Cooperative Institute for Research in Environmental Sciences

Response of Northern hemisphere extratropical cyclone activity and associated precipitation to climate change, as represented by CCSM3

The projected effects of rising CO2 levels on Northern Hemisphere extratropical cyclone activity and cyclone-associated precipitation have been examined for September through May, using output from version 3 of the Community Climate System Model (CCSM3). A cyclone identification algorithm was applied to a 5 member ensemble of CCSM3 20th and 21st century output, along with a method of isolating precipitation produced by each cyclone. Mean seasonal statistics describing cyclone activity and the character of associated precipitation were calculated over several study regions for twenty year periods. The dominant change in cyclone activity is a marked mid-latitude decrease in cyclone frequency during autumn, winter, and spring. Few significant changes in cyclone intensity were identified (storm track shifts).. Total daily precipitation from these events is found to increase into the 21st century, largely due to increases in available atmospheric moisture with rising temperatures. This thermodynamic increase in precipitation leads to large rises in total seasonal cyclone-associated precipitation over high latitudes, while over mid-latitudes the thermodynamic increase is offset by the dynamic effect associated with decreased cyclone frequency.