Relatively cool waters just below the surface of the Gulf of Mexico – within 150 miles of land – cause the most intense hurricanes to almost always lose intensity before they hit that part of the U.S. coastline, according to a new study analyzing years of observational data. The findings could help scientists more accurately forecast hurricanes.
Forecasters observed that weak hurricanes tended to intensify in this region but the strongest of storms usually weakened, so that both normally approached a mid-range hurricane when reaching the coast. The research, a collaboration that included the National Hurricane Center in Miami and Colorado State University’s Atmospheric Science department, appears in a recent issue of the Journal of Weather & Forecasting. The paper is available at http://journals.ametsoc.org/doi/pdf/10.1175/2010WAF2222369.1.
Researchers used data from satellites above the Earth, ships in the ocean, weather balloons and computer models to understand the physical reasons for observations made by the National Hurricane Center (NHC).
The lead author is Ed Rappaport, deputy director of the NHC. Co-authors from Colorado State are Andrea Schumacher, a research scientist with the Cooperative Institute for Research in the Atmosphere or CIRA, and Mark DeMaria, a National Oceanic and Atmospheric Administration research meteorologist who is based at CIRA. CIRA is a collaboration between NOAA and Colorado State’s Department of Atmospheric Science.
While results of the study can be applied only to the U.S. Gulf coast, Schumacher notes that over the past 30 years, two-thirds of U.S. landfalling hurricanes came ashore there.
“NHC forecasters can begin applying these results to their forecast process for these important storms right away,” Rappaport said. “We’re hopeful that follow-up collaborations between NHC and CIRA scientists will lead to more advanced forecast applications of this information for the Gulf.”
“It’s something special about the Gulf of Mexico,” DeMaria said. “This behavior is because of strong gradients in ocean properties in the Gulf of Mexico. In the center of the Gulf, deep, warm water comes out of the tropics, but closer to the northern Gulf coast, warm water does not extend as deep below the surface. When hurricanes move over that water, high surface winds tend to mix cooler water up to the surface, which can lessen a storm’s intensity.”
A secondary factor in the weakening of storms is the stronger upper-level winds upon approach to the northern Gulf coast, which also act to reduce storm intensity.
“You can get really intense hurricanes in the Gulf of Mexico,” Schumacher said. “In that last stretch before they hit land, the strongest hurricanes weaken unless you have unusual conditions.”
For example and in rare cases, ocean currents can bring deep, warm water closer to the northern Gulf coast, reducing the time spent by hurricanes over waters where the subsurface temperature is cool. Although ocean measurements were sparse at the time, this situation may have occurred for Hurricane Camille in 1969, which maintained Category 5 intensity right up to the Mississippi coast. This is a particularly troubling pattern, DeMaria said.
The study looked closely at the potential relationship between a storm’s initial intensity up to two days before landfall and its landfall intensity.
Colorado State researchers have worked with the National Hurricane Center on an ongoing basis to develop and improve forecasting techniques.