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Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales PowerPoint PPT Presentation


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IRI for climate prediction. Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales. A. Giannini (IRI) R. Saravanan (NCAR) and P. Chang (Texas A&M). Reflections on : Seasonal climate prediction - sensitivity of climate to boundary conditions Recent trends in climate

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Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales

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IRI

for climate prediction

Oceanic forcing of Sahel rainfall

on interannual to interdecadal time scales

A. Giannini (IRI)

R. Saravanan (NCAR) and P. Chang (Texas A&M)


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-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


Outline

-statistical analysis

-local land-atmosphere dynamics

-tropical ocean teleconnections


Statistical analysis

Principal Component Analysis

of July-September precipitation

over tropical Africa (1930-2000)


spatial signature of JAS African rainfall variability

Gulf of Guinea patternSahel pattern


temporal signature and relation to surface temperature


land-atmosphere interaction


precipitationsurface temperature

vertically-integrated moisture convergenceevaporation


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 consistent

with 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 1987

Yulaeva and Wallace 1994

Chiang and Sobel 2002

Neelin et al 2003


Long-term trends and African climate variability

Webster 1972

Gill 1980

Matsuno 1966


Response to diabatic heating in the equatorial Indian Ocean


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?


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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