ACCESS NUMBER: 32916 (M04PC00019)

STUDY TITLE: Direct Observations of Ocean Currents over the Western Slope in the 

Gulf of Mexico 

REPORT TITLE: Full-Water Column Current Observations in the Western Gulf of 

Mexico 

CONTRACT NUMBER: 32916 (M04PC00019) 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Western 

FISCAL YEARS OF PROJECT FUNDING: 2008; 2009; 2010 

COMPLETION DATE OF REPORT: December 2010 

COSTS: FY 2008: $123,000; FY 2009: $321,000 

PROJECT MANAGER: J. Sheinbaum. 

AFFILIATION: Centro de Investigación Científica y de Educación Superior de 

Ensenada (CICESE) 

ADDRESS: Carretera Ensenada Tijuana No. 3918, 22860, Ensenada, B.C. México 

PRINCIPAL INVESTIGATORS*

: J. Sheinbaum, J. Ochoa, J. Candela and A. Badan 

KEY WORDS: Western Gulf of Mexico, Loop Current eddies, eddy-eddy interactions 

BACKGROUND: The Western Gulf of Mexico owes a large portion of its circulation 

variability to the translation of Loop Current Eddies (LCEs). These eddies are warmcore anti-cyclonic features produced by a detachment process of the Loop Current in 

the Eastern Gulf of Mexico. They have typical diameters of 200 km and maximum 

current speeds of 1 m/s at the surface. Their size and swirl motion generally diminishes 

with depth remaining coherent up to 800-1,000 m depth. The westward or 

southwestward translation of these eddies occurs at rates on the order of 2 to 5 km/day, 

is irregular, accompanied by cyclones of smaller or similar size, and it ends up with a 

collision against the Gulf’s western shelf. As a result of satellite-based measurements, 

the amount of information at the surface is quite ample, but there is a lack of deep-water 

observations of these eddies as they interact with the shelf. 

OBJECTIVES: To measure and document the breakup and ultimate dissipation of 

LCEs in their collision process against the western shelf of the Gulf of Mexico. 

Particular emphasis is given to the full-water column signature of the anti-cyclonic and 

cyclonic eddies that participate in this process. To determine average currents and 

temperatures and their principal scales of variability over the experiment area. 
ACCESS NUMBER: 32916 (M04PC00019)

DESCRIPTION: Fourteen months, from August 2004 to November 2005, of direct 

current measurements off the coast of Tamaulipas, Mexico, in the NW Gulf of Mexico 

capture the evolution of currents and temperatures as several eddies transit a five 

mooring array. Three of these moorings are located at 500, 2000 and 3500 m bottom 

depth at a latitude near 25o 22’ N, and the remaining two to the southwest of the one at 

2,000 m and along the 2,000 m isobath resembling a tilted T. The separation among 

close-by moorings is of about 80 km. The coverage of the water column is ample with 

Acoustic Doppler Current Profilers (ADCPs), all looking upward and single level current 

meters all registering temperature too. In the 500 m mooring a single 75kHz ADCP, 

from 7 m above the bottom, sampled the full column. In each of the other moorings two 

75 kHz ADCPs were deployed near 450 and 1200 m below the surface, with single level 

current meters recording near 700 and 1500 m depth, and a 300 kHz ADCP located 

about 8 m above the bottom to measure near-bottom currents. The deepest mooring 

was also instrumented with single level current meters close to 2,000, 2,500 and 3,000 

m. These observations were complemented with satellite altimetry. LCEs Titanic (T), 

Ulysses (U) and Vortex (V) impacted the mooring area at the beginning, middle and end 

of the observational record. Cyclones labeled C1 and C4 had very complicated 

histories since their generation, involving several intensification and merging events. 

They had a strong impact on the mooring observations. During September-October 

2004, LCE Titanic affected the two southern moorings while cyclone C1 was over the 

eastward moorings. Late November 2004 marks the arrival of eddy Ulysses which was 

blocked by cyclone C1 and together with cyclone C5 split the eddy in two, forming 

Ulysses-S and Ulysses-N eddies at the end of December 2004. Cyclonic circulation 

dominated the mooring area during January and February 2005. In March 2005 eddy 

Ulysses-S moves northward along the slope and Ulysses-N stretches to the west so 

that by April the two branches merge again and for the following 7 months (until mid 

September) this “re-merged” eddy Ulysses defines the circulation over the array. The 

last three months of observations are marked by the presence of cyclone C4, the result 

of several merging events including cyclone C1, and the arrival of LCE Vortex. 

SIGNIFICANT CONCLUSIONS: The high degree of consistency between the mooring 

observations and the altimetry data clearly indicate that flow variability in the study area 

is determined by the passage, formation, regeneration and dissipation of both cyclonic 

and anti-cyclonic eddies. With surface geostrophic currents derived from altimetry, we 

find some eddy events reaching all the way to the bottom and some others in which 

substantial veering of the current occurs between the surface and 800-1,000 m. Strong 

wind events do leave their signature (and may even be responsible for the formation of 

some eddies) but arrival of LCEs, eddy-eddy and eddy-topography interactions appear 

to be the key features that explain most of the observations in the sense that every 

notable event can be traced to the presence of an eddy, cyclonic or anti-cyclonic. The 

centroid of the observed LCEs always remained in waters deeper than 2,000 m. High 

and low sea level values are clearly correlated with warm and cold temperature 

anomalies as deep as 1,200-1,500 m. 

STUDY RESULTS: Besides the main conclusions other results are worth noticing. 

Weak but persistent mean currents near and above the bottom-boundary layer are 

directed toward the southwest in all three 2,000 m moorings and stand out above the 
ACCESS NUMBER: 32916 (M04PC00019)

 

variability. In contrast to other studies, these observations do not show the coherent 

intensification of current variability from 1,000 m towards the bottom, although other 

current features still suggest the presence of topographic Rossby waves (Donohue et 

al. 2008). The energetic motions are sub-inertial but near-inertial and supra-inertial 

fluctuations are ubiquitous in all moorings. Deep moorings show that there is a relative 

increase of high frequency contribution to the total kinetic energy in the 800-1,200 m 

depth interval. The sub-inertial currents dominate the observed variability representing 

more than 60% of the total variance at all depths except for a mid-water interval on the 

southernmost mooring between 1000 and 1,300 m depths where the ratio of sub-inertial 

to total variance gets as low as 47%. A coherent signal with a period of 20 days is also 

found within the mooring array at this depth-range. Using ADCP measurements of 

vertical motion of zooplankton, the mean diel cycle, from near the surface up to 1200 m 

depth is determined. Similar to the well known behavior from 500 m up to the surface, 

the deep diel cycle is phase locked to the sunlight cycle. The downward peak velocity 

occurs earlier closer to the surface; at 220 m it reaches a maximum of 150 m/h half an 

hour before sunrise, while at 900 m it peaks at only 30 m/h one hour and a half after 

sunrise. The cycle is nearly symmetric to solar noon, with upward peak velocities 

happening earlier at depth, a pattern expected if the vertical migration is triggered by 

critical light levels that reach greater depth closer to noon. Consequently with such 

differences in timing, at greater depth the nocturnal shallower stay is longer for the 

deeper migrating biota. The peak vertical migration velocities show relative maxima at 

250 and 1,100 m depth, a sign of increased biological activity. 

STUDY PRODUCT: Sheinbaum, J., J. Ochoa, J. Candela, and A. Badan. 2010. Fullwater column current observations in the western Gulf of Mexico: Final report. U.S. 

Dept. of the Interior, Bureau of Ocean Energy Management, Regulation and 

Enforcement, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study BOEMRE 

2010-044. pp 119 

*

 P.I.’s affiliation may be different than that listed for Project Managers.