ten taken as a rough measure of the destructive potential of storm winds (Knutson et al., 2010). Whether the frequency of the most intense (category 3-5) cyclones should be expected to increase is less certain, in that it is also affected by the potential for reduction in total cyclone numbers.

An increase in rainfall within tropical cyclones is projected by arguments based on the increase in atmospheric water vapor. Typical values in models are on the order of an 8% increase per degree warming for the rainfall within 100 km of the storm center (Knutson et al., 2010).

4.4
CIRCULATION AND RELATED FACTORS

As discussed in Section 4.2, increasing greenhouse gas concentration is expected to cause an increase in atmospheric moisture of about 7% per degree C global temperature increase. In general, this will lead to greater rainfall in the tropical monsoon regions and drier conditions in the subtropical regions. Wet regions over the tropical oceans should also see increases in precipitation, although the positioning of these centers may shift due to coupled atmosphere-ocean interactions. The increase in atmospheric moisture is also expected to increase precipitation in the mid-latitude storm track regions by roughly 5-10% per degree C global temperature increase; however, it is not clear that storminess will increase.

Arguments based on first principles also indicate that increasing greenhouse gas concentrations should cause the mid-latitude jets and attendant storm tracks to shift poleward (e.g., Hartmann et al., 2000). Since in the atmosphere the temperature increase due to increasing greenhouse gases is confined to the troposphere and the tropopause is higher in the tropics than in high latitudes, increased greenhouse gases causes the poleward pressure gradient at the tropopause in mid-latitudes to increase. This, in turn, causes changes in the wave-mean flow interactions that shift the jets and their associated storm tracks poleward. This poleward shift in the jets and stormtracks should persist as long as the greenhouse gas concentrations remain elevated. A poleward shift in the jets and stormtracks due to global warming is a robust feature of the climate models (Miller et al., 2006; Hegerl et al., 2007; Meehl et al., 2007), with a zonal mean poleward shift of about 100 km per 3ºC global temperature increase.

As a result of the poleward shift in the storm tracks, precipitation will likely increase on the poleward—and decrease on the equatorward side—of the location the climatological jets in the present climate. As noted in Section 4.2, this is indeed the pattern of precipitation change that is found across



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