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The Atlantic Hurricane Activity Transition of 1994/95
Landsea, C.W, Bell, G.D., Gray, W.M. and Goldenberg, S.B.  1998.  The extremely active 1995 Atlantic hurricane season: environmental conditions and verification of seasonal forecasts.  Monthly Weather Review 126: 1174-1193.

What was done
The 1995 Atlantic hurricane season was one of near-record tropical storm and hurricane activity.  During the preceding four years (1991-94), however, average tropical storm and hurricane activity over the Atlantic basin was the lowest since the keeping of reliable records began in the mid-1940s.  The authors studied the meteorological characteristics of these two periods to determine what might have caused the remarkable upswing in storm activity.

What was learned
In the words of the authors, "perhaps the primary factor for the increased hurricane activity during 1995 can be attributed to a favorable large-scale pattern of extremely low vertical wind shear throughout the main development region."  They also note that "in addition to changes in the large-scale flow fields, the enhanced Atlantic hurricane activity has also been linked to below-normal sea level pressure, abnormally warm ocean waters, and very humid values of total precipitable water."  An additional local environmental factor that they say "likely contributed to the active 1995 hurricane season" was the westerly phase of the stratospheric quasi-biennial oscillation, which typically enhances Atlantic basin hurricane activity.  Possibly the most important factor of all, however - which tends to influence several of the other factors - was the "dramatic transition from the prolonged late 1991-early 1995 warm episode (El Niņo) to cold episode (La Niņa) conditions."  This transition contributed to what they call "the dramatic reversal" of weather characteristics "which dominated during the [prior] four hurricane seasons."

What it means
"Some have asked," say the authors, "whether the increase in hurricanes during 1995 is related to the global surface temperature increases that have been observed over the last century, some contribution of which is often ascribed to increases in anthropogenic 'greenhouse' gases."  Their answer?  "We conclude that such an interpretation is not warranted."  Why?  Because the various factors described above seem sufficient to explain the observations.  "Additionally," as the authors write, "Atlantic hurricane activity has actually decreased significantly in both frequency of intense hurricanes and mean intensity of all named storms over the past few decades," and "this holds true even with the inclusion of 1995's Atlantic hurricane season."

Reviewed 15 May 2002