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

Abstract

Hydrographic data from the Gulf of Mexico (gulf) provide evidence that a western boundary current was set up by the interaction of an anticyclonic Loop 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 cyclonic rings were shed as the anticyclone transferred vorticity to the surrounding slope water. During July‐August 1985, the ring triad weakened and evolved into a ∼900‐km‐long, north flowing, along‐slope, western boundary current and cyclonic‐anticyclonic ring pairs distributed throughout the central and western gulf. This western boundary current attained maximum northward flow speeds of 25 cm s−1 and an 8.3‐Sv mass transport between 94°–96°W at 25°N. Our March‐August 1985 observations 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 ∼50%. Thus the western boundary current's evolutionary period, from its gestation to its absolute decay, is estimated to be of the order of 300 days. Although the presence 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 corresponding (January, February and May, June 1985) monthly mean synoptic wind stress curl distributions reveal that these constitute competing forcing mechanisms for the gulf's regional circulation. However, when very strong local forcing such as large eddies are present, the wind‐driven background circulation is overwhelmed by such eddy forcing.