Modelled ENSO-driven precipitation changes under global warming in the tropical Pacific

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1 Modelled ENSO-driven precipitation changes under global warming in the tropical Pacific Christine Chung Scott Power Julie Arblaster Harun Rashid Greg Roff

2 ACCESS SST-forced experiments Annually-repeating SSTs with added Composite El Nino SST anomalies (Chung et al 2014, Clim Dyn) Composite La Nina SST anomalies (Chung & Power 2014, Clim Dyn) Zonal SPCZ (1982/1983 & 1997/1998) SST anomalies (Chung & Power, J Clim, accepted) Time-varying SSTs ( ) (in prep) All experiments run for 20C and 21C SSTs 21C SSTs = 20C SSTs + CMIP3 MMEM warming pattern CO2 increased for 21C runs (346 -> 730 ppm)

3 Annually repeating SST experiments - anomalies Anomalies added to climatological SSTs (Hurrell et al 2008) Anomaly pattern multiplied 1-4 x

4 Annually repeating SST experiments EN LN 0EN 0LN 1EN 1LN 2EN 2LN 3EN 3LN 4EN 4LN

5 Annually repeating SST experiments EN LN GW increases rainfall & eastward shift along equator, enhances southward shift of ITCZ -> effects of EN enhanced across tropical Pacific GW increases rainfall along equator, enhances drying of ITCZ & southward shift of SPCZ but effects of LN reduced over central-eastern equatorial Pacific

6 Annually repeating SST experiments zonal SPCZ events (82/83 & 97/98) GPCP Model (3EN) Only increasing amplitude of canonical EN SST anomalies does not accurately reproduce zonal SPCZ events

7 Zonal SPCZ events - Strong EN SST anomalies What is the difference between the 3xEN SST anomaly and the observed 82/83/97/98 SST anomalies? 3xEN SSTA - 82/83/97/98 SST = SSTR

8 Zonal SPCZ events - Modelled precip response 3EN 3EN + 0.3xSSTR 3EN + 0.6xSSTR 3EN + 1xSSTR 3EN + 1.3xSSTR

9 Zonal SPCZ events - Modelled precip response clim clim + 0.3xSSTR clim + 0.6xSSTR clim + 1xSSTR clim + 1.3xSSTR

10 Zonal SPCZ events rainfall profiles ITCZ SPCZ

11 Time-varying SST experiments Annually repeating SST experiments have limitations: Soil moisture buildup Can't analyse time evolution of events Time-varying SST experiments: 10-member ensemble using SSTs 20C and 21C Annually repeating Time-varying

12 Time-varying SSTs Neutral All EN All LN

13 Changes in precip anomalies 20c anoms 21c anoms 20c anoms In 21C, effect of EN/LN increased over western equatorial Pacific, but decreased over eastern Pacific between 0-10N Change in EN anomalies consistent with CMIP3 MMEM (Power et al 2013)

14 Summary Linear increases in EN/LN SST anomalies can generate large nonlinear rainfall responses in the tropical Pacific, more so for EN events EN eastward shift of max precip over equator, nonlinear increase in rainfall, SPCZ more (but not fully) zonal LN westward shift of max precip over equator, ITCZ dries, SPCZ more diagonal Precipitation response is sensitive to warming pattern, less sensitive to CO2 The spatial structure (not just amplitude) of the SST anomalies plays a key role in generating zonal SPCZ events When GW SST pattern is added Rainfall along the equator increases, especially over the western Pacific ITCZ moves southward, drying to the east of the SPCZ EN/LN intensified over west & central Pacific

15 Christine Chung Thank you Thank you

16 EOF analysis zonal SPCZ Increase in amplitude of events, but no significant increase in frequency of zonal SPCZ events Causes - MMEM warming pattern insufficient? Coupling needed?