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

and interannual variability

Andrew Turner,

Pete Inness, Julia Slingo

NCAS-Climate, University of Reading, UK

Motivation #1

• How will the Asian summer monsoon (which affects more than 2 billion people) change with increased greenhouse gas forcing?

The model & datasets• HadCM3 -atmosphere 3.75°lon x 2.5°lat -ocean 1.25° x 1.25°• L30 used rather than L19 - more realistic

intraseasonal tropical convection (Inness et al. 2001) and better convective response to high SSTs (Spencer & Slingo 2003).

• 100 year integrations at pre-industrial CO2 (control) and 2xCO2.

• ERA-40 Reanalysis (1958-1997).• All –India Rainfall (AIR) gauge dataset;

Parthasarathy et al. (1994).

The effect of climate change

HadCM3

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

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

HadCM3

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

HadCM3

Motivation #2

• Correct simulation of the basic state in the tropics essential for accurate seasonal prediction of precipitation variability (Sperber & Palmer 1996).

• Systematic biases could have an enormous influence on predictions of the future climate (Federov & Philander 2000).

• Test the effect of systematic biases at 2xCO2 using limited area heat-flux adjustments.

Heat flux adjustments• Traditionally used in older models (e.g. HadCM2)

to prevent climate drift; HadCM3 does not have this problem.

• Used here to counteract biases in the mean state.

• 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

Amplitude of annual 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

Heat flux adjustments

• Same heat flux adjustments used as in 1xCO2 experiment (Turner et al. 2005).

• Assume that systematic model biases will remain consistent (there is no dataset for comparison).

• 100 year integrations of HadCM3FA compared at 1xCO2, 2xCO2.

The effect of climate change

HadCM3FA

HadCM3

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

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

Monsoon & ENSO variabilityHadCM3

HadCM3

HadCM3FA

HadCM3FA

1.221.51

2.052.17

0.941.05

1.211.32

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

Instantaneous correlation of summer (JJAS) Indian rainfall with Nino-3 SST

(in 21-year moving window )

Consistent with the findings of Annamalai et al. 2006

Summary

Future monsoon simulation:

• Tendency to stronger monsoons in future climate scenario, irrespective of flux correction.

• Increased interannual variability using both dynamic and rainfall indices.

• Increased climate change signal when biases are removed.

SummaryFuture monsoon-ENSO relationship:

• Monsoon-ENSO teleconnection more susceptible to bias removal than greenhouse warming.

• Stronger biennial character to flux-adjusted future ENSO.

• Large amplitude variations across decadal timescales under fixed CO2 forcing suggest recent changes in the observed record may not be due to climate change.