A Look at Currents in the Gulf of Mexico in 1999 - 2000

Abstract

Abstract Oceanographic conditions in the Gulf of Mexico have been extensively studied from the summer of 1999 through the fall of 2000. These studies included cradle to grave surveys of Juggernaut Eddy; surveys in the Caribbean by several consortia; oceanographic surveys in support of hurricane research in the Gulf; measurements of inflow/outflow through the Yucatan Strait; and regional nowcasts and hindcasts of circulation. Several interesting phenomena have been observed during this period, including energetic bottom currents in excess of 2 knots and bottom furrows on the continental slope; the intrusion of one of the strongest eddies in a decade into the north-central Gulf leases; and strong midwater column currents in SE Ewing Bank (~140 cm/s). This paper describes the observational data available and summarizes ongoing efforts to understand the oceanographic conditions during this period. Eventually, it is hoped that these efforts will lead to a better estimation of offshore structure design currents; an understanding of the dynamical causes of the strong mid-water-column currents; determining whether there is a link between the strong bottom currents at the base of the Sigsbee Escarpment and the Loop Current; and assessing the skill of the regional forecast/nowcast/hindcast oceanographic models. Introduction The importance of the Gulf to offshore mineral exploration and production is well known. But what may not be as wellappreciated is the fact that, because of the wealth of observational data and observational and modeling resources that have been brought to bear on the region in recent years, a better understanding and prediction of its circulation is more feasible today than ever before. In fact, the Gulf is an ideal test bed for exploring ideas on monitoring and predicting oceanographic conditions in a coastal/marginal sea. The Gulf's circulation is dominated by the Loop Current, an energetic current of warm subtropical water that enters the Gulf through the Yucatan Strait, extends northward, then loops around to the south and ultimately exits the Gulf through the Florida Strait. The position and strength of this Loop Current (LC) exhibits considerable variability. As first suggested by Ichiye1 and later observed by Cochrane2, the LC may extend so far north, often nearly to the Mississippi Delta, that the circulation closes off and a large warm-core (anticyclonic) eddy is shed. Like the LC, these Loop Current eddies (LCE) also have strong currents, but unlike the LC, they are not constrained to the eastern Gulf and typically drift westward at a few kilometers per day. Often, the westward drift can interfere with offshore operations. This happened in 1999 when British Borneo reported the shutdown of deepwater operations at Ewing Bank 965 due to the LC during August and September. This shutdown contributed to a $20 million increase in development costs for the Morpeth Field3. Currents in the LC and newly detached LCEs can be quite strong. Using an acoustic Doppler current profiler (ADCP), Cooper et al4 surveyed two LCEs in 1983 and recorded 223 cm/s (4.3 knot) surface currents.