The 23rd Conference on Hurricanes and Tropical Meteorology

J6.6
RELATIONSHIPS BETWEEN DECADAL-SCALE FLUCTUATIONS IN VERTICAL SHEAR FROM NCEP/NCAR REANALYSIS DATA AND ATLANTIC BASIN TROPICAL CYCLONE ACTIVITY

Stanley B. Goldenberg, NOAA/AOML/HRD, Miami, FL; and C. W. Landsea

Numerous studies have elucidated decadal-scale fluctuations in Atlantic basin tropical cyclone activity. The years from the mid-1940Õs through1969 are classified as an "active" period, both with respect to numbers of major hurricane (maximum sustained surface winds of at least 50 m/s) activity and in overall activity, especially in the deep tropics. The years 1970 through 1994
have been classified as an "inactive" or "below-average" period. Of course, in both periods, there still exists substantial interannual variability as well. Some investigators have speculated that the hyperactive years 1995 and 1996 ushered in another active dedade of Atlantic tropical cyclone activity. Two of the indicators of the recent decadal-scale shift are shifts in the sign of the "North-South Atlantic SST dipole" as well as in vertical shear measured in the "main development region" (MDR: 10-20 degrees north) in the Atlantic hurricane basin. Studies have shown that the changes in vertical shear are probably the main physical mechanism responsible for the decadal-scale shift in activity and that these vertical shear changes are probably related to the SST shifts. Until recently, analyses of were only available back to 1968. This prevented any real sampling of the overall vertical shear conditions during the previous active period. The recent completion of the NCEP/NCARÊReanalysis project now allows examination of these wind fields from 1958 through the present, providing a continuous record to examine the relationship between the long-term wind fluctuations and changes in tropical cyclone activity. The current study will examine these fluctuations and whether or not they support the speculation that the Atlantic has indeed shifted back to an active period.

The 23rd Conference on Hurricanes and Tropical Meteorology