Lagrangian observations of an anticyclonic ring in the western Gulf of Mexico

Abstract

This analysis documents, for the first time, the movement and velocity characteristics of an anticyclonic ring. The ring was pinched off from the Loop Current in the fall of 1980 and moved into the western Gulf of Mexico. Lagrangian measurements obtained from satellite-tracked drifters show that typical speeds of the near-surface currents associated with this ring are about 50 cm/s. There is also a surprising amount of higher-frequency current fluctuations. These include diurnal and semidiurnal tides, a basin tidal resonance, and a free gravity mode. 1. INTRODUCTION It is well known, but not widely appreciated, that a dominant characteristic of the circulation of the western Gulf of Mexico is the presence of intense anticyclonic structures. See, for example, Figure 3 from Sweitzer [1898]. More recently, Austin [1955] showed general regions of relatively high dynamic topography in the western Gulf. Nowlin and McLelland [1967] and Nowlin [1972] established the presence of a region of relatively high dynamic topography centered near 23.5øN, 95øW. Elliott [1979, 1982] used several historical, quasisynoptic data sets to establish the presence of strong anticyclonic rings in the western Gulf. Merrell and Morrison [1981] showed that in April 1978 an intense cyclone, anticyclone pair was present in the western Gulf, with an eastern transport between their centers, away from the Mexico coast of about 30 sverdrups (sv = 106 m3/s). This is a surprisingly high transportmapproximately the same as that entering the Gulf of Mexico through the Yucatan Straits. In April 1980, Brooks and Leqeckis [1982] observed a similar cyclone, anticyclone pair in the western Gulf.Ichiye [1962] hypothesized that the anticyclonic structures in the western Gulf were current rings shed by the Loop Current. If so, we would expect the rings to migrate westward until they reach the western continental slope, where they should dissipate. Such a mechanism would transport enormous amounts of potential and kinetic energy into the western Gulf. Elliott [1979] documented that anticyclonic rings definitely move to the western boundary of the Gulf after separating from the Loop Current. Moreover, he demonstrated that the rings play an important role in the western Gulfs heat and salt budgets. On the other hand, on the basis of Hellerman's [1977, 1978] data, Stur#es and Blaha [1976] and Blaha and Sturqes [1978] contended that the anticyclonic feature centered near 23.5øN, 95øW was due principally to the balance of the curl of the wind stress with the planetary vorticity. However, Elliott [1979], using a finer spatial grid than that of Hellerman, showed that the wind stress curl was often positive rather than negative as required by Sturqes and Blaha [1976]. Finally, it is perhaps noteworthy that the numerical simulations of Hulburt and Thompson [1980] also produced westward migrating rings and cyclone, anticyclone pairs in the western Gulf in the absence of wind stress.

Death of three Loop Current rings

“…However, as the rings approach the western basin the character of drifter trajectories often changes significantly. Figures 1 and 2, taken from Kirwan et al (1984a) and Hamilton et al (1999), show that drifters cease steady anticyclonic rotation in the vicinity of 93W in favor of accelerated westward translation with occasional very large anticyclonic loops. This region of so-called 'stalls' and 'spurts' often coincides with abrupt shifts in elliptical-eddy model parameter fits.…”

“…Along the western and northwestern slope, the drifters described by Kirwan et al (1984a) and Hamilton et al (1999) often entrained briefly in anticyclonic structures. However, the number of revolutions was usually far less than similar trajectories from young rings in the eastern Gulf.…”

Eddy energy and shelf interactions in the Gulf of Mexico

“…The modeling study by Smith [1986] suggests that the western boundary imposes an asymmetry in the flow that results in northward movement of anticyclonic rings. 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].…”

The Motion of a Point Vortex near Large-Amplitude Topography in a Two-Layer Fluid

“…Indeed, motion of a LCE similar to that shown in Figs. 12 and 16 (i.e., parallel to topography) has been observed by Kirwan et al (1984) and to some extent in the numerical simulations of Sutyrin et al (2003).…”