Warm-Water Eddies In Gulf Intensify Hurricane Changes

4 October 2005

Scientists monitoring ocean heat and circulation in the Gulf of Mexico during Hurricanes Katrina and Rita have a new understanding of how these tropical storms can gain intensity so quickly: the Gulf of Mexico’s “Loop Current” is likely intensifying hurricanes that pass over eddies of warm water that spin off the main current.

The Loop Current is a horseshoe-shaped feature that flows clockwise, transferring warm subtropical waters from the Caribbean Sea through the Yucatan Straits into the Gulf of Mexico.

A positive outcome of a hurricane season like this is that we’ve been able to learn more about the Loop Current and its associated warm-water eddies, which are basically hurricane intensity engines.

—Nick Shay, University of Miami

This year, the Loop Current extended deep into the Gulf of Mexico during hurricane season. Currents at this time of year typically become unsteady and pinch off deep, warm eddies. The warm water then becomes ideal for hurricanes in the process of intensifying.

After Hurricane Katrina and a week before Hurricane Rita, Shay, Peter Black from the NOAA Atlantic Oceanographic and Meteorological Laboratory (AOML) and Eric Uhlhorn of the University of Miami/NOAA Cooperative Institute for Marine and Atmospheric Science, deployed Airborne Expendable Conductivity, Temperature and Depth profilers (AXCTDs); Current Profilers (AXCPs); and Bathythermographs (AXBTs) to obtain information on water temperature to depths of up to 3,300 feet (1,000 meters).

The AXCTDs and AXCPs, which were funded by NSF, are dropped from aircraft and measure salinity and currents.

Orange shows Gulf of Mexico Loop Current location; markings are where water temperature and other parameters were measured during Hurricane Rita. Credit: RSMAS/NOAA

Two days before Hurricane Rita, Black and Rick Lumpkin of AOML, and Peter Niiler of the Scripps Institution of Oceanography, deployed surface drifters that measure surface and subsurface thermal conditions while traveling clockwise around a Loop Current warm eddy just south of Louisiana. The eddy was lying in the path of Rita.

This represents one of the most comprehensive ocean-data sets where two major hurricanes passed through the same region. This series of observations is a testament to how new ocean observations are helping us understand hurricane intensity changes.

—Frank Marks, director of NOAA’s Hurricane Research Division

The last season with two Catgeory 5 hurricanes in the same basin was in 1961 (Carla and Hattie). The same phenomenon occurred the year before with Donna and Ethel.

Hurricanes Katrina and Rita also have much in common with Hurricane Opal, a category 4 storm that occurred a decade ago, on Oct. 4. During Opal, meteorologists first recognized the pivotal role that deep, warm eddies play in quickly building hurricane intensity. Opal encountered a warm-water eddy in the Gulf of Mexico and strengthened in intensity from Category 1 to Category 4 in just 14 hours.