Regional Trends in Extreme Events:
 The Recent Increase in Atlantic Hurricane Activity
 Mr. Stanley B. Goldenberg
 Hurricane Research Division/National Oceanic and Atmospheric Administration

        The years 1995-2004 have experienced the highest level of North Atlantic hurricane activity in the reliable record.  Compared with the generally low activity of the previous 24 years (1971-94), the last ten years have seen a doubling of overall activity for the whole basin, a 2.5-fold increase in major hurricanes („ 50 m s-1) and a five-fold increase in hurricanes affecting the Caribbean region.  As for the increase in major hurricane activity, this is not implying the basin is seeing stronger hurricanes, but rather more of the stronger storms.  The greater activity results from simultaneous increases in North Atlantic sea-surface temperatures (SST) and decreases in vertical wind shear.  It has been hypothesized that multidecadal changes in these oceanic temperatures are related to fluctuations in the intensity of the thermohaline circulation in the North Atlantic.
   An additional impact of the shift in Atlantic activity has been a dramatic increase in October hurricane (especially major hurricane) activity.    During the years 1965-1994, there were only two major hurricanes that developed after the first couple of days of October - Joan (October, 1988) and Kate (November 1985), only one of which affected the Caribbean.   Since the shift in activity in 1995, however, there have been eight of these late season major hurricanes in only ten years, with most of these affecting the Caribbean.
      Some have asked whether the increase in activity since 1995 is due to anthropogenic global warming. The historical multidecadal-scale variability in Atlantic hurricane activity is much greater than what would be expected from a gradual global temperature increase attributed to global warming, although it is possible that a small fraction of the increase in hurricane activity might be associated with the gradual, long-term SST increase.
Because of the multidecadal-scale nature of the Atlantic SST variability portrayed here, the shift since 1995 to an environment generally conducive to hurricane formation is not likely to change back soon.  This means that during the next 10-40 years or so, most of the Atlantic hurricane seasons are likely to have above average activity with a continuation of significantly increased numbers of hurricanes (and major hurricanes) affecting the Caribbean Sea, and basin-wide numbers of major hurricanes.  Tragic impacts of the heightened activity have already been felt, especially in the Caribbean, where over two dozen killer hurricanes have occurred since 1995 -- the deadliest being Hurricanes Georges (1998), Mitch (1998) and  Jeanne (2004).
      Government officials, emergency managers, and residents of the Atlantic hurricane basin should be aware of the apparent shift in climate and evaluate preparedness and mitigation efforts in order to respond appropriately in a regime where the hurricane threat is much greater than it was in the 1970s through early 1990s.
Primary reference:
Goldenberg, S.B., C.W. Landsea, A.M. Mestas-Nuñez, and W.M. Gray, 2001: The recent increase in Atlantic hurricane activity : Causes and implications.  Science, 293, 474-479.

For links to the Science article, the paper's supplement and to the NOAA press release about this study, go to www.aoml.noaa.gov/hrd/Goldenberg

 Stanley.Goldenberg@noaa.gov