The Loop Current in the Gulf of Mexico
In collaboration with Fabien Lefèvre, oceanographer at CLS
The loop current
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The Loop Current is part of the general surface circulation. It is an extension of the Caribbean Current, coming from northern South America then penetrating into the Caribbean Sea. It becomes the 'Loop Current' at the Yucatan Channel, between the Yucatan Peninsula and the island of Cuba. It penetrates with more or less strength into the Gulf of Mexico, flowing in a loop, then escapes through the Straits of Florida, flows along the coast of Florida where it becomes the Florida Current, and finally joins the Gulf Stream off Cape Hatteras. In the Yucatan Channel, the Loop Current can reach surface velocities of around one metre per second (i.e. several kilometres per hour). |
 Gulf of Mexico topography
From : CLS |
Eddy shedding
Once or twice a year, the Loop Current sheds a clockwise (anticyclonic) eddy. Its principal characteristics are:
- Rotation velocity : around one meter per second,
- Moving velocity : around a few kilometers per day,
- Diameter : 400 to 500 kilometers
- Vertical extension : up to 800 meters
- Duration : up to one year
Some of these eddies may be shed and then later re-connect with the Loop Current.
Altimetric, thermal and biochemical signatures
The Loop Current and its eddies have been closely observed from space for a long time. Their presence is indicated by:
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) Chlorophyll concentration on 4 August 2004 Credit : CLS |
) Eddy shedding between August 2004 and December 2004, as observed by satellite altimetry Credit : Ssalto/Duacs (click on the image to view animation -2.6 MB-) |
) Eddy shedding and re-connecting between February 2005 and May 2005, as observed by satellite altimetry Credit : Ssalto/Duacs (click on the image to view animation -1.9 MB-) |
Environmental and industrial issues
These eddies are of major concern to offshore industries in the area (mainly companies such as Exxon, Shell, BP). A few figures indicate how much is at stake: 25% of total U.S crude oil output and 33% of total natural gas production. The proportion of deep sea exploitation (at depths greater than 300 metres) has increased from 5% to 10% in ten years.
Such eddies make extraction operations more difficult and thus more dangerous, both for people and the environment. Firstly, on the surface, because deep offshore platforms are not secured to the sea bottom. Nowadays in deep waters, oil and gas are extracted by means of large vessels, called FPSOs (Floating Production, Storage and Offshore Loading), which are connected to the sea bottom with flexible pipelines. FPSOs do not have great storage capacities, so they need other vessels to dock with them to offload production. This involves delicate manoeuvres for which a detailed knowledge of the sea surface conditions is essential. It is also essential to have a good understanding of the deep sea conditions. From surface to bottom, the current velocity may change in intensity and direction, generating shearing currents which may damage, or even break the pipelines.
 Platform secured to the bottom Credit : Fugro GEOS |
 FPSO platform: the vessel docked with the FPSO offloads the oil or gas Credit : Fugro GEOS |
The contribution of operational oceanography
In this context, technologies which enable such events to be monitored and forecast are very useful. Satellite altimetry enables real time monitoring of the movement of eddies. With ocean models, therefore, forecasting the trajectory and intensity of such events becomes a real possibility.
Several companies working for platform operators have tested operational ocean circulation models in this area on actual cases. This is true of Ocean Numerics, which is supplying oceanic information based on both satellite and model information. The Hycom (Miami University, USA) regional model, nested in the Norwegian Topaz model is one of their sources of information, as is the ' PSY2 ' Mercator prototype (for the North Atlantic and Mediterranean at 1/15°).
The figures below demonstrate the ability of an ocean model such as the PSY2 Mercator prototype to forecast such a phenomenon.
On 18 August 2004, the Loop Current penetrated deep into the Gulf of Mexico. The seven-day Mercator forecast announced that the distortion of the loop was a precursor of unravelling, which was also confirmed by the two-week forecast. This prediction proved to be correct. An analysis made two weeks later showed an eddy in the process of shedding.
We do not yet know which applications might benefit from this forecast, but there is no doubt that such events will help build confidence in the reliability of ocean modelling.
In a next bulletin, the Mercator Ocean oceanography forecasters will describe how Mercator has analysed and forecast the most recent vortical phenomena which occurred in this area during the past year.
Useful links
- The Loop Current as observed by the first Mercator prototype, Newsletter #6
- The lifecycle of an anticyclonic eddy in the Gulf of Mexico, Interpreted oceanic bulletin #11
- A loping current, Image of the month, April 2003, Aviso website
- Loop current, Hycom Consortium web site
