Loop Current

This article is about the oceanic phenomenon. For the electrical signaling schemes, see Current loop. For the network analysis variable, see loop current.

Loop Current

A parent to the Florida Current, 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 east and south before exiting to the east through the Florida Straits and joining the Gulf Stream.

A related feature is an area of warm water with 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.

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.

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 °C (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).

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. [1]

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 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.

Process

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).

Sea level and sea temperature

Sea level is relatively easy to measure accurately using radars from satellites. Sea temperature below the surface is not as easy to measure widely, but can be inferred from the sea level since warmer water expands and thus (all other factors, such as water depth, being equal) a vertical column of water will rise slightly higher when warmed. Thus sea level is often used as a proxy for deep sea temperatures.

NOAA's National Data Buoy Center maintains a large number of data buoys in the Gulf of Mexico, some of which measure sea temperature one meter below the surface.

See also

• National Oceanographic Data Center

External links

• Near Real-Time Gulf of Mexico Satellite Altimetry Data
• Ocean Surface Currents explains The Loop Current
• LSU's Earth Scan Laboratory
• National Data Buoy Center (see data buoy #42003)
• Hurricane Rita Roars through a Warm Gulf
• CU-Boulder Researchers Chart Katrina's Growth In Gulf Of Mexico
• "Global Warming: The Culprit?" (Time Magazine, October 3, 2005, pages 42–46) - Loop Current mentioned
• ScienceDaily on Loop Current and hurricanes
• Chart of the mechanisms inside the Loop Current
• Atlantic Oceanographic and Meteorological Laboratory