Geoscience Reference
In-Depth Information
1 Introduction
Recent occurrence of extreme weather across the globe (including flooding and drought)
relates to shifts in atmospheric and oceanic circulation patterns (e.g., the mid-latitude jet
stream or El Ni ˜o Southern Oscillation, ENSO) yet heightens concern upon the potential
for human influence on and vulnerability to such events (e.g., Peterson et al.
2012
). The
frequency of damaging flooding and drought are projected to increase in the future based
upon detailed computer simulations of the climate system (Meehl et al.
2007
) and backed
up by basic physics (Held and Soden
2006
; O'Gorman et al.
2012
); it is also vital that
projected responses of the water cycle are verified, where possible, by careful use of
homogeneous, well-characterized observations (Trenberth
2011
).
The magnitude and rate of change in the regional hydrological cycle determine the
impacts suffered by infrastructure, agriculture and health. The distribution of rainfall (in
time and space), in particular for the extremes, is crucial in determining damage from
particular events. Time and space means of hydrological quantities may reflect aspects of
these changes, in particular for the amount of water potentially available for a region.
However, the relevance of changes in global mean quantities to local impacts is unclear.
Yet, without appreciation for the driving mechanisms at the largest spatial and temporal
scales, the robust nature of local projections is questionable at best. In this paper, our aim is
to identify the most important, robust and physically understandable responses in the
atmospheric hydrological cycle, exploiting climate model simulations and confronting
these with the globally available observational record.
2 Constraints upon Global Mean Precipitation Responses
The most robust climatic response to increasing temperatures is a rise in mean water vapor
near to Earth's surface at *7 %/K, in line with the Clausius-Clapeyron equation (see
Sect.
2.3
). Although changes in moisture are an important constraint upon regional changes
in precipitation and its extremes (discussed in Sect.
3
), it has been known for some time
that the total amount of precipitation (P) increases with warming at a slower rate than
water vapor (*2-3 %/K), responding instead to a changing heat balance of the atmosphere
(Manabe and Wetherald
1975
; Mitchell et al.
1987
; Allen and Ingram
2002
). The primary
physical basis for this is that a warming atmosphere radiates energy away more effectively,
particularly to the surface (e.g., Allan
2006
; Stephens and Ellis
2008
; Prata
2008
).
To maintain energy balance in the atmosphere, this additional atmospheric net radiative
cooling to space and to the surface (DQ
atm
) is primarily compensated for by extra latent
heating via precipitation (LDP) with changes in sensible heating of the atmosphere by the
surface, DSH, playing a more minor role (but see Lu and Cai (
2009
)):
LDP
¼
DQ
atm
DSH
;
ð
1
Þ
Surface evaporation (E) is similarly constrained by energy balance; Richter and Xie (
2008
)
find that climate models simulate small adjustments to boundary layer temperature,
humidity and wind speed that cause E to increase with warming below the rate expected
from the Clausius-Clapeyron equation (Lu and Cai
2009
) and almost identically to the
P changes (as expected from the trivial moisture holding capacity of the atmosphere
relative to the moisture fluxes).
As discussed by O'Gorman et al (
2012
), radiative cooling above the lifting condensa-
tion level is in fact more directly related to precipitation since, when heating is applied
