Part of the Gulf Stream, the Loop Current is a warm ocean current in the Gulf of Mexico that flows northward between Cuba and the Yucatán peninsula, moves north into the Gulf of Mexico, loops west and south before exiting to the east through the Florida Straits.
A related feature is an area of warm water called an "Eddy" or "Loop Current ring" that separates from the Loop Current, somewhat randomly. These rings then drift to the west at speeds of about 5 cm/s (0.18 km/h or 0.11 mph) and bump into the coast of Texas or Mexico.
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.
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.
The turbulent environment of hurricanes pulls up water from beneath the surface, often upwelling cooler water. Stronger hurricanes upwell deeper water. If the water in the lower levels is significantly cooler, the water will limit the hurricane's ability to strengthen, and may even cause it to weaken. But if the water is still warm at lower depths, then water being pulled to the surface remains warm, and the hurricane can increase in intensity if other atmospheric conditions are also conducive to strengthening. Meteorologists look for areas of deep warm water of at least 26 degrees Celsius (79°F). A continuous supply of warm water is one of several critical factors in enabling hurricanes to intensify beyond the initial level of a major hurricane (Category 3).
When a hurricane is traveling quickly over warm sea surface temperatures (SSTs), intensity might be maintained despite upwelling because the hurricane moves on before the cooler water impacts the hurricane.
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 / 305 km/h sustained winds were estimated to have occurred in a very small area to the right of the eye).
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.
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.
Before devastating Homestead, Florida, 1992's Hurricane Andrew briefly touched the Loop Current and made landfall as a Category 5. Hurricane Opal crossed a Loop Current eddy and went from a Category 1 to a Category 4 in 14 hours. Hurricane Ivan rode the Loop Current twice in 2004.
The process by which a hurricane increases in intensity is complex and not completely understood and consequently, difficult to predict. Passage over the Loop Current or the Eddy Vortex does not guarantee strengthening. Once Hurricane Rita left the Loop Current and passed over cooler water, it declined in strength, but the main factor in this weakening was an eyewall replacement cycle (ERC) occurring at that time. The ERC and other atmospheric factors are why Rita did not reintensify when subsequently passing over the Eddy Vortex.
Also of note: tropical depressions, tropical storms, and hurricanes gain strength from, but are not steered by, the temperature of the water. They are steered by the atmosphere, and the atmospheric level involved in steering a hurricane is different at different intensities (i.e., it relates to the minimum pressure of the hurricane).
Although hurricanes do increase in strength over warmer water, it has not been shown that the Loop Current specifically has been significantly warmer than in the past.
Kevin E. Trenberth of the National Center for Atmospheric Research, with others, wrote "It should be recognized that the issue is not black or white, but rather that global warming has a pervasive influence on ocean SST [sea surface temperature] and heat content, atmospheric temperature, water vapor, and atmospheric and oceanic general circulation patterns, all of which affect tropical cyclones in complex, not yet fully understood ways." and "in our view the growing body of evidence suggests a direct and growing trend in several important aspects of tropical cyclones, such as intensity, rainfall, and sea level [a proxy for sea temperature], all of which can be attributed to global warming."
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