ACCESS NUMBER: 31029 

STUDY TITLE: Cross-Shelf Exchange Processes and the Deep-water Circulation of the Gulf of 

Mexico: Dynamical Effects of Submarine Canyons and Interactions of Loop Current Eddies with 

Topography 

REPORT TITLE: Cross-Shelf Exchange Processes and the Deep-water Circulation of the Gulf 

of Mexico: Dynamical Effects of Submarine Canyons and Interactions of Loop Current Eddies 

with Topography 

CONTRACT NUMBER: 14-35-01-99-CT-31029 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA(S): Western Gulf of Mexico; Central Gulf of Mexico; Eastern 

Gulf of Mexico 

FISCAL YEAR(S) OF PROJECT FUNDING: 2000; 2001; 2002; 2003 

COMPLETION DATE OF REPORT: March 2004 

COST(S): FY 2000: $200,304; FY 2001: $173,224; FY 2002: $180,765; FY 2003: $186,054; 

CUMULATIVE PROJECT COST: $740,347 

PROJECT MANAGER(S): L.M. Rothstein, I. Ginis 

AFFILIATION: Accurate Environmental Forecasting Inc. 

ADDRESS: 165 Dean Knauss Dr., Narragansett, Rhode Island 02882 

PRINCIPAL INVESTIGATOR(S): S.A. Frolov, G.D. Rowe, L.M. Rothstein, I. Ginis 

KEY WORDS: Gulf of Mexico, numerical modeling, ocean currents, Loop Current Eddies, Loop 

Current Frontal Eddies, current-topography interaction 

BACKGROUND: Deep-water exploration and development is expected to progress rapidly 

under stimuli of high flow rates, high volume, and technological advances. In response to this 

expectation the U.S. Department of the Interior sponsored a multi-part numerical modeling study 

designed to improve our understanding of physical mechanisms controlling the ocean circulation 

in deep-water regions of the Gulf of Mexico. This report contains results of a 4-year processoriented numerical study focused on investigating the interaction of deep ocean circulation 

features, e.g. Loop Current Eddies (LCEs), with continental shelf and slope. 

OBJECTIVES: (1) To adapt eddy resolving numerical models to the rise, slope, and adjacent 

shelf of the northern Gulf of Mexico; (2) To reproduce the three-dimensional currents associated 

with eddy-slope, eddy-eddy, and Loop Current-eddy interactions as well as other known 

physical phenomena in the area; (3) To determine the optimal sampling scheme to modify 

ongoing field observations in the area. 

DESCRIPTION: The overall methodology adopted for this study is based upon a numerical 

experiment strategy of building model configurations with gradually increasing realism: start with 

relatively less complex models, which we hypothesize to represent the essential physics of the 

phenomenon, then gradually transition into a more complete set of physics while still retaining 
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the simplified setting of the problem as much as possible. A hierarchy of numerical experiments 

was constructed. The hierarchy is based upon the level of approximation used to derive the 

differential equations describing the fluid motion in each experiment and upon the degree of 

realism involved in the experiment design. The hierarchy consisted of four classes of 

experiments: intermediate equations (IE) experiments, extended intermediate equations 

experiments (IE+), idealized primitive equations (PE) experiments, and realistic PE 

experiments. The intermediate equations are a class of approximations to the system of 

primitive equations, which is similar but generally more accurate than the traditional quasigeostrophic approximation and remains valid for moderate Rossby and Froude numbers. 

The principal goal of the IE experiments was to develop an initial understanding and intuition of 

the system's principal dynamics. Another important goal of IE experiments was the 

development of initialization techniques for more sophisticated primitive equations models. IE 

models are capable of isolating the balanced component of the flow providing a very useful tool 

for initialization. The IE+ experiments extend the results of the IE experiments into domain 

configurations with bottom topography that penetrates the lower layer density interface. In many 

cases the IE+ model allows a significant increase in the realism of numerical experiments, e.g. 

by including realistic coastal topography, while at the same time retaining the computational 

efficiency and the clarity of the dynamical analysis of the IE model. The idealized PE 

experiments further increase the realism of the numerical simulations by including a realistic 

continuous stratification and a more complete set physics. The PE model used in this class of 

experiments was the Princeton Ocean Model (POM) configured with high (better than 5 km) 

horizontal and vertical (50 levels) resolutions. The final step in the hierarchy of numerical 

experiments was running the fully realistic PE model simulations. These simulations were 

designed primarily to verify the results of previous idealized experiments by enabling a direct 

comparison with observations. 

SIGNIFICANT CONCLUSIONS: It was demonstrated that deep eddies can form during LCE 

interaction with the western shelf, particularly in the region around 25W. The deep eddies are 

characterized by relatively strong currents (up to 20 cm/s) and large size (200-300 km in 

diameter) and have significant impact on the deep local environment. Deep eddies were also 

shown to significantly affect the evolution of LCEs interacting with the western shelf. It was also 

demonstrated that LCE interacting with a thermocline-penetrating shelf can form small to 

intermediate-scale cyclones as a result of water advection from the shelf. The cyclones 

generated by this process can reach considerable strength depending upon the exact shape of 

the bathymetry and the motion of the LCE. The intermediate layer, i.e., the density layer 

defined with moderate to weak stratification extending from approximately 400 to 1,200 meters, 

was found to play a central role in the process of LCE-topography interaction. 

STUDY RESULTS: LCE interactions with topography were studied in four regions: the central 

GOM, the northern GOM, the north-east corner of the GOM, and the western GOM. Several 

new dynamical mechanisms controlling the LCE-topography interactions were identified, and 

other more familiar mechanisms were clarified. Potential scenarios describing the evolution of 

an isolated LCE interacting with bottom topography in various regions of the GOM were 

identified. A real event of an LCE interacting with coastal topography was simulated in a 

realistic setting and the results were compared directly with observations. The previously 

developed understanding of dynamical processes involved in LCE-topography interactions 

obtained from more process-oriented experiments enabled a much clearer dynamical 

interpretation of the observed events. 

The investigation of physical processes within, and in the vicinity of, the DeSoto Canyon 

focused on the conceptual models of DeSoto Canyon circulation based on observational data 
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collected during the DeSoto Canyon Eddy Intrusion Study (1997-1999). In particular, three 

conceptual models were analyzed: 1) the direct interaction of an anticyclone with the canyon; 2) 

remotely generated eastward jets west of the canyon; and 3) the interaction with Loop Current 

frontal eddies. As the result of the analysis conclusions about the dynamical feasibility of the 

conceptual models were made and some of the physical mechanisms involved were identified. 

For each of the conceptual models the corresponding observed events were simulated, and the 

results compared with observations. 

STUDY PRODUCT(S): Frolov, S.A., G.D. Rowe, L.M. Rothstein, and I. Ginis, 2003. Cross-shelf 

exchange processes and the deep-water circulation of the Gulf of Mexico: Dynamical effects of 

submarine canyons and the interactions of Loop Current eddies with topography; final report. 

U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New 

Orleans, LA. OCS Study MMS 2004-017. Contract No. 14-35-01-99-CT-31029. 164 pp.