On the Eddy Structure in the Northern Gulf of Mexico - Implications of VMADCP Observations from 2005 to 2007
Authors: Guan Xiaorui Amy , Brown Andrew , Brown Andy 
Published: May 2011

From 2005 to early 2008, Vessel Mounted 300khz and 75khz ADCP (VMADCP) data have been collected by Fugro GEOS as part of operational support for oil and gas activities in the Mississippi Canyon, Green Canyon and Walker Ridge areas of the Gulf of Mexico. Despite the fact that these ship transects were not designed for scientific studies of any specific eddy, and were not long enough to cut though the eddy cores, most of the ship transects did pass through the current maximum of the Loop Current Eddy (LCE). The analyses of the VMADCP data shed light on some eddy features and their evolution.

The maximum observed Loop Eddy current speed is 2.75 m/s. The 1.0 m/s LCE speed contour can penetrate as deep as 299 m with a mean of 111 m, while it can expand as wide as 210 km with a mean of 59 km. Three types of LCE structures have been identified: shallow and wide, deep and narrow, and those in between. According to the eXpendable BathyThermographs (XBTs) temperature data across Eddy Zorro in June and July 2007, the 15°C contours are located around 300 m. An Emperical Orthogonal Functions (EOF) analysis method was also applied to identify the characteristics and major current profile modes. The distribution of relative vorticity, an important parameter of the eddy strength, was also presented. The distribution compared well with previous studies where the velocity field was derived from hydrographic data instead of direct velocity observations.

Screening of the VMADCP data reveals that eddy instability seems to follow the occurrence of deep cyclonic flow below the anticyclonic eddy for Eddy Vortex. It agrees with previous numerical models, which relate the eddy instability to the deep cyclonic flow. Instability could cause the eddy to distort, split or get reabsorbed by the Loop Current. An undisturbed eddy, however, may move westward or southwestward freely. For example, the Eddy Zorro's (April to August 2007) westward propagation speed seems to follow the simple two-layer analytical model results by Cushman-Roisin et al. (1990). Revisiting their model may help us understand the westward propagation of undisturbed eddies.

1. INTRODUCTION

The Loop Current (LC) and Loop Current Eddy (LCE) are the dominant features in the Gulf of Mexico. The Loop Current in the eastern Gulf episodically sheds anticyclonic warm core rings or LCEs, which are among the largest and most energetic mesoscale structures. The eddy shedding events occur at intervals of about 3 to 17 months (Sturges and Leben, 2000) with diameters of 150 to 300 km. They generally propagate westward or southwestward at 2 to 5 km per day with lifetimes of months to years (Elliott, 1982; Cooper et al. 1990; Forristal et al., 1992). The LCEs either propagate and dissipate into the western Gulf or reattach back to the Loop Current.

Previous theoretical and numerical studies have attempted to explain the general westward propagation of LCEs. The variation of Coriolis parameters f, i.e. planetary ß effect over the eddy induces the westward movement. At the same time, the planetary ß dispersion also causes an asymmetric pressure distribution. This non-linearity around the eddy results in a southward self-advection tendency (Hurlburt and Thompson, 1980; Killworth, 1983; Cushman-Roisin et al., 1990). The observed long ring lifetimes have encouraged the study of LCE stability. As the LCEs migrate southwestward after their separation from their parent LC, external perturbation and internal instability may develop.