The Evolution of Nelson Eddy, 1989
Authors: Schaudt K.J. , Forristali G.Z. , Biggs D.C. , Huh O.K. , Bole J.B. 
Published: October 2008

Nelson Eddy was the first major Loop Current Eddy to enter the deepwater lease areas offshore Louisiana during active drilling operations. This paper summarizes the history of this eddy, and the hydrographic and current measurements collected during its intrusion onto the shelf.

INTRODUCTION

Since open water drilling began in the late 1930's, the offshore industry has adapted to a changing environment. With the onset of drilling in water depths greater than 300 meters, high currents caused by the Gulf of Mexico Loop Current and its associated eddies have become an important consideration in both exploration and the eventual production activities.

The Loop Current and its associated eddies dominate the circulation in the central and eastern Gulf of Mexico (Figure 1). The waters forming this system originate in the equatorial Atlantic, eastern Atlantic and Caribbean, enter the Gulf through the Yucatan Straits, and eventually exit through the Florida Straits. This intrusion of warm tropical waters forms the Loop Current System.

The Loop Current's characteristics vary considerably with space and time. On the largest scales, the northward penetration of the Loop Current varies by hundreds of kilometers with a quasi-annual period. On a slightly smaller scale, as first suggested by Ichiye (1962), and later observed by Cochrane (1972), when the Loop Current is sufficiently far north (generally north of 26°N) a large anticyclonic eddy about 350 km across may be shed. In the following months these large eddies migrate westward at a few kilometers per day. On a still smaller scale, shingles, filaments and small eddies form along the boundary of the Loop Current or anticyclonic eddy fronts. Vukovich et al (1979) showed that the effects of these small meanders affect circulation to the 700 m depths and that these features propagate along the Loop Current boundary at 28 km/day.

Until recently, there were few direct measurements of the currents of the Loop Current and its associated eddies. Molinairi and Morrison (1988) estimated the current speeds associated with the Loop Current proper to be greater than 150 cm/sec (2.9 knots) from geostrophic calculations. Geomagnetic electrokinetograph (GEK) measurements indicated 204 cm/sec (4 knots) current speeds in the Loop Current proper (Nowlin, 1972).

Until the beginning of the Eddy Joint Industry project (EJIP) surveys, few measurements of the currents in large anticyclonic eddies were available. Nowlin and Hubertz (1972) estimated 88 cm/sec (1.7 knots) using indirect methods. Hubertzetal (1972) measured speeds greater than 100 cm/sec (1.9 knots) using GEKs. Cooper et al (1990) published measurements taken during the first EJIP surveys in 1983. During these surveys, an Acoustic Doppler Current Profiler measured the strongest currents near the southern edge of a recently detached eddy. The ADCP recorded surface current of 223 cm/sec (4.3 knots).

In recent years, oil industry and Minerals Management Service projects have conducted similar surveys throughout the U.S. portion of the Gulf of Mexico. Lewis et al (1989) describes the evolution of Fast Eddy in 1985 from drifting buoy data. Forristall et al (1991) presents the current measurements collected in Fast Eddy.