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Published byMadeleine Chandler Modified over 9 years ago
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Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales A. Giannini (IRI) R. Saravanan (NCAR) and P. Chang (Texas A&M) IRI for 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-mean of 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 vary seasonally, 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 Analysis of July-September precipitation over tropical Africa (1930-2000)
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spatial signature of JAS African rainfall variability Gulf of Guinea patternSahel pattern
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temporal signature and relation to surface temperature
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land-atmosphere interaction
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precipitationsurface 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 consistent with 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 1987 Yulaeva and Wallace 1994 Chiang and Sobel 2002 Neelin et al 2003
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Long-term trends and African climate variability Webster 1972 Gill 1980 Matsuno 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 scales is 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 rainfall when 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 monsoons connected? How is global warming going to affect tropical rainfall?
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