The role of the basic state in the ENSO-monsoon relationship and

implications for predictability

Andrew Turner,Pete Inness, Julia Slingo

Motivation• Asian summer monsoon affects more than 2

billion people in India, China and the rest of Southeast Asia.

• Regional agriculture reliant on the timing, duration and intensity of the ASM – GCMs increasingly used to predict these details.

• State of equatorial Pacific SSTs long regarded as an important predictor of the monsoon (e.g. Charney and Shukla, 1981).

• Coupled GCMs generating mean climate closer to observations are more likely to correctly simulate the interannual variability of tropical precipitation (Sperber and Palmer, 1996).

The model & datasets• HadCM3 3.75lon x 2.5lat (~T42). 100 year

integration.• L30 used rather than L19 - more realistic

intraseasonal tropical convection (MJO) and precip response to high SSTs (Inness et al., 2001; Spencer & Slingo, 2003).

• ERA-40 Reanalysis (1958-1997).• CMAP for tropical precipitation 1979-1997; Xie

and Arkin, 1997.• All –India Rainfall (AIR) gauge dataset;

Parthasarathy et al., 1994.

What’s wrong with the model?Summer DMI lag-correlated with Nino-3 SSTs

Mean summer surface temperature

HadCM3 mean summer (JJAS)

differences with ERA-40

Mean summer (JJAS) 850mb winds

HadCM3

differences with ERA-40

Mean summer (JJAS) precipitation

HadCM3

differences with CMAP

Heat flux adjustments• Traditionally used in older models (eg HadCM2) to

prevent climate drift; HadCM3 does not have this problem.

• Heat flux adjustments used here to study the effect of mean state error on the monsoon-ENSO system.

• Devised by Inness et al. (2003) to investigate the role of systematic low-level zonal wind and SST errors on the MJO.

• Coupled model run for 20 years, Indian and Pacific SSTs within 10S-10N relaxed back to climatology.

• Anomalous heat fluxes generate a mean annual cycle which is applied to a new 100 year integration (HadCM3FA).

Heat flux adjustments

• Large fluxes (up to 186Wm-2 at 120W) into the cold tongue.

• Much smaller (~30W.m-2) over Maritime Continent and Indian Ocean.

Annual Mean

Standard deviation of cycle

• Small annual cycle apart from upwelling region off African coast.

Improvements to the mean stateHadCM3FA mean summer (JJAS) surface temperature

differences with HadCM3

HadCM3 differences with ERA-40

Improvements to the mean state

differences with HadCM3

HadCM3FA mean summer (JJAS) 850hPa winds

HadCM3 differences with ERA-40

Improvements to the mean stateHadCM3FA mean summer (JJAS) precipitation

differences with HadCM3

HadCM3 differences with CMAP

The monsoon-ENSO teleconnection

• Stronger and better timed teleconnection with flux adjustments.

Lag-correlation of summer (JJAS) DMI with Nino-3 SSTs

The monsoon-ENSO teleconnection

• Monsoons feed back on Pacific system to further intensify ENSO.

Lag-correlation of summer (JJAS) Indian rainfall with Nino-3 SSTs

• Indian rainfall shares similar teleconnection pattern.

• ERA-40 has poor representation when compared to gauge dataset.

• Stronger and better timed teleconnection with flux adjustments.

The monsoon-ENSO teleconnectionComposite evolution of equatorial Pacific total SSTs during El Nino

• 10 warm events composited from each model integration.

• Warmest waters (absolute SSTs) are further east, past the dateline.

• Convection and hence the rising branch of the Walker circulation is repositioned.

• Warmer mean state means that even weak El Ninos in HadCM3FA may drive the teleconnection.

• See Turner et al. (2005)

HadCM3 HadCM3FA

The effect of climate changesummer (JJAS) 850hPa wind differences: 2xCO2-1xCO2

HadCM3

HadCM3FA

The effect of climate changesummer (JJAS) precipitation differences: 2xCO2-1xCO2

HadCM3

HadCM3FA

The effect of climate change

HadCM3FA

HadCM3

summer (JJAS) surface temperature differences: 2xCO2-1xCO2

The teleconnectionLag-correlation of summer (JJAS) Indian rainfall with Nino-3 SSTs

Future ENSO?irregular period

biennial period

SummaryCurrent climate:• Flux adjustments, whilst having some drawbacks, can

help correct mean state and have beneficial effect on monsoon predictability.

• Stronger teleconnection; more realistic Walker circulation & El Nino development.

• Flux adjustments highlight the danger in assuming a linear system, anomaly forecasting etc.

Future climate:• Tendency to stronger monsoons in future climate

scenario, irrespective of flux correction.• The sign and timing of the monsoon-ENSO

teleconnection may not be robust.• Flux adjustment raises questions relating to the nature of

ENSO in future climate.