the effect of doubled co 2 and model basic state biases on the monsoon- enso system andrew turner,...
TRANSCRIPT
The effect of doubled CO2 and model basic state biases on the monsoon-ENSO system
Andrew Turner,
Pete Inness, Julia Slingo
Walker Institute / NCAS-Climate
University of Reading, UKNCAS Advisory Group 30 January 2007
How will the Asian summer monsoon, crucial to the lives of more than 2 billion people, change with increased greenhouse gas forcing?
Motivation #1
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
summer (JJAS) 850hPa wind differences: 2xCO2-1xCO2
The effect of climate change
HadCM3
summer (JJAS) precipitation differences: 2xCO2-1xCO2
The effect of climate change
HadCM3
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.
Motivation #2
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 towards 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, and central Pacific.
Improvements to the mean state
HadCM3FA 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
summer (JJAS) surface temperature differences: 2xCO2-1xCO2
HadCM3
HadCM3FA
The effect of climate change
summer (JJAS) 850hPa wind differences: 2xCO2-1xCO2
HadCM3FA
HadCM3
The effect of climate change
summer (JJAS) precipitation differences: 2xCO2-1xCO2
HadCM3
HadCM3FA
Monsoon & ENSO variability
HadCM3
HadCM3
HadCM3FA
HadCM3FA
1.221.51
2.052.17
0.941.05
1.211.32
The teleconnection
Lag-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).
Turner et al. (2007a) submitted, Q. J. R. Meteorol. Soc.
Decadal variations
The ability to change regimes?
See Turner et al. (2007b), submitted Q. J. R. Meteorol. Soc.
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.
Summary
Future 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.