Differences in the frequency and the distribution of intense extratropical cyclone events in a model simulated doubled CO2 environment

A comparison of 20-year simulations employing the GFDL_CM2.0 (model) are run in order to determine the effects a doubled CO₂ environment (PCTTO2x) has on the population, distribution, and large-scale characteristics of intense cyclone events (ICEs) in the NH. ICEs in this study are NH extratropical cyclones with a central pressure of 970 hPa or less for at least a 24-hour period. It is found that changes to the frequency and distribution of ICEs prove to dramatically alter the heat and moisture transports throughout the NH. Through an investigation of the synoptic environments of the ICEs, and of the characteristics of the ICEs themselves, there is a suggestion in the PCTTO2x experiment, that the frequency, ferocity, and/or developmental mechanism of the strongest NH mid-latitude cyclones likely will be effected. The desire to recognize changes in ICEs results in the development of a cyclone tracking and counting process that was applied to the model output for both experiments. Climatologies of the synoptic environments in which the ICEs occurred and statistical relationships between the experiments were invoked as the primary diagnostic tools. To fully appreciate the 4% increase in the number of ICEs and ICE days in the PCTTO2x experiment, isolating their locations and distributions is vital. The majority of ICEs fall within the NH storm track (NHST), which appears to have northward shift and an eastward extension especially in the North Pacific in the PCTTO2x experiment. By far, the vast majority (96%) of all ICEs for both experiments occur within the calendar months of October through March and is termed the “6-month winter.” There appears to be a seasonal shift of ICEs from the latter months (JFM) of this period to the earlier months (OND) in the PCTTO2x experiment. December and January, however, undoubtedly are the months of most frequent ICE occurrence for both experiments. By examining the synoptic environment in which the ICEs develop, the causes behind the differences in ICE distribution and intensity in the PCTTO2x experiment can be determined. A reduced pole-to-equator temperature gradient in the global warming scenario presumably leads to reduced meridional baroclinicity and decreased poleward heat and moisture transport, especially at higher latitudes. A warmer climate induces a moister atmosphere to sustain the ICEs, especially in the low levels of the atmosphere. Decreased sea-ice at high latitudes in a doubled CO₂ environment dramatically alters the latent heat flux from the ocean to the atmosphere from near zero to much larger values. These mechanisms that promote ICE distribution and intensity differences between the two experiments are competing effects. The relative importance of each of these causes in the PCTTO2x and PreIndCtl environments requires a unique combination for the development, maintenance, and decay of ICEs in each type of environment.