ACCESS NUMBER: 20043

STUDY TITLE: Dynamics of the Loop Current in U.S. Waters

REPORT TITLE: Observations and Dynamics of the Loop Current

CONTRACT NUMBER: M08PC20043

SPONSORING OCS REGION: Gulf of Mexico

APPLICABLE PLANNING AREA: Eastern

FISCAL YEARS OF PROJECT FUNDING: 2009; 2010; 2011; 2012; 2013

COMPLETION DATE OF REPORT: August 2014

COSTS: FY 2009: $2,197,056; FY 2010: $965,142; FY 2011: $610,828; FY 2012: 

$1,091,271; FY 2013: $947,663; CUMMULATIVE PROJECT COST: $5,811,960

PROJECT MANAGER(S): Peter Hamilton, Scott McDowell

AFFILIATION: Leidos Inc. (formerly SAIC)

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

PRINCIPAL INVESTIGATOR(S)

*

: K. A. Donohue, P. Hamilton, R. R. Leben, L.-Y. Oey, 

D. R. Watts

KEY WORDS: Eastern Gulf of Mexico; Straits of Florida; Yucatan Channel; Campeche 

Bank; West Florida Slope; Mississippi Fan; Loop Current; Eddies; Observations; 

Currents; Temperature; Salinity; Pressure; Travel Time; Moorings; Pressure Equipped 

Inverted Echo Sounders (PIES); Numerical Modeling; Circulation; Dynamics; 

Instabilities; Meanders; Frontal Eddies; Eddy Separation; Inertial Oscillations; Satellite 

Oceanography; Sea Surface Height; Inter-Annual; Intra-Annual; Statistics.

BACKGROUND: The Loop Current (LC) is a branch of the major western boundary 

current system of the North Atlantic that includes the Florida Current and the Gulf 

Stream. It enters the Gulf of Mexico through the Yucatan Channel, makes a clockwise 

Loop through the eastern basin, and exits through the Straits of Florida between Cuba 

and Key West. This energetic and variable current is the major source of upper layer 

(surface to ~ 1000 m) eddies, and lower layer (below 1000 m) eddies and planetary 

waves throughout the deep waters of the Gulf. The LC, on an irregular cycle averaging 

9 to 11 months, extends to the northwest towards the Mississippi delta, and sheds a 

large warm, clockwise rotating eddy of ~ 200 to 400 km in diameter. A detached eddy 

may reattach to the LC several times, but eventually will separate and move into the 

western Gulf where it dominates circulation processes. These types of events are well 

known and have been studied using remote sensing and circulation models. However, 

there had been no previous attempts to directly observe the three-dimensional velocity 

and density structure of LC variability, eddy detachments and separations using in-situ 

measurements, with the goal of investigating these processes in detail. A deeper 
ACCESS NUMBER: 20043

understanding of LC circulation processes is important for the assessment of numerical 

circulation models that are used for predictions, investigations of dynamics, and oil spill 

research. 

OBJECTIVES: (1) To increase understanding of LC incursions into the Gulf; (2) To 

describe circulation processes that lead to detachments, reattachments, and 

separations during eddy shedding; (3) To understand how the LC interacts with and 

forces the lower-layer circulation; and (4) To incorporate observations from the Mexican 

sector of the Eastern Gulf into the investigation of circulation and dynamics.

DESCRIPTION: A moored array of nine full-depth and seven near-bottom moorings, 

and 25 bottom-mounted, pressure-equipped inverted echo sounders (PIES) was 

deployed in an array in the deep basin of the eastern Gulf for 30 months beginning in 

April 2009. The Mexican complementary study deployed nine full-depth moorings in two 

transects across the western Campeche Bank slope for 24 months beginning in June 

2009. The near-bottom moorings were equipped with one current meter 100 m above 

bottom. The full-depth moorings were equipped with a variety of instrumentation, 

including ADCPs, current meters, and temperature, salinity and pressure (T/S/P) 

instruments, distributed so as to adequately capture the velocity and T/S vertical 

structure of the strongly sheared and nearly depth-independent upper and lower layers, 

respectively. PIES measure bottom pressure, temperature and the acoustic travel time 

from bottom to the surface and back. The latter are converted to water column T/S 

profiles using the historical database of CTD casts that included calibration CTDs taken 

during the field operations. Additionally, historical remote sensing data, including 

altimeter measurements of sea-surface height (SSH), sea-surface temperature (SST) 

and ocean color were acquired through the period of the field studies. Finally, a 

numerical circulation model was used to investigate the dynamics using observations. A 

high rate of data return was obtained from the moored instruments in both the U.S and 

Mexican sectors, and data were made available to PIs from both studies for analysis 

and interpretation.

SIGNIFICANT CONCLUSIONS: The three observed LC eddy detachments in the 

records followed consistent scenarios with the northwestward extensions of the LC that 

developed large amplitude, southward-propagating meanders, with crest to crest 

distances of ~ 300 km, and periodicities of 40 to 60 days on the northern and eastern 

sides of the southeastward flowing arm of the current that is over deep water (> 2500 

m). Growth of these meanders was fed by baroclinic instabilities that involved 

interactions between a depth-independent (surface to bottom, i.e., barotropic), and a 

surface intensified velocity components. Initiation of baroclinic instability was triggered 

by lower-layer eddies propagating into the LC from the north-northeast part of the 

eastern basin. The growing barotropic eddies that dominated the lower layer, were 

steered by the topography of the Mississippi Fan and propagated southwestward across 

the neck of the LC to effect an eddy detachment. The east side meanders were not 

generated by LC frontal eddies propagating clockwise along the front from the 

Campeche Bank slope into deep water, as previous studies have suggested. These 

short ~ 10 day period frontal eddies appear to decay over deep water or are absorbed in 

to larger-scale meander troughs.
ACCESS NUMBER: 20043

A new history of LC eddy separations was constructed for 1978 to 1992 using prealtimeter datasets, and was combined with the 1993 to 2012 altimeter defined 

separations to give the longest time series yet available. Mean annual variability was 

examined and it was found that there is a significant higher probability of separations 

occurring around the spring equinox than in other months. LC growth and wane is a 

predominantly annual process with LC area being relatively large from January through 

July, and low in late fall and December. Mean annual variance of the LC area was found 

to co-vary with the dominant mode of monthly SSH anomalies along the southeast 

Atlantic coast of the U.S., through southward propagating coastal-trapped waves.

STUDY RESULTS: Three LC eddy formations and separations occurred during the field 

study. The three eddies were named Ekman, Franklin and Hadal with first detachment 

dates of July 6, 2009, June 8, 2010, and August 15, 2011, respectively. Ekman and 

Franklin were large eddies that behaved initially very similarly with detachment points in 

the southwest of the basin that followed the scenario given above. Ekman eventually 

separated after two reattachments to the LC when the detachment point moved to the 

northwest of the Campeche Bank. Franklin reattached four times after the initial 

detachment, and at each reattachment the LC drained area and energy from the eddy, 

so that after the final separation, Franklin was not a significant western Gulf LC eddy. 

Hadal extended much farther to the northwest than the other two eddies with three 

growth spurts with decreases in east-side meander activity between spurts. It separated 

on first detachment, and a free-running numerical model was able to hindcast the 

detachment date within about two weeks. For each eddy, lower-layer currents had large 

increases in variance over background, one to two months just prior to, or during the 

first detachments, which was accounted for by the baroclinic instability mechanism that 

drew on the large potential energy of the mean LC upper-layer flows. The propagation 

of short-period frontal eddies around the periphery of the LC was found to have very 

little influence on near bottom flows. There was significant growth of frontal eddies on 

the western side of the LC between the Yucatan northwards along the Campeche Bank 

slope, with decay occurring over deep water.

Strong inertial oscillations in the observed velocity field occurred during winter, and for 

hurricane Ida in November 2009. The inertial response to the northward transit of Ida 

was reasonably simulated using a regional numerical model and confirmed that the 

horizontal velocity shear of the LC front suppressed the inertial response of the upper 

water column compared with more quiescent waters.

STUDY PRODUCT: Hamilton, P., K. Donohue, C. Hall, R. R. Leben, H. Quian, J. 

Sheinbaum, and D. R. Watts. 2015. Observations and dynamics of the Loop Current. 

US Dept. of the Interior, Bureau of Ocean Energy Management, Gulf of Mexico OCS 

Region, New Orleans, LA. OCS Study BOEM 2015-006. 490 pp.

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