Loop Current - Effect On Hurricanes

Effect On Hurricanes

Around 1970, it was believed that the Loop Current exhibited an annual cycle in which the Loop feature extended farther to the north during the summer. Further study over the past few decades, however, has shown that the extension to the north (and the shedding of eddies) does not have a significant annual cycle, but does vacillate in the the north-south and east-west directions on an inter-annual basis.

The Loop Current and its eddies may be detected by measuring sea surface level. Sea surface level of both the eddies and the Loop on September 21, 2005 was up to 60 cm (24 in) higher than surrounding water, indicating a deep area of warm water beneath them. On that day, Hurricane Rita passed over the Loop current and intensified into a Category 5 storm with the help of the warm water.

In the Gulf of Mexico, the deepest areas of warm water are associated with the Loop Current and the rings of current that have separated from the Loop Current are commonly called Loop Current eddies. The warm waters of the Loop Current and its associated eddies provide more energy to hurricanes and allow them to intensify.

As hurricanes pass over warm areas of the Gulf of Mexico, they convert the ocean’s heat into storm energy. As this energy is removed from the seas, a wake of colder water can be detected along the hurricane’s path. This is because heat is withdrawn from the ocean mixed layer in a number of ways. For instance, sensible and latent heat are lost directly to the tropical cyclone across the air-sea interface. Also, the horizontal divergence of wind-driven mixed layer currents results in the upwelling of colder thermocline water. Finally, the turbulent entrainment of colder thermocline waters caused by wind stirring also results in the cooling of the surface waters . These are the reasons that the depth of the ocean mixed layer is more important in hurricane deepening than sea surface temperature. A thin veneer of warm surface waters will be more susceptible to hurricane induced cooling than waters with a larger mixed layer and deeper thermocline. Furthermore, models suggest that cyclones are more likely to reach a larger fraction of their maximum potential intensity over warm oceanic features where the 26°C isotherm extends beyond 100 meters .

An example of how deep warm water, including the Loop Current, can allow a hurricane to strengthen, if other conditions are also favorable, is Hurricane Camille, which made landfall on the Mississippi Gulf Coast in August 1969. Camille formed in the deep warm waters of the Caribbean, which enabled it to rapidly intensify into a category 3 hurricane in one day. It rounded the western tip of Cuba, and its path took it directly over the Loop Current, all the way north towards the coast, during which time the rapid intensification continued. Camille became a category 5 hurricane, with an intensity rarely seen, and extremely high winds that were maintained until landfall (190 mph (310 km/h) sustained winds were estimated to have occurred in a very small area to the right of the eye).

In 1980 Hurricane Allen strengthened to a category 5 hurricane while moving over the Loop Current, but it weakened before landfall in Texas.

In 2005 Hurricane Katrina and Hurricane Rita both greatly increased in strength when they passed over the warmer waters of the Loop Current. Hurricane Wilma of 2005 was expected to make its Florida landfall as a category 2 hurricane, but after encountering the southeastern portion of the Loop Current, it reached the Florida coast as a category 3 instead.

While not as infamous as Katrina, Hurricane Opal most definitively illustrates the deepening abilities of a warm core ring. After crossing the Yucatan peninsula, Opal reentered the Gulf of Mexico and passed over an eddy shed by the Loop Current. Within a fourteen-hour period, sea surface pressure dropped from 965 to 916 hectaPasals, surface winds increased from 35 to 60 meters/second, and the storm condensed from a radius of 40 kilometers to 25 kilometers. Prior to the storm, the 20°C isotherm was located at a depth between 175 to 200 meters, but was found 50 meters shallower after the storm had passed. While the majority of this hurricane induced cooling of the mixed layer was attributed to upwelling (due to Ekman divergence), another 2000 to 3000 watts/meter squared were estimated to be lost through heat flux at the air-water interface of the storm’s core. Furthermore, buoy-derived sea surface temperature readings recorded temperature dropping 2° to 3°C as Opal passed over Gulf Common Waters, but only 0.5° to 1°C as the storm encountered the more massive ocean mixed layer associated with the warm core eddy.

In 2008 Hurricane Gustav transited the Loop Current, but due to the current's temperature (then only in the high 80's-degrees-F) and truncated size (extending only halfway from Cuba to Louisiana, with cooler water in-between its tip and the Louisiana coast) the storm remained a category 3 hurricane instead of increasing strength as it passed over the current.

Hurricane Ivan rode the Loop Current twice in 2004.