Thomas R. Knutson

Geophysical Fluid Dynamics Laboratory

Atlantic Hurricanes and Climate Change

Room 811 AO&SS, October 12, 2009, 3:30 PM

Abstract

Research to date on greenhouse warming and Atlantic hurricanes indicates: i) It is premature to conclude that human activity--particularly greenhouse warming--has had a detectable impact on Atlantic hurricanes. ii) Model simulations indicate that 21st century greenhouse warming may lead to greater numbers of very intense Atlantic hurricanes and higher hurricane rainfall rates, but fewer hurricanes overall. Observations since 1950 shows a strong correlation, on multi-year time-scales, between local tropical Atlantic sea surface temperatures (SSTs) and measures of Atlantic hurricane activity, such as the Power Dissipation Index (PDI). Other studies show that Atlantic PDI is also well-correlated with other SST measures which don't increase sharply with greenhouse warming--such as tropical Atlantic SST relative to tropical mean SST. Century-long basin-wide observed records of very intense Atlantic hurricanes are considered unreliable, but tropical storm and hurricane counts have been used for century-scale climate trend studies. Unadjusted counts of tropical storms show a significant rise from the mid to late 1800s to present, though this change is primarily due to an increase in very short duration (< 2 day) storms in the database. We interpret this finding as likely arising from observing limitations. Our analysis of historical ship track records suggests that reporting coverage was likely too sparse to detect all tropical storms. After adjusting for this time-dependent bias, Atlantic tropical storm counts have no significant trend over 1878-2006. Two different atmospheric model frameworks developed at GFDL reproduce the observed rise and year-to-year variability in Atlantic hurricane counts (1980-2006) remarkably well when forced with observed SSTs (and in the case of the regional model, using large-scale atmospheric conditions from the NCEP Reanalysis). According to these models, the ensemble late 21st century climate change projected by 18 IPCC AR4 global models implies reduced numbers of Atlantic hurricanes and tropical storms, which we interpret as mainly arising from projected increases in vertical wind shear. Downscaling experiments with higher resolution models indicate that despite a projected reduction in hurricane numbers, the frequency and intensity of the strongest Atlantic hurricanes may increase substantially, particularly poleward of the shear increase, though the projected increase may not be detectable for a number of decades.