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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

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The effect of mid latitude sst on east african summer rains l.jpg

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 l.jpg

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?


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Composites

Observation: Relation between SST and rainfall

Correlation


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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.


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Low level Link

  • Both excess/deficit rains and warm/cold SSTs are associated with

    • Strong EALLJ

    • Strong monsoon trough

    • Westerly anomaly over Northeast Africa


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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


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Modelling approach

Observed SST forced HadAM3

  • HadAM3 (3.75ï‚´2.5, 19L) was used to investigate the observed link between SST and rainfall


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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?


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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


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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.


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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


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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


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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


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