ACCESS NUMBER: 00112

STUDY TITLE: A Lagrangian Approach to Study the Gulf of Mexico's Deep Circulation

REPORT TITLE: Deep Circulation in the Gulf of Mexico: A Lagrangian Study

CONTRACT NUMBER: M10PC00112

SPONSORING OCS REGION: Gulf of Mexico

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

FISCAL YEAR(S) OF PROJECT FUNDING: 2011–2016

COMPLETION DATE OF REPORT: March 2017

COSTS: FY2011: $1,667,136; FY2012: $1,585,148; FY2013: $396,343; FY2014: 

$700,373.70; FY2015: $593,930; FY2016: $367,443.

PROJECT MANAGER(S): Peter Hamilton

AFFILIATION: Leidos, Inc.

ADDRESS: 615 Oberlin Road, Suite 100, Raleigh, North Carolina 27605

PRINCIPAL INVESTIGATOR(S)

*

: Amy Bower, Heather Furey, Peter Hamilton, Robert 

Leben, Paula Pérez-Brunius.

KEY WORDS: Gulf of Mexico; Escarpments; Mississippi Fan; Bay of Campeche; 

deepwater; circulation; floats, Lagrangian; RAFOS; APEX; Loop Current; eddies; 

observations; trajectories; currents; temperature; salinity; pressure; dynamics; 

instabilities; topographic Rossby waves; boundary currents; gyre; satellite 

oceanography; sea surface height; eddy paths; eddy statistics.

BACKGROUND: The deep circulation of the Gulf of Mexico is forced by instabilities of 

the Loop Current (LC), a branch of the Gulf Stream system, which enters through the 

Yucatan Channel and exits through the Straits of Florida. The northward extension of 

the LC towards the Mississippi delta becomes unstable and separates a large warm 

anticyclone, known as an LC eddy (diameter ~ 200–400 km), that translates across the 

western basin until it interacts with the Mexican slope and the mesoscale upper-layer 

eddy field. Initial LC eddy detachments, which may occur multiple times before fully 

separating and moving off to the west, are caused by baroclinic instabilities that transfer 

large amounts of kinetic energy to the lower layer (below ~ 1000 m), along the path of 

the extended LC front. The resulting deep eddies were thought to transfer deep energy 

towards the west through eddies and topographic Rossby waves (TRWs). These 

northwestward propagating TRWs impinge on the Sigsbee Escarpment at the base of 

the northern slope, and drive a deep westward-flowing boundary current. Before this 

study, evidence was very limited, from observations and models, that an anticlockwise 

escarpment boundary current might continue around the edge of the deep basin. In 

addition, the passage of LC eddies, and their interactions with slope topography and 
ACCESS NUMBER: 00112

smaller-scale upper-layer cyclones and anticyclones (diameters ~ 50–200 km), in the 

western basin was thought to influence the deep circulation, though there were few 

observations to validate circulations generated by numerical models. Dispersion of 

water mass properties and pollutants in the lower layer of the deep Gulf of Mexico 

basins will be a strong function of circulation processes found there.

OBJECTIVES: (1) To increase knowledge and understanding of the deep circulations in 

the Gulf; (2) To use a Lagrangian approach using deep Ranging and Fixing of Sound 

(RAFOS) and seven autonomous profiling (APEX) floats that could produce 

observations through the deep Gulf, without regard to national boundaries;(3) To 

develop Lagrangian statistics on dispersion and deep currents; and (4) To investigate 

the connections between upper- and lower-layer circulation processes. 

DESCRIPTION: RAFOS floats were tracked using sound sources deployed on 

moorings. Four sound sources were deployed in deep water, including one in the 

Mexican Exclusive Economic Zone to provide redundant coverage of the deep basins.

The original experimental design was to deploy up to 120 RAFOS floats, over an 18-

month period, in all parts of the Gulf, and track them over a three-year interval. 

Individual floats had designed mission lengths ranging from six months to two years. 

However, because of a number of manufacturing problems that produced float failures,

the deployments of 186 floats were spread out over two years; this resulted in 158 good 

trajectories and extended the observational part of the study to four years. The 

maximum mission length was 630 days, and 194 float-years of location positions (three 

per day per float) were eventually obtained. Nominal float depths were 1500 and 2500 

m. The latter were 22% of the total and deployed only east of 90°W. The second 

component of the study was the deployment of APEX floats. These were similar to 

those used by the US Argo program, with additions of hydrophones, for location tracking 

when submerged at 1500 m, and bio-optical sensors. Generally, conductivity, 

temperature, pressure (CTD) and bio-optical profiles were obtained every 14 days over 

the duration of the study (a total of 597). The satellite observational component of the 

study was the collection of altimeter-derived sea-surface height, along with highresolution sea-surface temperatureand ocean color over the four-year duration of the 

observations.

SIGNIFICANT CONCLUSIONS: For the first time, observations of deep circulation have 

been obtained throughout the deep basins in a single four-year interval, during which 

five LC eddies separated and transited the western Gulf. In the east, there was 

confirmation of the recent BOEM LC Dynamics Study that baroclinic instability is the 

dominant mechanism for transferring kinetic energy to the deep lower layer. Developing 

deep anticyclones and cyclones, displaced down stream of their respective meander 

crests and troughs, were observed on the north and east sides of an extended LC on a 

number of occasions. The circulation in a deep cyclone was measured with multiple 

floats just before LC eddy Kraken’s detachment. The cyclone was not in solid body 

rotation, with maximum speeds ~ 20 cm s-1

, and a central rotation period of ~ 8–10 

days. A deep cyclone was also tracked from northeast to southwest across the 

developing neck of a LC eddy detachment, confirming previous observations from fixed 

arrays. A new kind of deep anticyclone was observed under the leading edge of a 
ACCESS NUMBER: 00112

northward-extending LC. These “tip” anticyclones appear to rapidly dissipate into TRWs 

as do eddies under the LC when the LC separates an eddy and retracts towards the 

Yucatan Channel. Strong rectilinear motions, over several months, were observed over 

the deep Mississippi Fan that were attributed to TRWs, with evidence of propagation of 

momentum to the west along the isobaths. There was little evidence of deep eddies that 

transfer mass from the eastern to the western basin. The western basin had much lower 

levels of eddy activity and most of the circulation was related to an anticlockwise 

escarpment boundary current which, for the first time, was shown to be a fairly 

continuous flow from south of the Mississippi delta, south along the Mexican slope, and 

around the Bay of Campeche to the northeastern point of the Campeche-Yucatan shelf, 

whence it partially contributed to the deep, mean flows of the eastern basin. The

boundary current, though variable and interrupted by reversals, was also a source of 

deep eddies in the western basin. Along the steep Campeche escarpment, the 

northeastward flowing boundary current becomes unstable and detaches anticyclones 

that propagate westward towards the Mexican slope. This eddy generation mechanism 

appears to be independent of any upper-layer mesoscale eddy activity. A second new 

major circulation feature was a large anticlockwise gyre over the deepest part of the 

western basin that was distinct from the boundary current. Other results showed that 

upper-layer LC eddies displace lower water column parcels toward deeper water, and 

possibly also generate TRWs. This behavior has not been observed previously in the 

Gulf. Other significant results are the lack of a connection between the eastern and 

western basins, and an indication that the cyclonic boundary current has some 

recirculation between the Campeche escarpment and the northern slope. Relatively 

few floats released in the east and west made excursions to the western and eastern 

basins, respectively, and certain regions such as the southeast under the LC and the 

Bay of Campeche had almost no, or small, float excursions to other regions, over 18-

month intervals. The database of CTD profiles from the APEX and available US Argo 

floats allowed the water mass distribution to be measured across the whole basin, from 

the surface to > 1000 m, for the first time since the 1970s. The distribution and 

characteristics of Antarctic Intermediate Water and Subtropical Underwater, mixed layer 

depth, and thermocline depth were mapped and related to the presence of LC eddies 

and upper-layer cyclones.

STUDY RESULTS: Maps of basin-wide deep mean flows that resolve boundary 

currents and smaller-scale structures, along with variances and kinetic energy. A deep 

eddy census was developed for both cyclones and anticyclones. Large-scale dispersion 

between sub-regions was calculated using the floats as passive particles. Descriptions 

and dynamics of small groups of floats that respond to the LC and the transits of LC 

eddies across the western Gulf. Description and characterization of the LC and five LC 

eddies using satellite altimetry. Comparisons of float velocities with the Hybrid 

Coordinate Ocean Model numerical circulation model. Statistics of Gulf hydrography 

and the mapping of water mass characteristics, and their influence on lower watercolumn dynamics.

STUDY PRODUCT(S): Hamilton, P., A. Bower, H. Furey, R.R. Leben, P. PérezBrunius. 2017. Deep Circulation in the Gulf of Mexico: A Lagrangian Study. U.S. Dept. 
ACCESS NUMBER: 00112

of the Interior, Bureau of Ocean Energy Management, Gulf of Mexico OCS Region, New 

Orleans, LA. OCS Study BOEM 2017-081. 289 pp.

 *

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