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University of Oklahoma (2014)

Atmospheric Moisture Transport Associated with the West African Monsoon System : An Observational Study and Evaluation of a WRF Dynamical Downscaling Simulation

Issa Lele, Mouhamadou

Titre : Atmospheric Moisture Transport Associated with the West African Monsoon System : An Observational Study and Evaluation of a WRF Dynamical Downscaling Simulation

Auteur : Issa Lele, Mouhamadou

Université de soutenance : University of Oklahoma

Grade : Doctor of Philosophy (PhD) 2014

Résumé partiel
The intraseasonal variability of the low-level atmospheric moisture transport and its divergence associated with the West African monsoon (WAM) has been investigated for the 1960-2008 period using the NCEP-NCAR reanalysis data. The climatology of the moisture flux integrated from the surface to 850 hPa, shows moderate southerly moisture transport prior to the WAM onset (March-April), associated with cross-equatorial flow in the South Atlantic Ocean and southwesterly flux exceeding 200 kg m-1 s-1 over the Guinea coast just after the WAM onset (May-June). On the intraseasonal time-scale, the study indicates that besides the transport/divergence by the mean flow, the transport/divergence by the intraseasonal climate (timescale >10 day) anomalies dominates the total atmospheric moisture transport over West Africa. There is also a relatively strong and uniform moisture transport by the synoptic (timescale <10 day) anomalies over the Guinean coast but its associated moisture divergence was relatively small. It is also shown that the zonal component is the largest contributor to the mean moisture transport into the Sahel while over the Guinean coast, the meridional transport contributes the most, and the prevailing strong westerly transport extends polward to 20oN. Large scale moisture flux convergence is suggested to be the primary source of water vapor needed to support enhanced convection over West Africa. In the second part of the study, the active and break monsoon periods have been analyzed. Results showed that intraseasonal wet (dry) spells in the Sudan-Sahel region are the manifestations of the superposition of the 2-9, 10-25, and 30-90 day intraseasonal oscillation associated with enhanced (suppressed) moisture transport and convection. In general, wet (dry) spells over the Sudan-Sahel region are associated with dry (wet) spells over the Guinean coast. Lag regression between the intraseasonal wet/dry spell anomalies and 30-90 day filtered moisture transport/divergence anomalies shows that the 30-90 day timescale has a very large zonal scale encompassing both the Sudan and the Sahelian zone. The suppressed convection over West Africa leads to an increased surface temperature and surface pressure gradient. These effects act to further strengthen the monsoon winds and moisture transport towards the ITF, and increased convection in the continental Intertropical convergence zone.

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