CMIP5 projection of significant decrease in extratropical cyclone activity over North America under global warming

Mid-latitude cyclones and storm tracks form an important part of the global circulation. Cyclones passing along the storm tracks dominate the day-to-day weather variability, and changes in storm track activity or location strongly impact regional climate variations. Baroclinic waves that form the storm tracks are also responsible for transporting much of the heat, moisture, and momentum poleward in the mid-latitudes. Thus how the storm tracks may respond to global warming under increasing greenhouse gas forcing is of significant interest to scientists and policy makers alike.

In this study, projections of storm track changes over the continental U.S. and southern Canada made by 23 CMIP5 models have been compared to changes projected by 11 CMIP3 models. Overall, under RCP8.5 forcing, CMIP5 models project much more significant decreases in North American storm track activity than CMIP3 models under SRES A2, with the largest decrease in summer and the smallest decrease in spring. The decrease is found both in temporal variance and cyclone statistics, with the frequency of strong cyclones projected to decrease by 15.9%, 6.6%, 32.6%, and 16.9% for winter, spring, summer, and fall, respectively. There is a strong consensus among the 23 models regarding the sign of the projected change, with less than 20% of the models projecting changes in the opposite sign in any of the storm track parameters examined. Nevertheless, there are also significant model-to-model differences in the magnitude of the projected changes.

Projected changes in the large scale flow have been analyzed to understand the physical mechanisms that may have contributed to the differences between CMIP5 and CMIP3 projections, as well as model-to-model differences between CMIP5 projections. Projections in precipitation changes have also been examined and the results indicate that the projected decrease in storm track activity may have significant impacts on water resources over eastern and southern U.S.