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Long-Term Hurricane Climatic Variability

Team Leader:
Stanley Goldenberg (HRD)

Team Members:
Evan Forde (OD/AOML)
Christopher Landsea (HRD)
Howie Friedman (HRD)

Objective:

Use of objective techniques to analyze and understand hurricane climatic fluctuations for the North Atlantic basin on multidecadal time scales. At the present time, the work is focused on obtaining a better understanding, hopefully eventually leading to better predictability, of the nature and cause for multidecadal-scale fluctuations in October hurricanes.

Introduction and Methodology:

The years 1995-2002 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 eight 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. The greater activity results from simultaneous increases in North Atlantic sea-surface temperatures 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, only one of which affected the Caribbean. Since the shift in activity in 1995, however, there has been an average of one October major hurricane per year -- essentially a ten-fold 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.

Previous work has established the physical mechanism for the multidecadal-scale shift in overall activity (see accomplishments). Current work is using hurricane, oceanic and atmospheric climate data to investigate the dramatic shift in October activity. Using best-track historical hurricane data, plots will be made of the interannual and intraseasonal distribution of activity for various eras. SST data and NCAR/NCEP Reanalysis data in various formats are used to analyze the climatic conditions associated with the increased activity.

Accomplishments:

NCAR/NCEP Reanalysis data have been obtained in MATLAB format (netcdf) for 1948-2002 for upper-(200-mb) and lower-level (850-mb) and vertical shear (200-mb minus 850-mb) winds for the Atlantic basin. Atlantic hurricane track data have been updated through 2002. Programs have been written and preliminary plots made of the intraseasonal distribution of activity for eras of above- and below-average activity. Time series plots of various measures of October activity have been examined. Numerous presentations were made and a manuscript published (Goldenberg et al. 2001) detailing the nature and and causes of the overall increase in Atlantic basin activity. Presentations have been given at various conferences on the preliminary results on the nature of the increased October activity.

References:

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.

Future Work:

  • Continue using intraseasonal plots to elucidate the basic nature of the long-term changes in October activity for various strengths.
  • Do various analyses with the Reanalysis upper- and lower-level and vertical shear wind data to examine the nature of the climatic conditions that were conducive to the increased October activity.
  • Prepare a manuscript detailing the results for publication in the refereed scientific literature.
  • After completion of October study, it is hoped that a more in depth study could be accomplished concerning the overall shift in Atlantic activity through a statistical analysis of the teleconnections governing the interplay between multidecadal shifts in Atlantic SSTs and vertical shear. A better understanding of these links could lead to a realization of long-term predictability of future shifts in activity.


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Last modified: 3/31/2004

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