Complete Report
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 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.
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 low-frequency 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 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) 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 along-isobath
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. 530 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 2001-0xx, U.S. Dept. of the Interior, Minerals Management Service,
Gulf of Mexico OCS Region, New Orleans, LA.
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 AREA(S): 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 $
