ACCESS NUMBER: 30910 

STUDY TITLE: Deepwater Physical Oceanography Reanalysis and Synthesis of 

Historical Data 

REPORT TITLE: Deepwater Physical Oceanography Reanalysis and Synthesis of 

Historical Data: Synthesis Report 

CONTRACT NUMBER(S): 1435-01-98-CT-30910 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREAS: Western, Central, and Eastern Gulf of Mexico 

FISCAL YEAR(S) OF PROJECT FUNDING: 1998, 1999, 2000, and 2001 

COMPLETION DATE OF REPORT: June 2001 

COST(S): FY 1998 $261,576; FY 1999 $313,923; FY 2000 $135,747; FY 2001 

$47,301; CUMULATIVE PROJECT COST: $758,547 

PROJECT MANAGER: Worth D. Nowlin, Jr. 

AFFILIATION: Texas A&M University 

ADDRESS: Department of Oceanography, 3146 TAMU, College Station, TX 77843-

3146 

PRINCIPAL INVESTIGATORS*: Steve DiMarco, Matt Howard, Ann Jochens, Lakshmi 

Kantha, Worth Nowlin, and Robert Reid 

KEY WORDS: Physical oceanography, Gulf of Mexico, circulation, hydrography, 

currents, Loop Current, eddies, rings, topographic Rossby waves, tropical cyclones, 

extratropical cyclones, furrows 

BACKGROUND: The Minerals Management Service (MMS) Gulf of Mexico Outer 

Continental Shelf Region sponsored a workshop on 22-24 April 1997 in New Orleans, 

LA, to identify issues related to the expansion of oil and gas operations into the 

deepwater Gulf of Mexico and to provide recommendations on studies needed to fill 

gaps in knowledge. One recommendation was that a synthesis of historical physical 

oceanographic data be completed prior to implementation of any major observational 

study of the deepwater physical oceanography. Based on that recommendation, MMS 

funded the Deepwater Physical Oceanography Reanalysis and Synthesis of Historical 

Data Study in the Gulf of Mexico. 

OBJECTIVES: (1) To create an inventory of physical oceanographic data and compile 

it into a single database on a CD-ROM. (2) To conduct analyses and interpretations of 
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the physical oceanographic data to identify physical processes and phenomena. (3) To 

produce a climatology of the processes from available data and analyses and to 

prioritize the processes in terms of importance to improved understanding, simulation, 

and prediction of deepwater circulation. (4) To provide criteria and constraints useful in 

design of future field observations and numerical modeling efforts. 

DESCRIPTION: The study area is bounded by the northern Gulf shelf edge 

(approximately the 200-m isobath) and the 25°N latitude line and extends from sea 

surface to sea floor. The three project components were: assembly of physical 

oceanographic data from the deepwater Gulf of Mexico; identification and prioritization 

of physical processes and phenomena operating in the deepwater Gulf; and 

development of recommendations for experimental designs for future field studies. An 

inventory of data sets was compiled. Data included were current measurements from 

moored single-point current meters and acoustic Doppler current profilers; hydrographic 

observations from bottles, conductivity-temperature-depth sensors, mechanical 

bathythermographs, and expendable bathythermographs; and drifting buoy trajectories. 

As many of these data sets as possible were obtained from MMS, the National 

Oceanographic Data Center, other federal and state agencies, national laboratories, 

universities, Mexican institutions, and the private sector, particularly the oil and gas 

industry. Data were quality controlled. Data provided without restrictions on use were 

compiled into a database and put on a CD-ROM with a descriptive technical report. A 

high resolution numerical model of Gulf circulation provided output for 1993-1999. Ten 

processes were identified and prioritized as requiring additional study. They are 

discussed in Significant Results below. Specific measurement programs, systematic 

monitoring operations, and numerical model enhancements were recommended to 

improve our understanding of priority processes. These are discussed in Significant 

Conclusions below. 

SIGNIFICANT CONCLUSIONS: General conclusions are: (1) The general surface 

Gulf circulation is rather well described and understood in terms of forcing and 

response. (2) There appear to be adequate data to characterize surface features, such 

as the Loop Current and anticyclonic eddies, but not to understand the processes of 

eddy evolution and decay. Moreover, models and their inflow conditions are not yet 

adequate to hindcast or forecast them with the needed degree of skill. (3) There are 

inadequate data to fully characterize the deep subsurface processes, which include 

barotropic deep events in the form of topographic Rossby waves or deep eddies, or to 

determine the influence of surface processes, such as Loop Current Eddy separation 

events, on such deep processes. (4) Verification of mid-water jets remains 

problematical; a few events have been documented, but data quality remains in doubt. 

(5) There is no convincing evidence of topographically generated near-inertial motion in 

the Gulf. (6) Knowledge of the general deep Gulf circulation is based principally on 

model output and physical speculation. However, the sparse, existing data sets tend to 

confirm the speculation and model results. (7) Processes responsible for mega-furrows 

near the Sigsbee Escarpment in the north-central Gulf are not understood. 
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Three types of recommendations are made: specific measurement programs, 

systematic monitoring operations, and model improvements. Specific measurement 

programs are: a moored array near the Sigsbee Escarpment in the north-central Gulf 

designed to measure barotropic deep motions propagated along this boundary; a thin 

array of current moorings over the abyssal plain to detect and describe deep eddy pairs 

and surface-intensified eddies propagating across the Gulf; a Lagrangian float 

experiment in the deep basin designed to obtain statistics regarding the deep circulation 

and its variability and to give indications of deep flow through the Yucatan Channel; and 

an experiment to characterize currents and determine processes responsible for the 

mega-furrows in the north-central Gulf. Systematic monitoring operations are: to 

enhance the environmental observations obtained on drill vessels and production 

platforms of the petroleum industry; to monitor surface currents with pairs of high 

frequency radars; to use automated sea level and meteorological stations on both sides 

of the Yucatan Channel and between Cuba and Key West to monitor the transport of 

the inflow and outflow; and to obtain regular surface estimates of velocity and 

temperature and subsurface measurements of temperature and salinity by inclusion of 

the Gulf of Mexico in operational surface drifter, ship-of-opportunity XBT, and Argos 

profiling float deployment programs. To enhance the capabilities of numerical models 

for providing needed environmental information, periodic, careful comparisons should 

be made between results from the many available models and improvements should be 

made to Gulf of Mexico circulation models, including notably better boundary conditions 

for regional models and improved data assimilation capabilities. 

STUDY RESULTS: General hydrography and circulation of deepwater Gulf. The 

circulation within the Gulf of Mexico is driven principally by two sources of energy. The 

main source consists of the Yucatan Current and other circulation features that enter 

the Gulf from the Caribbean Sea through the Yucatan Channel. Effects are seen as a 

Loop Current, current rings that detach from the Loop Current and their subsequent 

distribution of energy throughout the Gulf, and effects on the deep circulation within the 

basin of ring separation from the Loop Current. Transport estimates for the Loop 

Current system are approximately 30 Sverdrups with seasonal fluctuations of about 

10%. The second major energy source is wind stress forcing. Effects are seen as lowfrequency regional circulation patterns forced by low-frequency regional wind patterns 

and as episodic currents forced by high frequency atmospheric events including tropical 

cyclones and extratropical cyclones. Although thermohaline forcing is known to be 

important over the Gulf shelves, no thermohaline forcing of consequence or significant 

water mass formation are known to occur in the deepwater Gulf. 

Large scale circulation and its variability. Large scale circulation was examined using 

geostrophic shear fields from hydrography, statistics from the near-surface drifter 

velocity field, the Gulf model circulation, and altimeter sea surface height fields. Current 

data, model output, and prior hydrographic studies indicate 800-1000 m is a reasonable 

depth estimate for separation of surface-intensified upper ocean currents from nearly 

barotropic deep currents within the Gulf. The dominant feature in the mean fields of the 

upper layer is the Loop Current. A closed anticyclonic feature is often present within the 
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Loop Current. In addition to the strong inflow on the west side of the Yucatan Channel, 

the drifter field shows an outflow just west of Cuba. The largest variability is in the 

region of the Loop Current and eddy separations in the northeastern Gulf. Variance 

ellipses generally are aligned parallel with the inflow and outflow limbs of the Loop 

Current. Both the sea surface height field and surface dynamic topography have a 

slight lowering of sea level around the margins relative to the center, suggesting slight 

anticyclonic circulation. The fields exhibit evidence of anticyclonic circulation in the west 

central Gulf, centered about 24°N 95°W, that is consistent with the forcing of the surface 

circulation by the annual cycle of wind stress curl. The fields also indicate cyclonic 

circulation in the Bay of Campeche. The dynamic topography indicates there is a 

cyclone beneath the Loop Current and that the circulation in the western Gulf is cyclonic 

at depth. Model results also show a cyclone beneath the Loop Current, most 

predominantly on the 1000-m and 2000-m surfaces. They also indicate cyclonic flow 

around the basin near the 2000-m to 3000-m isobaths. The strong effect of the 

bathymetry on the directionality of the variability is seen in the orientation of the model 

variance ellipses along isobaths at the margins. 

Identification of energetic current events. The inventory of identified processes and 

phenomena was conducted in three steps: identification of energetic currents, 

identification of possible processes/phenomena, and categorization of currents by class. 

Each time series of currents was examined for the occurrence of energetic currents. 

Energetic current events were considered to have occurred when the magnitudes of 

currents over the period of an event were considerably greater than the background 

currents for the region and/or for situations where the currents had characteristics 

known to be associated with particular classes of events. The possible processes and 

phenomena were identified from the literature and from the character of the energetic 

currents observed. Five broad classes of energetic current events were identified: Loop 

Current and surface-intensified eddies; deep barotropic and bottom-trapped motions; 

atmospheric storm generated motions; internal waves generated by topographic 

influence; and mid-water current jets. Current records were compared and, where 

appropriate, matched in time and space to known occurrences of the Loop Current, 

Loop Current Eddies, other anticyclonic or cyclonic rings, tropical storms, hurricanes, 

extratropical (winter) cyclones, frontal passages, and other energetic wind events. The 

energetic portions of each record were then inventoried according to type of 

phenomena or process. 

Climatology of processes and phenomena. The climatology of processes and 

phenomena consisted of general current statistics, current roses and persistence tables, 

current speed versus depth by region, record-length velocities and variances, eddy 

kinetic energy distributions, energy spectra, vertical empirical orthogonal functions, and 

a detailed examination of the statistics for each event type. The categories of physical 

processes and phenomena identified for prioritization and their priorities are: (1) deep 

barotropic & bottom-intensified motions, (2) general circulation—deep currents, (3) 

currents associated with furrows, (4) eddy induced currents, (5) Loop Current, (6) 

general circulation—surface currents, (7) subsurface, mid-water column motions, (8) 
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hurricane/tropical storm-induced motions, (9) other energetic wind event induced 

motions, and (10) topographically generated near-inertial motion. Prioritization was 

based on the need for additional data to improve understanding, simulation, and 

prediction of slope and subsurface circulation. Three major criteria were selected for 

use in determining the priority. These were: improved level of understanding, improved 

ability to simulate and predict, and ability to observe. 

Measurement system design criteria. The primary region of interest for gathering more 

data to characterize energetic phenomena and processes in the Gulf is the north-central 

slope and rise and northwest corner of the deepwater Gulf. The highest priority 

phenomena/processes are considered to be the currents in deep water. These include: 

(1) deep anticyclonic-cyclonic eddy pairs and topographic Rossby waves; (2) the deep 

general circulation; and (3) currents associated with furrows. Current and hydrographic 

data and model output were examined for the presence of the top three processes. 

Characteristics of these processes, in terms of spatial and temporal scales, were 

estimated. Deep eddies and topographic Rossby waves have time scales of 10 to 100 

days and spatial scales of hundreds of kilometers. The deep circulation has spatial 

scales ranging from eddy size (200-400 km) to basin scale, with time scales that are 

long but uncertain. Furrow fields have cross-isobath scales of ~35 km and alongisobath scales of over 100 km; time scales of currents associated with these fields are 

unknown. Measurement arrays were developed for each priority process using, among 

other factors, the characteristic scales to determine optimal spatial locations and 

temporal duration. 

STUDY PRODUCT(S): Nowlin, W. D., Jr., A. E. Jochens, S. F. DiMarco, R. O. Reid, 

and M. K. Howard. 2001. Deepwater Physical Oceanography Reanalysis and Synthesis 

of Historical Data: Synthesis Report. OCS Study MMS 2001-064, U.S. Dept. of the 

Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 

537 pp. 

DiMarco, S. F., M. K. Howard, and A. E. Jochens. 2001. Deepwater Data Report: 

Historical Physical Oceanographic Data from the Deepwater Gulf of Mexico. TAMU 

Oceanography Tech. Rpt. No. 00-D-01. Texas A&M University, College Station, TX. 

CD-ROM, Deepwater Physical Oceanographic Data Base for the Gulf of Mexico, 2001. 

Deepwater Physical Oceanography Reanalysis and Synthesis of Historical Data. OCS 

Study MMS 2001-064, U.S. Dept. of the Interior, Minerals Management Service, Gulf of 

Mexico OCS Region, New Orleans, LA. 

*P.I.'s affiliation may be different than that listed for Project Manager(s).