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

The role of the basic state in the ENSO-monsoon relationship and implications for predictability. Andrew Turner, Pete Inness, Julia Slingo. Motivation. Asian summer monsoon affects more than 2 billion people in India, China and the rest of Southeast Asia.

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

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  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. What’s 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. irregular period biennial period 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.

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