Transcript Document

IUGG meeting: JMS011 Monsoon Systems
The effect of doubled CO2 and model basic
state biases on the monsoon-ENSO system
Andrew Turner, P.M. Inness & J.M. Slingo
Introduction
Indian summer monsoon is vital to the lives of
more than 2 billion people across South Asia
through agricultural and, increasingly, industrial
users of water.
How characteristics of the mean monsoon and its
variation on different timescales may change in
the future is a key goal of climate research.
Changing predictability of the monsoon through its
teleconnection to ENSO must also be addressed.
Outline
Introduction
Model framework
Climate change and the mean monsoon
Interannual variability
How do systematic model biases affect the
result?
The monsoon-ENSO teleconnection
Model set-up
Hadley Centre coupled model HadCM3 run at
high vertical resolution (L30) which better
represents intraseasonal tropical convection1 and
has an improved atmospheric response to El
Niño2.
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.
1P.M.
2H.
Inness, J.M. Slingo, S. Woolnough, R. Neale, V. Pope (2001). Clim. Dyn. 17: 777--793.
Spencer, J.M. Slingo (2003). J. Climate 16: 1757--1774.
2xCO2 response of HadCM3
Summer climate of HadCM3 2xCO2
Response to 2xCO2
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
Interannual variability
Interannual variability is projected to increase
at 2xCO2 (+24% using Webster-Yang
dynamical index*).
DMI (JJAS)
seasonal rainfall PDF
1xCO2
2xCO2
Increased likelihood of very wet seasons.
* P.J. Webster & S. Yang (1992). QJRMS 118: 877—926.
Interannual variability
Greater difference between extreme monsoon
seasons at 2xCO2.
strong-weak monsoon precip and 850hPa wind
1xCO2
2xCO2
Model set-up
Hadley Centre coupled model HadCM3 run at
higher vertical resolution (L30), which better
represents intraseasonal tropical convection1 and
has an improved atmospheric response to El
Niño2.
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.
1P.M.
2H.
Inness, J.M. Slingo, S. Woolnough, R. Neale, V. Pope (2001). Clim. Dyn. 17: 777--793.
Spencer, J.M. Slingo (2003). J. Climate 16: 1757--1774.
Systematic biases in HadCM3
Summer climate of HadCM3 1xCO2
HadCM3 minus observations
Monsoon-ENSO teleconnection:
lag-correlations
DMI (JJAS) vs. Niño-3
The monsoon-ENSO teleconnection is weak and mis-timed
in HadCM3.
Flux adjustments at 1xCO2
Flux adjustments are
calculated by relaxing IndoPacific 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.
Systematic biases in HadCM3
& their reduction in HadCM3FA
Results from A.G. Turner, P.M. Inness, J. M.
Slingo (2005) QJRMS 131: 781-804
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
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, eg in HadCM2*).
* M. Collins (2000). J. Climate 13: 1299-1312.
2xCO2 response of HadCM3
Summer climate of HadCM3 2xCO2
Response of HadCM3 2xCO2
2xCO2 response of HadCM3FA
Summer climate of HadCM3FA 2xCO2
Response of HadCM3FA to 2xCO2
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, in press.
Monsoon-ENSO teleconnection:
lag-correlations
DMI
Indian rainfall
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.
Monsoon-ENSO teleconnection:
moving correlations
Variations of correlation strength
in models are of similar amplitude
to those seen in observations
despite fixed CO2 forcing.
HadISST vs. All-India gauge data
See also AR4 models in
Annamalai et al. (2007).
DMI
rainfall
Summary
Projections of the future climate show enhanced mean
monsoon consistent with other modelling studies.
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 than climate change.
Large amplitude variations occur in the modelled
monsoon-ENSO teleconnection despite fixed CO2 forcing.
Thank you!