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warmer T, but the dynamical structure is unlike the present-day cases and in fact delivers a
smaller 3-day P total (112 mm) than the present-day event simulated in Fig.
8
b (152 mm).
While this is just an illustration, it nevertheless highlights the potential importance of both
dynamical and thermodynamic factors in influencing future changes in extreme rainfall. The
thermodynamic climate change component of such events is likely to be robust with the
Clausius-Clapeyron relation a reliable constraint. However, small changes in the jet stream
and storm track regions (and the overall dynamical character of intense rainfall-producing
events) may dominate the local response and therefore regional projections of the occurrence
of damaging flooding and drought remain a substantial challenge.
4 Conclusions
Global precipitation is projected to rise in the future primarily to maintain balance with
enhanced atmospheric radiative cooling as temperatures increase (Manabe and Wetherald
1975
). This slow, well-constrained response of around *2-3 %/K is modulated by more
rapid adjustments to the radiative forcings, that are themselves responsible for current and
future warming, but which directly influence atmospheric radiative cooling (Andrews et al.
2010
). An additional influence appears to relate to how far from equilibrium the climate
system is (McInerney and Moyer
2012
). This is governed by the differing timescales of
adjustment by the land and ocean and the associated changes in energy and moisture fluxes
between them (Cao et al.
2012
).
Regional changes in the hydrological cycle, of more importance for climate impacts, are
more strongly tied to (i) changes in moisture transports which are well constrained by the
Clausius-Clapeyron equation linking saturation vapor pressure with temperature and (ii)
small spatial movements in large-scale circulation systems which are highly uncertain.
This also applies to the local, extreme precipitation events which are strongly linked to the
rises in low-level moisture of around 7 %/K but are also influenced by changes in the
nature and spatial distribution of intense rainfall events.
The combination of the global changes in precipitation of around *2-3 %/K and
increased low-level moisture of *7 %/K leads to a general trend toward the wet regions
(Inter Tropical Convergence Zone and higher latitudes) becoming wetter and the dry
subtropics getting drier. In the margins, although projected responses appear ambiguous,
there are physical grounds for anticipating relatively small changes in P, signifying a
greater consensus on regional projections than previously thought (Power et al.
2011
).
Nevertheless, local changes are highly dependent upon small shifts in position of large-
scale atmospheric circulation patterns and in the physical nature of rainfall regimes, in
particular for extremes. Hence, determining accurate responses in the hydrological cycle at
the scales required for impact models may be beyond the predictive capacity of climate
modelling. Thus, determining robust, large-scale, robust responses in the hydrological
cycle (e.g., Martin
2012
) remain a crucial tool in understanding and planning for a
changing water cycle in the future.
Acknowledgments The comments and suggestions of Paul O'Gorman and two anonymous reviewers
helped to improve the paper. This work was undertaken as part of the PAGODA and PREPARE projects
funded by the UK Natural Environmental Research Council under grants NE/I006672/1 and NE/G015708/1
and was supported by the National Centre for Earth Observations and the National Centre for Atmospheric
Science. A. Bodas-Salcedo was supported by the Joint DECC/Defra Met Office Hadley Centre Climate
Programme (GA01101). GPCP v2.2 data were extracted from
http://precip.gsfc.nasa.gov/gpcp_v2.2_data.
html
,
and CMIP5 and AMIP5 data sets from the British Atmospheric Data Centre (
http://badc.nerc.ac.uk/
