CGD's Dr. Haiyan Teng

Abstract

Climatology and interannual variations of wintertime extratropical cyclone frequency in CCSM3 20th-century simulation are compared with the NCEP/NCAR reanalysis during 1950-1999. CCSM3 can simulate the storm tracks reasonably well, although the model produces slightly less cyclones at the beginning of the Pacific and Atlantic storm tracks and weaker poleward deflection over the Pacific. One CCSM3 21st-century A1B scenario realization indicates there is significant increase in the extratropical cyclone frequency on the US west coast and decrease in Alaska. Meanwhile, cyclone frequency increases from the Great Lakes region to Quebec and decreases over the US east coast, suggesting a possible northward shift of the Atlantic storm tracks under the warmer climate. The cyclone frequency anomalies are closely linked to changes in seasonal mean states of the upper-troposphere zonal wind and baroclinicity in the lower troposphere. Due to lack of 6-hourly outputs, we cannot apply the cyclone tracking algorithm to the other 8 CCSM3 realizations. Based on the linkage between the mean state change and the cyclone frequency anomalies, it is likely a common feature among the other ensemble members that cyclone activity is reduced on the East Coast and in Alaska as a result of global warming.

Figure caption: 9-member ensemble mean a) 300 hPa zonal wind anomalies and b) 780 hPa Eady growth rate maximum in JFM 2080-2099 relative to 1980-1999. Stippling indicates at least 6 out of the 9 members agree on the sign of the anomalies.