1. The Indian Summer Monsoon and Climate
Change
Andrew Turner
with Pete Inness & Julia Slingo
RMetS meeting, Wednesday 20 June 2007
2. Introduction
Indian summer monsoon affects the lives of more
than 2 billion people across South Asia, and
provides more than 75% of total annual rainfall.
Agricultural and, increasingly, industrial
consumers require reliable source of water,
together with an appropriate forecast on seasonal
and intraseasonal timescales.
How monsoon characteristics may change in the
future is a key goal of climate research.
4. Model set-up
Hadley Centre coupled model HadCM3 run at
high vertical resolution (L30).
This better represents intraseasonal tropical
convection1 and has an improved atmospheric
response to El Niño2.
1P.M. Inness, J.M. Slingo, S. Woolnough, R. Neale, V. Pope (2001). Clim. Dyn. 17: 777--793.
2H. Spencer, J.M. Slingo (2003). J. Climate 16: 1757--1774.
Control (1xCO2) and future climate (2xCO2)
integrations used to test the impact of increased
GHG forcing.
Further integration of each climate scenario to test
the role of systematic model biases.
5. 2xCO2 response of HadCM3
Summer climate of HadCM3 2xCO2 Response to 2xCO2
6. The monsoon in IPCC AR4 models
Annamalai et al. (2007):
Of the six AR4 models which reasonably
simulate the monsoon precipitation climatology
of the 20th century, all show general increases
in seasonal rainfall over India in the 1pctto2x
runs (including HadCM3 L19).
H. Annamalai, K. Hamilton, K. R. Sperber (2007). J. Climate 20: 1071--1092
7. Interannual variability
Exceptional seasons of persistent flood or
drought have devastating economic and
human consequences.
Interannual variability is projected to increase
at 2xCO2 (+24%), particularly through
increased likelihood of very wet seasons.
PDF of seasonal
rainfall over India
in HadCM3.
8. Intraseasonal variability
Intraseasonal monsoon variations are
arguably of most importance to local
populations, active and break events
bringing intense rains and short droughts to
monsoon regions.
The extended and
intense break of July
2002 contributed to
nationwide drought
with 19% reduction in
JJAS rainfall from
climatology.
Source: www.tropmet.res.in/~kolli/MOL
9. Intraseasonal variability
Changes to active-break cycles at 2xCO2:
break events
Break events defined where AIR
daily precip falls 1σ below the
mean.
More intense break events over
India at 2xCO2 (and active events,
not shown).
Various indices tested.
Break event precipitation
anomalies to annual cycle:
2xCO2 minus 1xCO2
Caveats?
10. Intraseasonal variability
Changes to heavy precipitation
Levels of heavy
precipitation
increase at upper
percentiles in 2xCO2
climate.
Changes are
beyond those due to
the change in mean
precipitation.
Precipitation values at upper percentiles
1xCO2 2xCO2
11. Model set-up
Hadley Centre coupled model HadCM3 run at
higher vertical resolution (L30 vs. L19).
This better represents intraseasonal tropical
convection1 and has an improved atmospheric
response to El Niño2.
1P.M. Inness, J.M. Slingo, S. Woolnough, R. Neale, V. Pope (2001). Clim. Dyn. 17: 777--793.
2H. Spencer, J.M. Slingo (2003). J. Climate 16: 1757--1774.
Control (1xCO2) and future climate (2xCO2)
integrations used to test the impact of increased
GHG forcing.
Further integration of each climate scenario to test
the role of systematic model biases.
12. Systematic biases in HadCM3
Summer climate of HadCM3 1xCO2 HadCM3 minus observations
13. Flux adjustments at 1xCO2
Flux adjustments are
calculated by relaxing Indo-
Pacific SSTs back toward
climatology in a control
integration.
The heat fluxes required for
the relaxation are saved and
meaned to form an annual
cycle.
Annual cycle applied to the
equatorial band of a new
integration*.
Annual Mean
Amplitude of annual cycle
* After: P.M. Inness, J.M. Slingo, E. Guilyardi, J. Cole (2003). J. Climate 16: 365-382.
14. Systematic biases in HadCM3
& their reduction in HadCM3FA
Maritime Continent cooled; cold tongue warmed
Coupled response: reduced trade wind errors and monsoon jet
Reduced convection over Maritime Continent & other precip errors opposed
HadCM3 minus observations HadCM3FA minus HadCM3
Results from A.G. Turner, P.M. Inness, J. M.
Slingo (2005) QJRMS 131: 781-804
15. Flux adjustments at 2xCO2
Assume systematic biases will still be
present in the future climate.
Assume that the adjustments necessary to
correct these biases will be the same.
Same annual cycle of flux adjustments used
at 2xCO2 (in common with previous studies
where adjustments were necessary to
combat drift).
16. 2xCO2 response of HadCM3
Summer climate of HadCM3 2xCO2 Response of HadCM3 2xCO2
17. 2xCO2 response of HadCM3FA
Summer climate of HadCM3FA 2xCO2 Response of HadCM3FA to 2xCO2
18. Monsoon precipitation response
Systematic bias
seems to mask full
impact of changing
climate
Taken from A.G. Turner, P.M. Inness, J.M. Slingo (2007). QJRMS, accepted, due out soon
19. Monsoon-ENSO teleconnection:
lag-correlations
Flux adjustments have dramatic impact on the
teleconnection, particularly when measured by Indian rainfall.
The impact of increased GHG forcing is less clear but the
teleconnection is generally robust.
DMI Indian rainfall
20. Summary
Projections of the future climate show robust /
enhanced mean monsoon consistent with other
modelling studies.
Intraseasonal and interannual modes of variation
are more intense at 2xCO2, potentially leading to
greater impacts of the monsoon on society.
Systematic model biases may be masking the
true impact of increased GHG forcing.
The monsoon-ENSO teleconnection, useful for
seasonal prediction, remains robust. Indeed
model error has more impact.
