ACCESS NUMBER: 31026 

STUDY TITLE: Study of Subsurface, High-Speed Current Jets in the Deepwater 

Region of the Gulf of Mexico 

REPORT TITLE: Subsurface, High-Speed Current Jets in the Deepwater Region of the 

Gulf of Mexico: Final Report

CONTRACT NUMBER(S): 1435-01-99-CT-31026 

SPONSORING OCS REGION: Gulf of Mexico 

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

FISCAL YEAR(S) OF PROJECT FUNDING: 2000; 2001; 2002; 2003; 2004 

COMPLETION DATE OF REPORT: July 2004 

COST(S): FY 2000: $6,627.67; FY 2001: $42,257.45; FY 2002: $62,968.44; FY 2003: 

$56,555.46; FY 2004: $48,755.04. CUMULATIVE PROJECT COST: $217,164.00 

PROJECT MANAGER(S): Dr. Steven F. DiMarco, Dr. Worth D. Nowlin, Jr. 

AFFILIATION: Texas A&M University 

ADDRESS: 3146 TAMU, College Station, Texas 77843-3146 

PRINCIPAL INVESTIGATORS: Dr. Steven F. DiMarco, Dr. Worth D. Nowlin, Jr., Dr. 

Matthew K. Howard, and Prof. Robert O. Reid 

KEY WORDS: Gulf of Mexico, physical oceanography, ocean current, jets, ADCP, 

numerical models, extreme events 

BACKGROUND: There are four major classes of energetic currents in the Gulf of 

Mexico that are of primary concern to offshore petroleum operators. These are: 1) 

currents resulting from energetic, episodic atmospheric events, 2) currents associated 

with the Loop Current and related eddies, 3) vertically coherent currents below 1000 m 

(e.g. those believed to be associated with topographic Rossby waves), and 4) highspeed sub-surface intensified currents. 

Based on observations, high-speed subsurface intensified currents, also known as jets, 

typically have temporal durations on the order of a few hours to one day, have 

subsurface speed maxima that can exceed 4 knots (200 cm/s), have peak speeds that 

occur between 150-350 m below the surface, and have little or no surface expressions. 

Offshore operators design drilling and production systems to account for forces exerted 

by currents at all depths; therefore frequency, persistence, and speed characteristics of 

jets are important design criteria. 
ACCESS NUMBER: 31026 

OBJECTIVES: The objectives of the study are to characterize subsurface jets that 

occur in the Gulf of Mexico and speculate on the physical mechanisms responsible for 

their generation. 

DESCRIPTION: There are several activities to achieve these goals which include: 1) 

identify and acquire data sets believed to contain subsurface jets, 2) characterize each 

identified jet and jet environment (through collection of ancillary data such as satellite, 

meteorological, and CTD data), 3) examine relationships between jets occurrence and 

potential forcing mechanisms, 4) identify and analyze jets found in numerical model 

output, 5) attempt to identify physical and other mechanisms responsible for jet 

generation. Additional objectives are, 6) to provide guidance to MMS as to how data 

collection should be improved on oil and gas industry platforms, 7) apply analysis 

schemes to outputs of model runs both with and without data assimilation, 8) consider 

internal/inertial and filamentary causal mechanisms. 

SIGNIFICANT CONCLUSIONS: Thirteen candidate cases of jets were identified in our 

observational database. The candidate jets occur over a twelve-year period (1990-

2001); this period is coincident with the period in which acoustic Doppler current profiler 

(ADCP) records have become available. Only profiling current meters possess the 

necessary vertical resolution to detect and measure a jet event. One of the jets consists 

of an inertial wave packet caused by Hurricane Georges that propagated downward to 

at least 500 m depth. Several of the observed jets have unusually large (> 10 cm·s-1) 

vertical velocities, suggesting possible measurement error. 

We have not ruled out the possibility that the measurement limitations of acoustical 

instruments may be responsible for certain biases in the data record. In particular, we 

have simulated a non-homogeneous flow field passing a realistic alignment of off-axis 

acoustic beams (typical of standard acoustic current instrumentation). We find that 

current inhomogeneities, both vertical and horizontal, can significantly affect the 

estimates of both horizontal and vertical current velocities. Further, for beam angles of 

20°, oppositely directed vertical velocity components in each beam path can correspond 

to the appearance of horizontal velocities 2.74 times the vertical velocity magnitude. 

Such inhomogeneities could be caused by structural interference, internal waves, or 

motions attributed to ship/rig thrusters and could masquerade as energetic features in 

data records. 

Some candidate mechanisms seem more likely than others. For example, the more 

likely mechanisms include: (a) motions derived from the Loop Current and associated 

eddies in the form of filaments and meanders, (b) motions due to eddy/eddy and/or 

slope-shelf/eddy interaction, (c) manifestations of internal waves with unusually large 

speeds, e.g. internal soliton, (d) the combined effects of transient surface winds and 

deep flow over an undulating sea bed, (e) reversed geostrophic flow, (f) inertial wave 

packets, and (g) frontal instabilities and the development of small-scale (15-25 km) 

preferentially cold-core features along frontal boundaries. Unlikely candidates (and 
ACCESS NUMBER: 31026 

reasons) include association with: coastal buoyancy fronts (too far from shore and river 

plume), upwelling (slower peak speeds and usually surface-trapped motions), and 

undercurrents (no evidence these exist in the Gulf of Mexico). 

STUDY RESULTS: Although most of the observations of jets have serious 

instrumentation and data quality issues associated with the current measurements, 

there is sufficient evidence to assert that unusually high-speed and short-lived subsurface current events exist. 

Model outputs were analyzed for the presence of subsurface jets in several numerical 

circulation models of the Gulf of Mexico including two versions of the Princeton Ocean 

Model. Model output were searched for jet events in the vicinity of the north-central Gulf 

of Mexico observational jet candidates and at comparable water depths and placement 

over the slope. However, model jets could be associated with motions of filamentary 

structures extending from the Loop Current or eddies associated with the Loop Current. 

We believe the filaments are caused by the interaction of the Loop Current with bottom 

topography and/or eddy-eddy interaction. We note the apparent disparity of time scales 

between jets observed in the real world and the jets found in the model output. The 

observed jets usually have time scales of the order of several hours to 1 day; we see no 

evidence in the model output of jets lasting less than one day. Model jets seem to occur 

higher in the water (not deeper than 200 m) and last 1-3 days. 

Recommendations are made regarding general oceanographic data collection as well 

as specific data collection strategies for capturing subsurface jets. The general 

recommendations include: the use of downward-looking long range (38 kHz) ADCPs on 

rigs, the routine collection of temperature and salinity profiles and meteorological data at 

such sites, the collection of near-bottom current data in deep water, the use of simple 

and portable acquisition system, regular backup and archival procedures for data, and 

the telemetry of data to a central onshore data facility. Recommendations specific to 

the detection and study of jets include: establish long term moored measurements in 

regions thought to contain jets, obtain temperature and salinity profiles during events, 

carry out targeted ship surveys during instability events (i.e., cyclone formation from 

eddy-topography interaction), and use standardized collection procedures. We 

recommend targeted field studies using moored instrumentation as well as ship surveys 

to capture jet events and provide evidence of their generation mechanisms and 

estimates of their spatial and temporal scales. Without such information, the real 

causes will remain questionable.

STUDY PRODUCT(S): DiMarco, S. F., M. K. Howard, W. D. Nowlin, Jr., and R. O. 

Reid, 2004. Subsurface, high-speed current jets in the deepwater region of the Gulf of 

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

Region, New Orleans, LA. OCS Study MMS 2004-022. Contract No. 1435-01-99-CT-31026. 

98 pp.