Ocean Observing in the 4th Dimension--Using Autonomous Gliders for Operational Surveillance of the Gulf of Mexico

Abstract

Abstract The ocean circulation in the Gulf of Mexico is highly dynamic in fourdimensions (4D). The circulation varies throughout the interconnected shelf, slope, and deep ocean domains (3 dimensions) and varies greatly in time (the4th dimension). Obtaining timely, comprehensive, and accurate forecasting ofthis 4D system remains a challenge for operators on the outer continental shelf(OCS). Long-range autonomous underwater gliding vehicles (AUGVs) enablecollection of high spatial resolution sections through the ocean repeatedly, autonomously, and at a very low cost compared to conventional methods. Usingthese new tools, in-situ data can be continuously assessed to provide a moreintegrated description of the ocean state. We describe a series of gliderobservations obtained during extended (3.4 months) observational campaigns inthe eastern Gulf of Mexico. One survey period took place from April-August2011, covered a linear distance of 1400 nm (2700 km), and collected over 1000vertical profiles to 1000 m. The deployment spanned the Loop Current (LC)growth phase and detachment of Eddy Hadal. Several cross-sections were obtainedthrough the LC front, cyclonic frontal eddies, and in close proximity toworking platforms in lease areas of the NGOM slope. The in-situ fields of oceandensity help to delineate the subsurface boundaries of these dynamic features, and their geostrophic velocity structure can be inferred from lateral densityvariations. Sustained in-situ glider observations throughout the eastern Gulfof Mexico will improve the knowledge and forecasting vital for efficientday-to-day operational management decisions of the offshore petroleumindustry. Introduction At basin scales, the ocean circulation in the GOM is dominated by the LoopCurrent. The Loop Current extends northward from the Yucatan Channel into theGOM and connects to the Florida Straits. At times, it extends well north overthe upper continental slope before retroflecting to the southeast and exitingthe Gulf. Periodically the LC retroflection will pinch and form a closedrecirculation of water that may ultimately detach (or shed) as a Loop CurrentEddy (LCE) at intervals from 3–18 months (Forristall, et al., 1992, Leben etal., 2005, Sturges et al., 2005). The LC and LCE are deep structures extendingdown to nearly 1000 m, and both exhibit strong surface currents in excess of3.75 knots (2 m/s). The LC and its energetic eddies will, at times, exert astrong influence on the NGOM upper slope and shelf due to the interconnectednature of circulation. A variety of physical mechanisms can potentiallyinfluence LC intrusion and eddy separation, including instability of the LCwithin the GOM and upstream effects propagating though the Yucatan Channel. Loop Current Frontal Eddies (LCFEs) typically form along the western part ofthe LC and amplify in an unstable manner as they propagate downstream along theLC (Vukovich and Maul, 1985), exerting a strong influence on the LC positionand evolution. Strong ocean currents encountered in any part of the OuterContinental Shelf (OCS) and deeper waters can significantly threaten the safeand efficient completion of deepwater drilling operations, creating unexpecteddowntime and added risk. Deepwater operators on the OCS require more accurateprediction on the LC and LCE location, intensity, and migration at meso- andeven smaller scales (100 m - 10 km).