The effect of mid-latitude SST on East African summer rains

1. The effect of mid-latitude SST on East African summer rains Gulilat Tefera Diro, David Grimes and Emily Black Africa group meeting- February 25, 2008

2. Outline • Observational approach • Relation between rainfall and global SST • Correlation and Composite analysis • Possible mechanisms for the observed teleconnections between mid-latitude Pacific SST and rainfall • Modelling approach • Observed SST forced AGCM (to assess the model capabilities) • Composites of SST based on model rainfall anomaly • Idealised SST experiment • Does the mid-latitude SST directly affect East African summer rain?

3. Composites Observation: Relation between SST and rainfall Correlation

4. The link between SST and rainfall via large scale features • Both excess (deficit) rains and warm (cold) SST are associated with a northward • (southward) shift in the ITCZ. • The mid latitude SSTA during September is linked to rainfall over Ethiopia via the north/south displacement of ITCZ.

5. Low level Link • Both excess/deficit rains and warm/cold SSTs are associated with • Strong EALLJ • Strong monsoon trough • Westerly anomaly over Northeast Africa

6. Warm/cold Mid-latitude northwest Pacific SST - north/south shift in ITCZ • Strong/weak EALLJ • Strong/weak westerly over northeast Africa • Strong/weak Azores high (NAO) Excess/deficit rainfall

7. Modelling approach Observed SST forced HadAM3 • HadAM3 (3.752.5, 19L) was used to investigate the observed link between SST and rainfall

8. Relation between model rainfall and observed SST Question: does the model (HadISST forced HadAM3) rainfall anomaly also linked to the mid-latitude Pacific SST anomaly?

9. Does the mid latitude SST directly affect rainfall? • Idealised SST experiments: • The control run: the AGCM is forced with climatological SST • The warm SSTA run: the model is forced with an idealised warm SSTA over the mid-latitude Pacific and climatological SST elsewhere • The cold SSTA run: the model is forced with an idealised cold SSTA pattern over the northwest mid-latitude Pacific and climatological SST elsewhere The SSTA imposed in June

10. Result from the idealised Experiment • Impact on the total Kiremt (JJAS) rainfall • The mid-latitude northwest Pacific has an impact on the total JJAS rainfall over NE Africa although it is a bit far north.

11. Result from the idealised Experiment • The impact of mid-latitude SSTA on the total rainfall • Cold SSTA over mid-latitude northwest Pacific causes a southward shift in the ITCZ over Africa

12. The impact on the sub-seasonal rainfall • The mid-latitude northwest Pacific SST directly affect September rainfall • This implies it affects the total rainfall by affecting early cessation of the rainy season

13. Summary • The mid-latitude northwest Pacific in September correlates significantly with the eastern Ethiopian summer rainfall • This has an implication that the mid-latitude SST can be used as forecasting the cessation of Kiremt rainfall. • Observational study suggests that the SST and rainfall are linked by among other things via the ITCZ • The modelling study suggests that the mid-latitude northwest Pacific affects the rainfall over Africa via the north-south shift in the ITCZ. • The modelling study also suggest that mid-latitude northwest Pacific SST directly affect the eastern Ethiopian rainfall during September. • This has an important implication that the mid-latitude SST can be used as a predictor to forecast the early cessation of the rainy season