Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales
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Oceanic forcing of Sahel rainfallon interannual to interdecadal time scales
A. Giannini (IRI)R. Saravanan (NCAR) and P. Chang (Texas A&M)
IRIfor climate prediction
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Reflections on:
Seasonal climate prediction- sensitivity of climate to boundary conditions
Recent trends in climate- are they related to global warming?
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r = 0.60
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The model used is NSIPP1(version 1, NASA Seasonal to Interannual Prediction Project)
Bacmeister et al 2000; Moorthi and Suarez 1992;Koster and Suarez 1992; Takacs and Suarez 1996
nsipp.gsfc.nasa.gov
Results shown here are from the ensemble-meanof a 9-member ensemble of integrations
forced over 1930-2000 with:1) observed, interannually-varying
sea surface temperature (SST); 2) constant atmospheric CO2 (350ppm); and
3) vegetation cover prescribed to varyseasonally, but not interannually
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Outline
-statistical analysis-local land-atmosphere dynamics-tropical ocean teleconnections
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Statistical analysis
Principal Component Analysisof July-September precipitationover tropical Africa (1930-2000)
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spatial signature of JAS African rainfall variabilityG
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temporal signature and relation to surface temperature
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land-atmosphere interaction
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precipitation surface temperature
vertically-integrated moisture convergence evaporation
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surface temperature variability (1930-2000)
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Land surface-atmosphere interaction acts to amplify remotely-forced precipitation anomalies:
-through local recycling of moisture, and -by modifying moisture convergence patterns
A negative trend in precipitation is consistentwith a positive trend in land surface temperature
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forcing from the global oceans
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(Sahel rainfall is sensitive to global, tropical SSTs)
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long-term v. interannual variability
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ENSO's impact on Northern Hemisphere summer climate
Ropelewski and Halpert 1987Yulaeva and Wallace 1994Chiang and Sobel 2002Neelin et al 2003
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Long-term trends and African climate variability
Webster 1972Gill 1980Matsuno 1966
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Response to diabatic heating in the equatorial Indian Ocean
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Conclusions:
Variability in Sahel rainfall on interannual to interdecadal time scalesis strongly influenced by SST variability (ENSO and warming trends);
The current generation of state-of-the-art atmospheric general circulation models displays skill in the prediction of Sahel rainfallwhen forced with 'perfect' SST anomalies;
-how do we improve prediction of SSTs? (is it a coupled ocean-atmosphere problem?)
How do we incorporate consideration of climate trends, e.g.trends attributed to climate change, in our predictions?
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What next?
How can we improve predictions of sea surface temperature?
How are the African and Indian monsoonsconnected?
How is global warming going to affecttropical rainfall?