Environmental Engineering Reference
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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 argu-
ments 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 subtropi-
cal 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 green-
house 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 3°C 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 Sec-
tion 4.2, this is indeed the pattern of precipitation change that is found across
