ACCESS NUMBER: 30660-19952 

STUDY TITLE: A Modeling Study of Gulf of Mexico Deep Water Circulation, 

Ventilation, and Transport 

REPORT TITLE: Lagrangian Study of Circulation, Transport, and Vertical Exchange in 

the Gulf of Mexico 

CONTRACTING NUMBER(S): 14-35-0001-30660-19952 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA(S): Northern Gulf of Mexico 

FISCAL YEARS(S) OF PROJECT FUNDING: 1997, 1998, 1999, 2000, 2001, 2002. 

COMPLETION DATE OF REPORT: November 2002 

COST(S): FY1997: $10,504; FY 1998: $47,022; FY 1999: $36,649; FY 2000: $98,761; 

CUMULATIVE PROJECT COST: $135,410 

PROJECT MANAGER(S): S.Welsh, M.Inoue 

AFFILIATION: Coastal Studies Institute, School of the Coast and Environment 

ADDRESS: Louisiana State University, Baton Rouge, LA, 70803 

PRINCIPAL INVESTIGATOR(S)*: S.Welsh, M.Inoue 

KEY WORDS: Gulf of Mexico; Loop Current rings; deep circulation; ventilation; vertical 

exchange 

BACKGROUND: Increased activity associated with offshore oil and gas exploration 

and production in the outer continental shelf and slope regions of the northern Gulf of 

Mexico calls for a better understanding of the ocean circulation in these regions and the 

potential environmental effects of those activities. Moving into deep water places the oil 

and gas operations closer to strong surface-intensified currents associated with the 

Loop Current and Loop Current eddies. Sub-surface current measurements over the 

northern continental slope provide evidence of vigorous currents and Topographic 

Rossby Waves. Along the continental slope there is an abundance of benthic 

communities associated with numerous hydro-carbon seeps. Understanding how deep 

energetic currents and eddies interact with the bottom topography is important to ensure 

that deepwater activities are safe and environmentally sound. The deep water of the 

Gulf of Mexico below the deepest sill depth is completely isolated from outside, but it is 

well-ventilated. The renewal of deep water should take place only via vertical exchange 

of water with the water above the sill depth. 
ACCESS NUMBER: 30660-19952 

OBJECTIVES: (1) To characterize the deep circulation in the Gulf of Mexico which 

appears to be dominated by vertically homogeneous eddies associated with the surface 

anticyclones; (2) To investigate transport, mixing, and ventilation processes in the deep 

water using Lagrangian techniques; (3) To estimate the residence time of deep water in 

the Gulf of Mexico; (4) To examine vertical exchanges over the outer shelf and slope in 

the northern and western Gulf of Mexico where Loop Current rings interact with the 

bathymetry and eventually decay. 

DESCRIPTION: The study area featured the entire Gulf of Mexico, although the model 

domain extends into the western Caribbean and the western North Atlantic Ocean to 

facilitate forcing inflow and outflow. The project was conducted in three parts: (1) 

numerical model development and verification; (2) tracer particle experiments at various 

depth levels; and (3) examination of the model mean fields and energetics. The 

Modular Ocean Model version 1.1, a three-dimensional, primitive-equation, numerical 

model was chosen for this study. The model grid has a horizontal resolution of 0.1o by 

0.1o and the boundaries of the model domain are 98.0°W to 72.0°W and 15.0°N to 

31.0°N. There are 20 levels in the vertical with a minimum depth of 12.5 m and a 

maximum depth of 3850 m. NODC global mean hydrographic data is used to initialize 

the model temperature and salinity. The model is forced at the surface with seasonal 

averages of temperature, salinity, and wind stress. A target value of 28 Sv is chosen to 

represent the annual-mean volume transport through the southern Straits of Florida. 

The Lagrangian technique of seeding and tracking tracer particles is used to examine 

detailed transport and mixing processes. Several different experiments were designed 

that feature different starting positions of the tracer particles depending on the region or 

process of interest. The positions of the tracers were analyzed to identify characteristic 

patterns in the particle paths and correlate the particle paths with physical processes or 

circulation features. The model temperature and horizontal velocity were saved at 

intervals of 1, 3, or 10 days for 15 years at selected levels and all prognostic variables 

are saved at every grid point every 30 days. Animations were created of the particle 

positions superimposed on the model flow fields for model verification and analysis. 

SIGNIFICANT CONCLUSIONS: The vertical exchange of particles in the deep water 

below 1000 m occurs over the steep slopes in the eastern basin of the Gulf of Mexico 

where the Loop Current is constrained by the bathymetry. Vertical motion of particles is 

observed as the Loop Current moves back and forth off the Campeche Bank, producing 

a net upward motion as particles are swept back toward the slope. Pronounced vertical 

motion of tracer particles also occurs over the steep slopes of the central and western 

Gulf as a result of deep eddies interacting with the bathymetry. In the upper layer 

above 800 m, the most active region of vertical migration of particles is in the northwest 

portion of the Gulf where Loop Current rings spin-down. The gradual upward motion of 

particles in the relict rings is likely due to the shoaling of the isotherms as the rings 

decay. There is limited communication between the eastern and central basins across 

approximately 88oW in the deep water. During a 15-year simulation in which the waters 
ACCESS NUMBER: 30660-19952 

below 2200 m were seeded with nearly 20,000 particles, fewer than 10% of particles 

were transported across 88oW in either direction. The deep motions appear to ‘feel’ the 

bottom, and the advective properties of deep eddies may change as they squeeze 

through the deep channel. 

The descent of particles is observed near the entrance to the Straits of Florida along the 

northern side of Cuba. From the 12-year mean temperature and salinity fields, it 

appears that relatively cold water flowing over the sill of the Yucatan Channel, hugs the 

eastern side of the channel and then flows northward along the eastern side of the 

basin. The cold water follow the topography back to the west at progressively greater 

depths. Relatively warm water appears to upwell over the northern Campeche Bank at 

the same depths. The Loop Current flows over the sill at a depth of 1400 m into waters 

depths greater than 3000m. The upwelling may be caused by the Loop Current 

entraining water just below sill depth on the northern side of the sill as the Loop Current 

moves over deeper water. A cyclonic current that follows the 3000-m isobath is present 

in the 12-year mean velocity fields . A conceptual model for ventilation of the deep 

water has developed from this study that features downwelling in the far eastern basin, 

a deep mean cyclonic current, and upwelling over the Campeche Bank. 

STUDY RESULTS: The numerical ocean model is able to realistically reproduce 

observed upper-layer circulation features, with special attention given to the 3-D 

structure of the Loop Current and Loop Current rings. Two important aspects of the 

model forcing are the temporal and spatial structure of the inflow through the Yucatan 

Channel. These aspects were handled with care in order to realistically simulate the 

vertical structure of the Yucatan/Loop Current system, as well as the variability in the 

shedding of Loop Current Rings. The implementation of a high resolution grid was also 

essential in the simulation of vertical motion, which is primarily a product of currenttopographic interaction. Lagrangian analysis of the tracer particles has indicated that 

the greatest vertical exchange below the surface layer is occurring in the eastern basin 

due to the bathymetric constraints on the Loop Current. Rapid vertical motion of 

particles is observed where deep currents impinge directly on steep topography, such 

as in the northern Gulf. Slow ascent of particles is observed in the northwestern Gulf as 

Loop Current rings decay. The model eddy kinetic energy fields provide evidence of 

Topographic Rossby Wave propagation along the slope in the northern Gulf of Mexico. 

STUDY PRODUCTS: Welsh, S. and M. Inoue, 2002. Langrangian Study of Circulation, 

Transport, and Vertical Exchange in the Gulf of Mexico. OCS Study MMS 2002-064 

Contract 14-35-001-30660-19952. U.S. Dept. of the Interior, Minerals Management 

Service, New Orleans, LA, 61pp. 
ACCESS NUMBER: 30660-19952 

NUMERICAL MODEL DOMAIN FOR LAGRANGIAN STUDY OF CIRCULATION, TRANSPORT, AND 

VERTICAL EXCHANGE IN THE GULF OF MEXICO.