34 



Meteorological effects on shelf circulation can be large. The general southeasterly wind flow most of the year 

 can be interrupted by severe tropical storms during summer and early fall, and by cold frontal outbreaks 

 ("northers") during winter. Front generation has been observed to shift from the coast in fall to the shelf break 

 during winter. Since 1983, MMS has supported further development of the Gulf of Mexico circulation model 

 developed by Hurlburt and Thompson. While providing accurate representations of deep-water circulation, 

 including LCE's, the model is not optimal for representing shelf circulation. Several presentations on the wide 

 variety of models that have been applied to Gulf regions indicated that, although seldom noted in the literature, 

 there have been notable successes in simulating specific regions or events, such as hurricanes. 



The prospects for major physical measurement or modeling activities by any other agency in the northwestern 

 Gulf of Mexico during the target period are slim. No significant new initiatives were described, but a number 

 of operational efforts will take place, including modeling work in Galveston Bay by the National Ocean Service, 

 an increase in offshore meteorological stations supplying data to the National Weather Service, and continued 

 hydrographic survey work by Texas A&M University, the Mexican Navy, and the Mexican Institute of Electrical 

 Investigations. The MMS intends to share data and to coordinate program activities with other agencies working 

 in the area, mainly by means of rapid data archiving at the National Oceanographic Data Center. 



RECOMMENDED PROGRAM OBJECTIVES 



Program needs as outlined above can be met through six studies outlined below. 



Title: Northwestern Gulf of Mexico Circulation Modeling Study 



The overall objective of the circulation modeling effort in any MMS Region is to provide reliable ocean 

 current information that can be used in environmental assessment activities, principally the Oil Spill Risk 

 Assessment (OSRA) Model. The specific aim is the development of a numerical modeling approach that 

 embraces the various physical processes over continental shelves and the adjacent slope (as affected by major 

 circulation systems further offshore). These processes have diverse spatial and temporal scales, ranging 

 from major current systems, e.g. the Loop Current, and eddies (several hundred kilometers/several months), 

 or continental shelf waves (several hundred kilometers/days) to major river plumes (kilometers/hours). 

 Characteristic "excursion" values of wave-like motions, such as tides and inertial currents, lie intermediate 

 between these extremes. To simulate these features will require a broad suite of models and modeling 

 exercises, emphasizing aspects of the physics involved and performed at appropriate temporal and spatial 

 scales. 



The work of modeling the Gulf would take place within an overall program of concurrent measurements 

 along the TEXLA shelf (the area of primary offshore oil and gas activity), and would include the use of 

 previously collected data from the 1982-1986 field measurements program. Tides, realistic topography and 

 coastline, true (non-deterministic) thermodynamics, fluvial input, wind forcing at all relevant frequencies, and 

 meso-scale circulation features such as eddies and "squirts and jets" will be included in the model system, 

 as appropriate. In line with current practices, the model system will also produce discrete surface particle 

 trajectories for designated suites of "launch points" at various times throughout the multiyear simulations. 



Title: Texas-Louisiana Shelf Circulation and Transport Processes Study 



• To identify key dynamical processes governing circulation, transport and cross-shelf mixing on the 

 TEXLA shelf, 



• To upgrade existing empirical evidence on the same processes, fill in gaps in the evidence, synthesize 

 the evidence into a scheme of circulation, and quantify transports and mixing rates, 



• To develop conceptual models of small to large-scale processes and circulation features, from coastal 

 plumes and fronts to shelf-edge eddy exchange, and large-scale shelf circulation, all on event to seasonal 

 scales. (The conceptual models would support and assist in the development of a hierarchy of numerical 

 models, described elsewhere.) 



