Geoscience Reference
In-Depth Information
Changes in M
F
normalized by DT [kg/(s hPa K)] between cold and warm months are
plotted for ERA Interim in Fig.
5
a. This shows increased inflow at low levels (maximum at
*950 hPa), but a significant compensating increase in mid-level outflow at around 600-
700 hPa as discussed by Zahn and Allan (
2011
). A similar response is evident for twentieth
century (20C, 1959-1989) and twenty-first century (21C, 2069-2099) time slice simu-
lations from a high resolution (0.5) climate model, ECHAM5 (Roeckner et al.
2003
), as
described by Zahn and Allan (
2012
). However, the model simulations indicate a higher
altitude for the maximum changes in moisture outflow and less vertical structure, which
may be influenced by the higher vertical resolution below altitudes of 500 hPa in ERA
Interim. Back and Bretherton (
2006
) suggest that meridional SST gradients determine the
altitude of outflow regionally and the nature of the differences in Fig.
5
a merits future
analysis. Also shown in Fig.
5
b is the 21C minus 20C change in M
F
into the wet region of
the tropics, normalized by DT
ΒΌ
3
:
0K. The vertical structure of the climate change
response of M
F
is very similar to interannual variability shown in Fig.
5
a where year to
year changes in T are relatively small (DT
0
:
6 K).
3.2 Precipitation Response in the Wet and Dry Regions of the Tropics
It does not simply follow from (
6
) that P rises in the wet regions and declines in the dry
regions. Nevertheless, since DE changes are expected to be more spatially uniform than DP
changes, there is a strong expectation that the wet regions will become wetter and the dry
regions drier in the tropics, borne out by recent analysis of climate models (e.g., Chou et al.
2009
) and limited observational evidence (e.g., Allan et al.
2010
).
Figure
6
demonstrates that climate models indeed simulate an increase in P in the wet
regions ([70th percentile of P ordered by intensity, defined each month such that the
precise location of the wet region varies with time) and static or declining P in other (dry)
regions of the tropics. This definition of wet and dry regimes is based upon the analysis of
Allan et al. (
2010
) but is somewhat arbitrary. Mean P is about 8 mm/day in the wet regions
while the remaining regions are not completely dry (P * 1 mm/day). GPCP observations
(displayed since 1988 due to inhomogeneities before this date when SSM/I data was not
available) also show this contrasting wet/dry response as discussed by Liu et al. (
2012
).
Since this response is contingent on warming (enhancing P - E), this explains the flat-
tening off of P responses in the RCP4.5 simulations after around 2060 as T asymptotes.
Nevertheless, as discussed previously, the precise transient DP response is strongly
influenced by the fast forcing processes (f DF) in addition to the slow response to warming,
kDT (Andrews and Forster
2010
; Wu et al.
2010
).
3.3 Extremes of Precipitation
Within the wet regions, during heavy rainfall events, the atmosphere is typically precip-
itating more water over the course of a day than is contained in the atmospheric column at
a particular location; this underlines the vital role of moisture convergence in determining
intense rainfall rates (e.g., Trenberth
2011
). While low-level moisture changes are indeed
thought to provide a strong constraint upon P intensification in a warming world, changes
in updraft velocity with warming explain the large range in simulated responses in the
tropics (e.g., O'Gorman and Schneider
2009
; Allan et al.
2010
; Sugiyama et al.
2010
).
Observed relationships provide a powerful constraint upon future simulated responses in
extreme
precipitation
(O'Gorman
2012
).
Figure
7
displays
observed
and
simulated
