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

1
The role of the basic state in the ENSO-monsoon
relationship and implications for predictability

• Andrew Turner,
• Pete Inness, Julia Slingo

2
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).

3
The model datasets

• HadCM3 3.75?lon x 2.5?lat (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.

4
Whats wrong with the model?
Summer DMI lag-correlated with Nino-3 SSTs
5
Mean summer surface temperature
HadCM3 mean summer (JJAS)
differences with ERA-40
6
Mean summer (JJAS) 850mb winds
HadCM3
differences with ERA-40
7
Mean summer (JJAS) precipitation
HadCM3
differences with CMAP
8
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 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).

9
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.

Standard deviation of cycle

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

10
Improvements to the mean state
HadCM3FA mean summer (JJAS) surface temperature
differences with HadCM3
HadCM3 differences with ERA-40
11
Improvements to the mean state
HadCM3FA mean summer (JJAS) 850hPa winds
differences with HadCM3
HadCM3 differences with ERA-40
12
Improvements to the mean state
HadCM3FA mean summer (JJAS) precipitation
differences with HadCM3
HadCM3 differences with CMAP
13
The monsoon-ENSO teleconnection
Lag-correlation of summer (JJAS) DMI with Nino-3
SSTs

• Stronger and better timed teleconnection with
  flux adjustments.

14
The monsoon-ENSO teleconnection
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.

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

15
The monsoon-ENSO teleconnection
HadCM3
HadCM3FA
Composite 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)

16
The effect of climate change
summer (JJAS) 850hPa wind differences 2xCO2-1xCO2
HadCM3
HadCM3FA
17
The effect of climate change
summer (JJAS) precipitation differences
2xCO2-1xCO2
HadCM3
HadCM3FA
18
The effect of climate change
summer (JJAS) surface temperature differences
2xCO2-1xCO2
HadCM3
HadCM3FA
19
The teleconnection
Lag-correlation of summer (JJAS) Indian rainfall
with Nino-3 SSTs
20
Future ENSO?
21
Summary

• Current 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.