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

Reflections on:

Seasonal climate prediction- sensitivity of climate to boundary conditions

Recent trends in climate- are they related to global warming?

r = 0.60

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

Outline

-statistical analysis-local land-atmosphere dynamics-tropical ocean teleconnections

Statistical analysis

Principal Component Analysisof July-September precipitationover tropical Africa (1930-2000)

spatial signature of JAS African rainfall variabilityG

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ea p

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temporal signature and relation to surface temperature

land-atmosphere interaction

precipitation surface temperature

vertically-integrated moisture convergence evaporation

surface temperature variability (1930-2000)

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

forcing from the global oceans

(Sahel rainfall is sensitive to global, tropical SSTs)

long-term v. interannual variability

ENSO's impact on Northern Hemisphere summer climate

Ropelewski and Halpert 1987Yulaeva and Wallace 1994Chiang and Sobel 2002Neelin et al 2003

Long-term trends and African climate variability

Webster 1972Gill 1980Matsuno 1966

Response to diabatic heating in the equatorial Indian Ocean

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?

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?