Transcript ENSO changes due to heat flux adjustment in current and future

The effect of doubled CO2 and model
basic state biases on the monsoonENSO 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
summer (JJAS) surface temperature differences: 2xCO2-1xCO2
HadCM3
The effect of climate change
summer (JJAS) 850hPa wind differences: 2xCO2-1xCO2
HadCM3
The effect of climate change
summer (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 10S-10N 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
Annual Mean
• Large fluxes (up to
186Wm-2 at 120W)
into the cold tongue.
• Much smaller
(~30W.m-2) over
Maritime Continent
and Indian Ocean.
Amplitude of annual cycle
• Small annual cycle
apart from upwelling
region off African coast.
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
HadCM3
HadCM3FA
The effect of climate change
summer (JJAS) precipitation differences: 2xCO2-1xCO2
HadCM3
HadCM3FA
Monsoon & ENSOHadCM3
variability
1.221.51
HadCM3FA
2.052.17
HadCM3
0.941.05
HadCM3FA
1.211.32
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 )
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.
Summary
Future monsoon-ENSO relationship:
• Monsoon-ENSO teleconnection more
susceptible to bias removal than
greenhouse warming.
• Stronger biennial character to fluxadjusted 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.