Projected significant decrease in storm track activity over North America in CMIP5 simulations

The mid-latitude 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 many people.

Storm track projections made by 23 CMIP5 models have been examined and compared to CMIP3 projections. The results indicate high degrees of consistency for Southern Hemisphere projections, but significant differences are found in the Northern Hemisphere, especially over North America. Specifically, CMIP5 simulations suggest significant decrease in storm track activity over North America during all 4 seasons, with over 90% of models agreeing on the sign of the projected change, while CMIP3 models only project significant (but much smaller) decrease in summer and no significant decrease for the other 3 seasons.

Preliminary analyses have been performed by examining changes in the large scale flow 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.