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2006 Gulf of Mexico Loop Current outlook

By: Dr. Jeff Masters, 2:41 PM GMT on May 08, 2006

The Loop Current is an ocean current that transports warm Caribbean water through the Yucatan Channel between Cuba and Mexico. The current flows northward into the Gulf of Mexico, then loops southeastward through the Florida Keys (where it is called the Florida Current), and into the Bahamas. Here, the waters of the Loop Current flow northward along the U.S. coast and become the Gulf Stream. With current speeds of about 0.8 m/s, the Loop Current is one of the fastest currents in the Atlantic Ocean. The current is about 200-300 km (125 -190 miles) wide, and 800 meters (2600 feet) deep, and is present in the Gulf of Mexico about 95% of the time. During summer and fall, the Loop Current provides a deep (80 - 150 meter) layer of vary warm water that can provide a huge energy source for any lucky hurricanes that might cross over.

Figure 1. The Loop Current flow northwards into the Gulf of Mexico. Every 6 - 11 months, a bulge in the current cuts off into a clockwise-rotating eddy that then drifts slowly west-southwestward towards Texas. Image credit: NOAA.

The Loop Current commonly bulges out in the northern Gulf of Mexico and sometimes will shed a clockwise rotating ring of warm water that separates from the main current (Figure 1). This ring of warm water slowly drifts west-southwestward towards Texas or Mexico at about 3-5 km per day. This feature is called a "Loop Current Ring", "Loop Current Eddy", or "Warm Core Ring", and can provide a key source of energy to fuel rapid intensification of hurricanes that cross the Gulf, in addition to the Loop Current itself. The Loop Current pulsates in a quasi-regular fashion and sheds rings every 6 to 11 months. When a Loop Current Eddy breaks off in the Gulf of Mexico at the height of hurricane season, it can lead to a dangerous situation where a vast reservoir of energy is available to any hurricane that might cross over. This occurred in 2005, when a Loop Current Eddy separated in July, just before Hurricane Katrina passed over and "bombed" into a Category 5 hurricane. The eddy remained in the Gulf and slowly drifted westward during September. Hurricane Rita passed over the same Loop Current Eddy three weeks after Katrina, and also explosively deepened to a Category 5 storm.

Figure 2. Current position of the Loop Current and Loop Current Eddy that cut off in March 2006. Image credit: Navy Research Lab.

So, a key question to ask this hurricane season, is when will the next Loop Current Eddy break off, creating a ready-made high-octane energy source for any hurricane that might pass by? Well, the behavior of the Loop Current over the past year can be viewed at Navy Research Lab's web site (51 Mb). This movie has arrows showing the direction of the current, plus a color coding that represents the height of the sea surface above mean level. The higher the height, the warmer the water (since warm water expands and thus raises the sea level where it is at). One can see near the beginning of the animation that the Loop Current Eddy that fueled Katrina and Rita breaks off from the Loop Current in July 2005, then slowly moves west-southwest to a point off the Texas coast by November, where it gradually dissipates. Another Loop Current Eddy breaks off eight months later in March, and lies south of Louisiana in early May. If the Loop Current maintains its 6 - 11 month periodicity shedding these eddies, the next eddy is due sometime between September and February. Oceanographic models can't forecast these events realiably, so we don't know when the most likely time is. Let's hope that this doesn't occur in September or October--we could do without another big eddy of warm water in the Gulf at the height of hurricane season, fueling explosive hurricane intensification!

The Navy web site offers a 1-month forecast of the Loop Current in the Gulf of Mexico. The current forecast hints that the Loop Current may shed an eddy at the beginning of June. I don't know how reliable these forecasts are, but keep in mind that currents are driven by winds, and wind forecasts are not reliable out more than about 10 days. Given that the Loop Current just shed an eddy in March, I'd be surprised if a new eddy comes off in June.

My next blog will be Wednesday.
Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.