Objective Assessment of Extratropical Weather Systems in Simulated Climates

Abstract

An automated weather system identification and tracking scheme is used to appraise the skill of the CSIRO9 GCM in replicating contemporary extratropical cyclone and anticyclone behavior, and to assess possible changes as a result of doubled CO2. Cyclones are identified as centers of cyclonic vorticity rather than pressure minima, which can vanish if the background pressure gradient increases. Comparison with an observational dataset from ECMWF revealed that the GCM control simulation realistically reproduced the present-day storm track locations, but with slightly fewer and generally weaker systems overall. These errors are consistent with the coarser resolution of the GCM and its underestimation of the strength and baroclinicity of the polar vortex in both hemispheres. Comparison between 1 and 2 ? CO2 GCM simulations revealed increases in both 500-hPa geopotential height and 1000?500-hPa thickness for doubled CO2. As in other studies, these changes are largest near the poles, resulting in weaker westerlies and reduced tropospheric baroclinicity. Decreases of 10%?15% in both cyclone and anticyclone activity consistent with these circulation changes are found. However, there is some evidence of increased winter cyclone activity near the downstream end of the principal storm tracks. There is also a general reduction in the number and strength of intense storms, despite generally lower central pressures, which arise from global-scale decreases in sea level pressure in the doubled CO2 atmosphere rather than from greater storm vigor. This underscores the need for GCM projections of midlatitude ?storminess? to employ more realistic measures of storm activity and intensity.