Interannual variations and future change of wintertime extratropical cyclone activity over North America in CCSM3 |
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| Authors: | Haiyan Teng Warren M Washington Gerald A Meehl |
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| Institution: | (1) National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307, USA |
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| Abstract: | Climatology and interannual variations of wintertime extratropical cyclone frequency in CCSM3 twentieth 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. As in the reanalysis, frequency of cyclones stronger than 980 hPa shows significant correlation with the
Pacific/North America (PNA) teleconnection pattern over the Pacific region and with the North Atlantic Oscillation (NAO) in
the Atlantic sector. Composite maps are constructed for opposite phases of El Nino-Southern Oscillation (ENSO) and the NAO
and all anomalous patterns coincide with observed. One CCSM3 twenty-first 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 eight 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. |
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