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

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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
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Motivation 1

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

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The model datasets

• HadCM3 -atmosphere 3.75lon x 2.5lat
• -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).

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The effect of climate change
summer (JJAS) surface temperature differences
2xCO2-1xCO2
HadCM3
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The effect of climate change
summer (JJAS) 850hPa wind differences 2xCO2-1xCO2
HadCM3
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The effect of climate change
summer (JJAS) precipitation differences
2xCO2-1xCO2
HadCM3
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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.

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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).

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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.

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Improvements to the mean state
HadCM3FA mean summer (JJAS) surface temperature
differences with HadCM3
HadCM3 differences with ERA-40
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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.

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The effect of climate change
summer (JJAS) surface temperature differences
2xCO2-1xCO2
HadCM3
HadCM3FA
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The effect of climate change
summer (JJAS) 850hPa wind differences 2xCO2-1xCO2
HadCM3
HadCM3FA
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The effect of climate change
summer (JJAS) precipitation differences
2xCO2-1xCO2
HadCM3
HadCM3FA
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Monsoon ENSO variability
HadCM3
1.22?1.51
HadCM3FA
2.05?2.17
HadCM3
0.94?1.05
HadCM3FA
1.21?1.32
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The teleconnection
Lag-correlation of summer (JJAS) Indian rainfall
with Nino-3 SSTs
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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
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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.

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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.