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. Observation : Relation between SST and rainfall Correlation Composites

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 Cold SSTA over mid-latitude northwest Pacific causes a southward shift in the ITCZ over Africa The impact of mid-latitude SSTA on the total rainfall

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