The effect of mid-latitude SST on East African summer rains
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Transcript of 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
Gulilat Tefera Diro, David Grimes and Emily Black
Africa group meeting- February 25, 2008
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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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Observation: Relation between SST and rainfallCorrelation
Composites
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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 (3.752.5, 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
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
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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