Why do hurricanes seem so muscle bound this year, and why has poor Florida been in the bulls eye so often?

Nearly every part of poor Florida, the Keys, Panhandle, west coast and east coast, have been battered by hurricane force winds during the past 6 weeks. It has been a dreadful month for Floridians. Not only have they had to endure a series of nasty hurricanes causing enormous damage, but in the aftermath of these storms, there has been serious shortages of water, gasoline, and electrical power. Charley, Frances and Ivan took different paths but nevertheless all converged on the Sunshine State. After this year, Florida may have to adopt a new nickname -- perhaps the Submerged State or the Plywood State.

The Tropical Prediction Center in Miami (part of the National Oceanic and Atmospheric Administrations Climate Prediction Center) issues a forecast in May of each year (updated in August) for the expected number and severity of Atlantic and Caribbean hurricanes during the upcoming season. This season, the outlook called for 12-15 tropical storms, with 6-8 becoming hurricanes -- an above average year. The predicted activity reflected the likely increase in hurricane activity that began in the mid 1990s. Among conditions deemed favorable for hurricane development includes warmer than normal sea surface temperatures in the tropical Atlantic and the absence of El Nino conditions in the tropical Pacific Ocean -- more about this later.

On average, by the end of August, 9 tropical storms and 6 hurricanes form in the Atlantic Ocean, the Caribbean Sea or the Gulf of Mexico. This year, there were 8 tropical systems in August alone (a new record), and September has also proven to be a potent month for hurricanes. Between 1995 and 2000, there were 41 hurricanes in the Atlantic Ocean and or Caribbean Sea and Gulf of Mexico, which is the most in any five year period since records were first kept in 1886. Compared to the period from 1971 to 1994, from 1995 to 2000, there was a doubling in overall hurricane activity in the Atlantic Ocean and a 2.5 fold increase in major hurricanes.

Actually, the dearth of canes between the 70s and early 90s likely enticed tens of thousands of people to make Florida their home. Thirteen million people now live in Florida's coastal counties. I wonder how many people are now making plans to move there? The point is, new developments, shopping areas and resorts have sprouted up all along the seacoast, compromising dunes and barrier islands of their ability to help fend off powerful storms. With more property there's more property to damage, thereby increasing the misery index when storms come ashore. Building on sand that was meant to shift is asking for trouble. But with so fews hurricanes menacing the southeast U.S. in the 70s and 80s, the might and fury wrought by past hurricanes seems to have been forgotten. With tropical systems cranking up to normal levels and higher in recent years, many of which took aim on the U.S mainland, people are now asking what happened?

Hurricanes are efficient heat engines. Their thermal energy is converted to powerful winds when evaporated seawater condenses into rain. They gain strength from the latent heat released as the huge amounts of evaporated sea water condenses aloft (latent heat of condensation). The released heat intensifies updrafts, which permits more water to be evaporated from the sea surface.

The waters of the tropical Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico, from about late July through September, are nearly always warm enough to support hurricane development (above about 80 degrees F or 26 C). This year as in the past several years, pockets of unusually warm water have been measured in the tropical Atlantic and Caribbean. Tropical systems are refueled and energized when they move over chambers of warm water, like cars pulling into filling stations along a highway.

There's no question that warmer water, as would be expected to occur in a global warming environment, could invigorate hurricanes. In saying this, though, it should be mentioned that some of the most devastating hurricanes occurred long before the globe began to noticeably heat up; hurricane Camille in 1969, for example. Back in the days when the Earth's temperatures weren't so warm, between about 1925 and 1965, Florida was lashed by 14 major hurricanes. On the other hand, as global temperatures were on the rise in the 70s, 80s and 90s, only a few major canes pounded the southeastern U.S.; Hugo in 1989, Andrew in 1992 and Opal in 1996.

Fortunately, warm water is not the only ingredient necessary for hurricane formation. Basically, tropical depressions and tropical storms will not evolve into full blown tropical hurricanes, having a stable eye, if winds aloft are not conducive to further development. In other words, if the vertical wind shear across the western, tropical Atlantic Ocean is stronger than normal, storm circulation is more likely to be shut off -- an organized system can't form if its intake and outflow mechanisms are compromised. This has not happened this year, but it's unknown how a warmer world will effect wind atmospheric wind shear.

In general, the cooler Pacific waters that are typical of La Nina years, promote upper-air circulation regimes that are more likely to result in languid winds above the Atlantic Ocean. In the case of El Nino circulation, the warmer Pacific temperatures tend to result in increased vertical wind shears over the tropical Atlantic. However, not every year is either a La Nina year or an El Nino year, and in fact, most years are neither. In 2001 and 2002, when hurricanes were more scarce, El Nino conditions were also rather feeble. Again this season, sea surface temperatures in the tropical Pacific have been lukewarm (weak El Nino), and hurricanes seem to form on a weekly basis.

An additional consideration is that so far this season, the upper-air easterly winds (African easterly jet stream) over western Africa have been particularly favorable for "kick starting" tropical waves or disturbances into weather systems that spin. It's thought that these winds are important for getting energy to weather disturbances as they propagate westward out of the Sahara region and into the open Atlantic. In recent weeks, Frances, Ivan, Jeanne, Karl and Lisa have all formed in the Atlantic Basin -- west of the Cape Verde Islands. Jeanne has been perhaps the most deadly storm this year, dumping copious amounts of rain on Haiti last week that resulted in over 1,000 deaths. This coquettish storm teased the Bahamas and Florida last week, and is now once again threatening Florida.

Also, this year the location of surface high pressure systems over the Atlantic Ocean have been oriented in such a way that storms forming in the south Atlantic are steered toward Florida. In previous years, these "highs" was situated further east, and thus many hurricanes were pulled up and out to sea, off the east coast of North America, by the clockwise circulation.

One other consideration is Florida's unique shape. It's basically an extended coastline (1,197 miles or about 1,915 km) with little interior lands. It sort of sticks out like a thumb -- a throbbing thumb this year. Florida's form allows it to be subjected to storms that approach from the Atlantic Ocean, the Caribbean and the Gulf Of Mexico. North Carolina, which also has a long coast, only has to deal with hurricanes that form in the Atlantic Ocean. Moreover, Florida's flatness doesn't help matters when hurricanes roar ashore. No point in the entire state is more than 350 ft (107 m) above sea level! As has been sadly proved this year, Florida can indeed be an inviting place for hurricanes to pommel.

For more about hurricane prediction see the Tropical Meteorology Project
http://tropical.atmos.colostate.edu/forecasts/ and
the Climate Prediction Center http://www.cpc.noaa.gov/products/outlooks/hurricane.html
For more about future hurricanes see the Washington Post for September 19, 2004 "Warning in the Winds" (B1).

Thanks to Dr. Marshall Shepherd for reviewing this.

This week's question comes from Dr. Jim Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for EIGHT YEARS! Dr. Foster now shares the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.