Collision of a loop current anticyclonic ring against the continental shelf slope of the western Gulf of Mexico

Abstract

A Loop Current anticyclonic ring --•330 km in diameter and extending to a depth of > 1500 m was observed to collide in January of 1984 against the continental shelf slope of the western Gulf of Mexico between 21.5 ø and 23øN. The collision occurred precisely at the time we conducted our Argos 84-1 hydrographic cruise in the western gulf (26o00 ' to 19ø20'N) aboard the R/V Justo Sierra. The Caribbean Subtropical Underwater (SUW) was used as a tracer to identify the Loop Current anticyclonic ring within the western gulf. The collision was identified from temperature and salinity distributions and from the dynamic topography distribution relative to 500 m. The ring' s collision zone was identified by the presence of a horizontal baroclinic flow divergence, to the east of Tamiahua, that divides the surface circulation into northward and southward baroclinic currents parallel to the western gulf's continental shelf break, with speeds of 85 and 32 cm s -1, respectively. Horizontal divergence and vertical convergence (ring asymmetries) resulted at the focus of the anticyclonic ring's collision and originated the alongshore self advection and northward translation of the colliding anticyclone. Upon colliding the anticyclonic ring shed approximately one third of its volume (--•2 x 10 4 km3), mass, and transferred angular momentum to the south flanking water mass, thus generating a cyclonic ring to the south of the collision zone. The observed alongshelf southward current results from mass conservation and volume continuity requirements associated with the anticyclonic ring's volume shedding and most probably constitutes the colliding ring' s potential vorticity conservation mechanism. The weakening of the anticyclonic ring's relative vorticity due to the collision is most likely made up by gain of vorticity from lateral shear in the northward and southward current jets parallel to the continental shelf break. The core of both the anticyclonic and cyclonic rings had typical SUW salinity (>36.5%0) and temperature (--•22.5øC) values. The rings were separated by a 5 x 10 4 km 2 divergence zone occupied by Gulf Common Water (GCW). The SUW was absent within the collision zone to the east of Tampico (22.3øN, 97.8øW). The GCW within this divergence zone resulted from the convective mixing and dilution of the SUW with less saline (36.1 -< S -< 36.3%0) water from the uppermost layer of the thermocline. Hence the collision of Loop Current anticyclones against the western continental shelf slope of the gulf constitutes a primary mechanism by which 30 Sv of SUW are converted to GCW in the Gulf of Mexico. On the other hand, the coastal and continental shelf water temperature and salinity distributions that resulted from the ring's collision indicate that the offshore GCW mass intrudes the continental shelf to the east of Tamiahua and is diluted by low-salinity coastal water within the western continental shelf. This GCW mass intrusion most probably constitutes a principal and efficient exchange mechanism between the western gulf's cont...

On the variability of the flow along the Meso-American Barrier Reef system: a numerical model study of the influence of the Caribbean current and eddies

“…1 Topographic map (with depth contours in meters) of the western North Atlantic Ocean and the maior currents and features of the region. The area of interest of this study, the WCS, is indicated by the arrow show the important interaction between eddies and shelf circulation have been done in regions such as the western Gulf of Mexico (Vidal et al 1992), but not in the WCS. Caribbean eddies have been studied in the past by observations and models (Carton and Chao 1999;Murphy et al 1999;Andrade and Barton 2000;Oey et al 2003), but their influence on the MBRS is still not clear.…”

Eddy energy and shelf interactions in the Gulf of Mexico

“…Both northward and temporary eastward movement of Loop current eddies in the western gulf have been observed [Kirwan et al, 1984;Lewis et al, 1989]. Upon colliding with the shelf break, anticyclonic eddies can shed a significant portion of their volume and angular momentum to the south, setting up adjacent cyclonic rings [Vidal et al, 1992]. Advection of shelf water offshore on the north side of a warm ring can also produce an adjacent cyclonic eddy as relative vorticity increases with flow into deeper water [Smith, 1986].…”

Dominant Circulation Patterns of the Deep Gulf of Mexico

“…After separation from the LC, these eddies travel westward until they dissipate due to processes not yet fully understood, which involve vertical mixing induced by air-sea fluxes and enhanced submesoscale activity around the rings' periphery, and interactions with other mesoscale structures and with the western boundary (e.g., Biggs et al 1996;Hamilton 2002;Vidal et al 1992Vidal et al , 1994LaCasce 1998;Mahadevan et al 2008).…”