slope. During this collision, two cyc lonic rings were shea as the anticyclone transferred vorticity to the surro unding slope water. During July-August 1985, the ring triad weakened and ev olved into a similar to 900-km-long, north flowing, along-slope, western bo undary current and cyclonic-anticyclonic ring pairs distributed throughout the central and western gulf. This western boundary current attained maximu m northward flow speeds of 25 cm s(-1) and an 8.3-Sv mass transport between 94 degrees-96 degrees W at 25 degrees N. Our March-August 1985 observation s reveal that the residence time and decay period of LC anticyclones in the western gulf may exceed 150 days. Within this time period the western gulf 's cyclonic-anticyclonic vorticity field decayed similar to 50%. Thus the w estern boundary current's evolutionary period, from its gestation to its ab solute decay, is estimated to be of the order of 300 days. Although the pre sence of a western boundary current in the gulf has been attributed to the annual wind stress curl cycle [Sturges, 1993], our analyses of the western gulf March and July-August 1985 ring-driven geostrophic circulation and cor responding (January, February and May, June 1985) monthly mean synoptic win d stress curl distributions reveal that these constitute competing forcing mechanisms for the gulf's regional circulation. However, when very strong l ocal forcing such as large eddies are present, the wind-driven background c irculation is overwhelmed by such eddy forcing.