MMM SEMINAR NCAR
Hurricanes in a Warming World:
Genesis to Revelation
Peter J. Webster
Georgia Institute of Technology
and
Greg J. Holland
Mesoscale and Microscale Meteorological Division,
National center for Atmospheric Research
During the last decade, there has been a quickening of the hurricane rate especially from a statistically significant increase in intensity coupled to near linear trends in SST, which has occurred in conjunction with nearly constant frequency of hurricanes. This has led to some debate on the degree to which the changes in intensity are due to global warming, natural variability or a combination of both factors. The Cheliah-Bell “multidecadal tropical mode” is shown to be a statistical artifact. For the North Atlantic Bryden et al (2005, Nature) show that the thermohaline circulation (THC) has distinctly slowed during the last decade. Using W. M. Gray’s arguments, hurricane frequency should therefore have decreased substantially during the last decade, whereas it has increased. Natural variability remains an unlikely candidate to explain the entire changes in hurricane intensity and frequency. Finally, we turn to the record hurricane season of 2005. There are two reasons why the season was anomalous: the hurricane season was longer than normal, a trend that has been occurring for a considerable time; and the low level flow has changed to produce strong negative stretching deformation regions which allow easterly wave to be rapidly concentrate into tight cyclonic vortices trapped in the vertical and enhance genesis and growth to major hurricanes. In 2005, 30% of African waves became hurricanes compared to 10% prior to 1995. We speculate on what this may mean for future hurricane seasons.
Refreshments
NCAR-Foothills Laboratory
Bldg 2, Rm1022