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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. Outline. Observational approach Relation between rainfall and global SST Correlation and Composite analysis

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the effect of mid latitude sst on east african summer rains

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

outline
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?
the link between sst and rainfall via large scale features
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.

low level link
Low level Link
  • Both excess/deficit rains and warm/cold SSTs are associated with
    • Strong EALLJ
    • Strong monsoon trough
    • Westerly anomaly over Northeast Africa
slide6
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

modelling approach
Modelling approach

Observed SST forced HadAM3

  • HadAM3 (3.752.5, 19L) was used to investigate the observed link between SST and rainfall
relation between model rainfall and observed sst
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?

does the mid latitude sst directly affect rainfall
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

result from the idealised experiment
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.
result from the idealised experiment11
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

the impact on the sub seasonal rainfall
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
summary
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