Distribution of salinity, dissolved oxygen, nitrate, phosphate, and silicate are presented for the western Gulf of Mexico in 1978. Plots of chemical concentration versus potential density are used to identify the presence of gulf water, tropical Atlantic central water, Antarctic intermediate water, Caribbean midwater, and upper North Atlantic deep water within the upper 1600 m of the western Gulf of Mexico. Property distributions are closely related to the current regime described by Merrell and Morrison (1981). Also, we present values for tritium, carbon 14, radium 226, and radon 222 that were taken at a deep station located at 23°02′N and 92°28′W.

In April 1978 the circulation pattern in the western Gulf of Mexico was dominated by a cyclonic circulation feature centered near 25°20'N, 95°20'W and an anticyclonic circulation feature centered near 23°30'N, 95°50'W. The north‐south scales of both gyres are approximately 225 km, while the east‐west scales are greater than 450 km. An eastward geostrophic transport of 29.7 × 106 m3/s is found between the centers of the cyclone and the anticyclone. We believe that both the anticyclone and the cyclone migrated to the western Gulf from the Loop Current region of the eastern Gulf. The anticyclone is probably supplemented by a wind‐driven circulation as described by Blaha and Sturges, and an eastward extension of the wind‐driven current which flows to the south along the lower Texas shelf may intensify the cyclone and/or anticyclone. Also, this extension of the Texas shelf current may tend to confine the anticyclone to the south and the cyclone to the north of the current's location.

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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.

A model developed primarily for the analysis of drifter data is discussed. The model is based on a parametric representation of the drifter velocity in terms of Monge potentials, two of which are constants following the motion of the fluid• In the application presented here, one of the potentials is taken as the frequency of rotation about a ring by a parcel. A more restricted interpretation of the velocity gradient invariant for horizontal flow is also discussed. The other potential is taken as the streamline of a parcel that is locally approximated as a conic section. For the restricted interpretation, solutions to the model equations are critically dependent on the relative sizes of the squares of the vertical vorticity and the total deformation rate. This model differs from drifter cluster models in that each drifter provides independent estimates of the vorticity and deformation rates. Application is made to path data from three drifters that were seeded in a warm core ring in the Gulf of Mexico in November 1980. The model provided estimates of the ring translation and swirl velocities along with the ring geometry. The analysis showed that the ring was persistently elliptical with the major axis aligned in the east-west direction. A satellite infrared photo on January 21, 1981, confirmed this orientation. 1. INTRODUCTION There have been varied uses of surface drifter data in analyzing upper ocean motions. The early studies of Molinari and Kirwan 1-1975] and Okubo and Ebbesmeyer 1-1976] emphasized the use of relative displacements of drifters in clusters to deduce diffusive and small-scale kinematic properties. More recently, Kirwan et al. [1978], McNally et al. [1983], Molinari [1983], and Richardson [1983] have used drifter paths to infer qualitative characteristics of the large.-scale ocean circulation. Using Lagrangian techniques, however, quantitative dynamical assessments of ocean currents are generally lacking. Two reasons for this may be the slippage effect caused by windage and the lack of a quantitative parametric model for interpreting the Lagrangian data. The latter problem is addressed here. A candidate parametric model that relates the drifter velocity to invariant flow properties is proposed. This relation connects the model to dynamical processes. The approach taken here differs from previous parametric models in that data from a single drifter supplies estimates of the vorticity and other kinematic properties. One theoretical basis for the model presented here is an Eulerian variational principle first proposed by Sasaki [1955, 1957] for inviscid compressible rotational flows. Stephens [1965] utilized this principle for studying numerical weather prediction. More recently, Katz and Lynden-Bell [1982] and Henyey [1983] have discussed applications of this principle to some oceanographic problems. Kirwan [1984] has summarized these results as well as discussed a simple application to two-dimensional flow parallel to surfaces of constant geopotential. As this application is used in interpreting s...