Author(s):
Robert E. Davis* - University of Virginia
Paul C. Knappenberger - New Hope Environmental Services
Oliver W. Frauenfeld - University of Colorado
Patrick J. Michaels - University of Virginia
Abstract:
Previously we demonstrated that sea surface temperatures (SST) and tropical cyclone intensity in the North Atlantic basin are not as closely linked as has been implied in several recent papers. Here, we broaden our investigation to examine aspects of the tropical environment beyond SST that have known impacts on tropical cyclone development, including, among others, vertical wind shear, vertical lapse rate, and layer-average specific humidity. We determine the monthly average value (upper air variables calculated from the 6-hourly data from both the NCAR-NCEP and the ERA-40 reanalysis data, SST data from NOAA NCDC ERSST) within 2.5º latitude x 2.5º longitude grid cells covering the North Atlantic basin region. Within each hurricane season, we calculate the areal extent of critical values of these variables, both individually and in combination, to determine how conditions critical for tropical cyclone development have evolved in the Atlantic basin over the past three to five decades.
When we examine the relationship between these environmental variables and tropical cyclone intensity and frequency, we find that although SSTs contribute significantly to tropical cyclone activity, their influence varies markedly over time, as does the importance of variables such as vertical wind shear and lapse rate. The variation of these influences indicates complex behavior over time, and do not suggest that human influences, acting on SST alone, can explain the recent increase in Atlantic hurricane activity. These findings highlight the complex nature of hurricane development and counter arguments of a simple cause-and-effect relationship between rising SSTs and stronger North Atlantic hurricanes.
