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Title : Water vapour and the Sarahan Heat Low : A new theory for interannual to decadal scale variability in Sahelian rainfall
Name of the speaker : Amato EVAN
Affiliation : University of California, San Diego - USA
Laboratory organizer : LATMOS
Date and time : 10-01-2013 10h00
Location : UPMC - 4 place Jussieu - Paris 5e - LATMOS - Tour 45-46, 4e étage - salle des séminaires
Summary :

This seminar is the presentation of A. Evan's work made in the frame of Labex L-IPSL.

Over the last century the intensity of the West African Monsoon has varied on interannual to multidecadal time scales. On seasonal time scales the strength of the monsoon is associated with dynamical processes over West Africa, while on decadal and longer time scales the circulation is sensitive to patterns of Atlantic and Indian Ocean temperature anomalies. However, these two theories explaining monsoonal/rainfall variability are at odds since ocean temperatures alone can not be used to predict seasonal to interannual variations in monsoon strength, and atmospheric processes alone can not explain coherent decadal-scale variability due to the short memory of the atmosphere. Additional, there is a long history of work suggesting that changes in Sahelian vegetation also influence the monsoon and regional precipitation, yet no work has been able to connect the roles of the oceans, dynamics over West Africa, and land surface processes.

Here I present initial results suggesting that a solution to this paradox may be found in small variations of water vapour within the Saharan Heat Low (SHL); a region of high surface temperature and low surface pressure that is a key dynamical element of the West African monsoon system. Analysis of observations, reanalysis data, coupled model output, and an idealized linear model suggest that interannual to decadal-scale variability of the intensity of the SHL are primarily radiatively forced by small changes of water vapour within the SHL region, via the greenhouse effect.

I suggest a new theory for explaining interannual to decadal-scale variability of the West African monsoon based on positive feedbacks between the vegetation changes in the Sahel, dust emission from West Africa, tropical Atlantic Ocean temperature anomalies, and the dynamics of the SHL.

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