Dr. Jeff Masters ·

Above: Ocean currents in the Gulf of Mexico as depicted by the U.S. Navy HYCOM model on July 21, 2018. Colors represent the speed of the current, in centimeters per second. Dark red colors are approximately 1.2 m/s (4 mph) in the Loop Current as it traverses the Florida Straits and up the U.S. East Coast (where it is called the Gulf Stream). The Loop Current was flowing directly from the Caribbean through the Florida Straits and up the east coast of Florida, while an clockwise-rotating eddy that broke off from the Loop Current in ealy July was spinning in the central Gulf of Mexico. A smaller counter-clockwise rotating cool eddy was just to its west, and another clockwise-rotating warm eddy farther to the west. Image credit: U.S. Navy.

There’s potential trouble cooking in the Gulf of Mexico for the coming peak portion of the Atlantic hurricane season: a near-record amount of heat energy in the ocean waters. Sea surface temperatures (SSTs) are near average, 29° – 30° C (84° – 86° F) in the eastern Gulf, where persistent cloud cover and windy conditions in recent days have acted to keep SSTs from warming to above-average levels. However, SSTs are 30° – 31°C (86° - 88°F) in the central and western Gulf, more than 1°C (1.8°F) above average. That’s a lot of heat in the waters for potential hurricanes to feast on.

Figure 1. SSTs in the Gulf of Mexico on July 25, 2018 ranged from 29° – 31° C (84° – 88° F), an average of 0.6°C (1° F) above average when summed over the entire Gulf. Image credit: Levi Cowan, tropicaltidbits.com.

But SSTs don’t tell the whole story. When a slow-moving hurricane traverses a shallow area of warm ocean waters, the hurricane’s powerful winds will churn up cold waters from the depths, cooling the surface and putting the brakes on any rapid intensification the hurricane may have had in mind. But when unusually warm ocean waters extend to great depth—down 100 meters or more below the surface—the hurricane’s churning winds simply stir up more warm water, allowing dangerous rapid intensification to occur if wind shear is low. Last year’s trio of great hurricanes—Harvey, Irma, and Maria—all underwent rapid intensification into major hurricanes when they were located over waters with above-average SSTs, where the warm waters extended to great depth. Thus, total Ocean Heat Content (OHC) is a key metric used to determine the potential for hurricane rapid intensification.

Figure 2. The Loop Current flows 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. According to Dr. Nick Shay of the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, these eddies maintain their core structure until they interact with the steep slope along the Texas shelf. A manifestation of that process is that they get elongated along the steep topography and start spinning down due to bottom friction. Image credit: NOAA.

Meet the Loop Current

In the Gulf of Mexico, the deepest warm water is found in the Loop Current--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 just south of the Florida Keys (where it is called the Florida Current), and then goes just west of the westernmost 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 1.8 mph (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 very warm water that can provide a huge energy source for any hurricanes aspiring to become rapidly intensifying major hurricanes.

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 2). This ring of warm water slowly drifts west-southwestward towards Texas or Mexico at about 2 – 3 miles 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.

Even when a Loop Current eddy has been separated from the Loop Current for more than a year, it can still provide a potent source of heat energy for a hurricane. Hurricane Harvey of 2017 was fueled by an old Loop Current eddy that had migrated to the coast of Texas, a full 16 months after it had broken off from the Loop Current. This heat energy contributed to Hurricane Harvey’s record rains. A 2018 paper by Kevin Trenberth of the National Center for Atmospheric Research, Hurricane Harvey Links to Ocean Heat Content and Climate Change Adaptation concluded: “The bottom line is that the total observed OHC change is remarkably compatible with the total energy released by precipitation and, unsurprisingly, reflects strong energy exchanges during the hurricane. Accordingly, the record high OHC values not only increased the latent heat which fueled the storm itself, likely increasing its size and intensity, but also likely contributed substantially to the flooding caused by rainfall on land. The implication is that if the OHC had been less, then the rainfall amounts would also have been less.”

Figure 3. A comparison of Ocean Heat Content (OHC) levels in the Gulf of Mexico in 2017 and in 2018. There are two Loop Current eddies in the Gulf this year, compared to one last year. OHC levels were at near-record levels both years. Image credit: University of Miami Rosenstiel School of Marine and Atmospheric Science.

Two Loop Current eddies in the Gulf for the 2018 hurricane season

A large warm eddy broke off of the Loop Current in February 2018, and has drifted slowly west-southwest during the year, arriving in late July to a position about 300 miles east of the Texas/Mexico and border and 400 miles south of western Louisiana (Figure 3). As if that weren’t enough, we had a new and larger warm eddy break off from the Loop Current in late June. That eddy is located a few hundred miles west of the southwest coast of Florida. Both of these eddies are capable of supplying major heat energy to tropical cyclones that might get loose in the Gulf. The total amount of heat energy in the Gulf right now is at near-record levels for this time of year—similar to last year’s levels, and far higher than what was observed during the nasty Hurricane Season of 2005.

Links

The behavior of the Loop Current over the past year can be viewed at Navy Research Lab's web site, with arrows showing the direction of the current. You can also see images with 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).

We've saved a copy of an animation of the Gulf of Mexico currents covering the period October 2005-October 2006. One can see at the beginning of the animation the Loop Current eddy that fueled Katrina and Rita sitting in the Gulf of Mexico south of Louisiana. This eddy moves slowly west-southwest to a point off the Texas coast by November, where it gradually dissipates. A smaller Loop Current eddy breaks off eight months later in March, and a large one just four months later, in July. The July 2006 eddy gets temporarily re-absorbed in September, then breaks free again by October.

For more information: A 1998 paper published in the Journal of Physical Oceanography titled, Loop Current Eddy Paths in the Western Gulf of Mexico provides some technical information on Loop Eddies. Horizon Marine, Inc. tracks and forecasts Loop Current eddies and provides a pay service for this.

Thanks go to my Mom for proof-reading today's post, and reminding me that I shouldn't end sentences with prepositions!