The effect of doubled CO2 and model basic state biases on the monsoon-ENSO system

Andrew Turner, P.M. Inness & J.M. Slingo

IUGG meeting: JMS011 Monsoon Systems

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.

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.

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*).

seasonal rainfall PDF

Increased likelihood of very wet seasons.

DMI (JJAS)

* P.J. Webster & S. Yang (1992). QJRMS 118: 877—926.

1xCO2

2xCO2

Interannual variability

Greater difference between extreme monsoon seasons at 2xCO2.

1xCO2

2xCO2

strong-weak monsoon precip and 850hPa wind

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.

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.

Systematic biases in HadCM3

Summer climate of HadCM3 1xCO2 HadCM3 minus observations

Monsoon-ENSO teleconnection: lag-correlations

The monsoon-ENSO teleconnection is weak and mis-timed in HadCM3.

DMI (JJAS) vs. Niño-3

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.

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

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

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

Monsoon-ENSO teleconnection: moving correlations

Variations of correlation strength in models are of similar amplitude to those seen in observations despite fixed CO2 forcing.

See also AR4 models in Annamalai et al. (2007).

HadISST vs. All-India gauge data

rainfallDMI

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!