April 15th, 2013
The authors simulated the response of sea surface temperature (SST) in the Atlantic Hurricane Main Development Region (MDR) to a doubling of CO2, using a cutting-edge global high-resolution coupled model developed at GFDL (CM2.5). This model has been shown to produce a very faithful simulation of the observed seasonal cycle and year-to-year (or interannual) variability in the tropical Atlantic. The skillful representation of Atlantic interannual variability enables the exploration of the response of interannual variability to increasing CO2 – in addition to exploring changes in the average conditions in the Atlantic.
This study reveals a significant increase in the amplitude of interannual variations of SST in the Atlantic MDR: the amplitude of these SST variations increases by about 25% in response to CO2 doubling. Further, the seasonal timing of the peak in interannual SST variations moves from northern hemisphere spring to early summer, moving that peak variability closer to the North Atlantic hurricane season (which begins in June). The changes to the interannual variations of the SST in the Atlantic Hurricane Main Development Region are due to an increase in effectiveness of the Wind-Evaporation-SST positive feedback.
A statistical refinement allows estimation of the impact of CO2 changes on Atlantic hurricane frequency. The average number of hurricanes is found to decrease in response to increasing CO2, consistent with previous studies. Nevertheless, the frequency of Atlantic hurricanes in the most active years is as high or higher after doubling CO2, because of the enhanced year-to-year variability in Atlantic MDR SSTs and its alignment with the Atlantic hurricane season.
Exploration of the observed record suggests some evidence for an enhancement of the interannual variation of SST in the Atlantic Hurricane Main Development Region in boreal early summer since the late 19th century, although there are substantial uncertainties in the observational record.
The combined impact of natural climate variations, changes in average conditions, and changes in variability is key to the response of climate extremes to increasing greenhouse gases. This change in the interannual variability of SST will be one of the factors in projecting the risk of extremes in Atlantic Hurricanes, and for drought (or flood) in the Sahel and South America in a future climate.