Looking back at the 2009 Atlantic hurricane season

For much of the past couple of decades, tropical cyclone activity has been above normal in the Atlantic basin as measured by the number of named storms which develop each year. Long-term averages for the Atlantic basin are as follows: 10 named storms (tropical cyclones of at least tropical storm strength) per year; and of those, six attain hurricane strength per year. By this measure, since 1990, there have been six years when below normal seasons occurred, and four of them were prior to 1995. More recently, only 1997, when eight named storms were observed, and 2006 when 10 named storms were observed were at or below normal. On the other side of the ledger, 2005 was the jackpot year when a record 28 named storms were observed.

Prior to the 2009 season, many of the researchers who provide predictions of the number of storms, and have shown some skill in doing so, forecast another above normal year for the Atlantic, although not dramatically so, and certainly not even approaching the anomalous numbers of 2005. These forecasts are based on several factors which were selected based on some knowledge of how tropical cyclones develop. These include rainfall in Africa through the previous season, which gives some indication of atmospheric moisture content, sea surface temperature data, and upper level pressure and wind data. 

In the mainstream media, much has been made of a connection between global warming and hurricane formation. While it is true that higher sea surface temperatures over a larger area would provide a larger area (possibly for a longer season) where hurricanes could develop, the process of hurricane formation is much too complex to tie it to only that one (albeit rather important) parameter. 

In 2009 the Atlantic hurricane season has, so far (as of mid-October), been below the long-term average in terms of the number of named storms with a total of eight. This is significantly below the pre-season forecast. This naturally leads to the question: “What happened?”

One factor that happened was El Niño. The warming of equatorial sea surface temperatures in the Pacific which began in June 2009 produced stronger than normal westerly winds in the upper atmosphere extending into the tropical and subtropical Atlantic. These strong winds (often referred to as “upper level wind shear” in the mainstream media) are strongly inhibitive to the formation of tropical cyclones, and can also lead to weakening and dissipation of existing systems. In fact, there were several tropical cyclones this season which developed in the eastern Atlantic and moved west with plenty of warm water available, but when they encountered these stronger than normal upper level winds they weakened and dissipated. 

The formation of tropical cyclones is a complex process which is not completely understood. It is very tempting to try to simplify the matter, and state that warmer sea surface temperatures will automatically produce more hurricanes, or more intense hurricanes, but, as has been shown this year, this cannot be done. In fact, the entire earth-atmosphere system is very complex, and the effects of global warming on different aspects of our climate, and on the climate as a whole. have not been definitively determined. Further research and open minds are needed.

For now, though, residents along the East Coast and the Gulf Coast can count their blessings after a quiet hurricane season.

About the Author 

Ken McKinley earned a bachelor’s degree in atmospheric science from Cornell University in 1980, and attended graduate school in meteorology at the Massachusetts Institute of Technology. After working as a meteorologist for nearly 10 years he founded his own meteorological consulting firm, Locus Weather, in Camden, Maine in 1991. 

A large portion of his business at Locus Weather involves providing custom weather forecast services for oceangoing yachts, both racers and cruisers. Ken serves as an instructor for the Ocean Navigator School of Seamanship, and also as an adjunct instructor at the STAR Centers for Professional Maritime Officers in Dania, Fla., and Toledo, Ohio, and for MITAGS in Baltimore, Md. He has also taught meteorology at Maine Maritime Academy. He resides in Rockport, Maine with his wife and two sons. Ken’s Web site is: www.locusweather.com 

By Ocean Navigator