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IOVWST Meeting
Portland, Maine, USA, 2019

Oceanic Origin of Precipitation Jump in the Sahel

Presenting Author: Dr. W Timothy Liu

Observation from the Tropical Rain Measuring Mission confirms a rather abrupt transition of rainfall at 8°N latitude in the western Africa between the Sahara and the Gulf of Guinea. North of this latitude, rainfall peaks in August, and south of it, rainfall peaks in June. There were postulations on this precipitation jump through data analysis and numerical model, but no satisfactory explanation of the phase shift of the annual cycle is evident. Using the moisture transport integrated over the depth of the atmosphere (IMT) over ocean derived from QuikSCAT data, we show that the IMT comes onshore from the Atlantic only around August and lags those coming from the Gulf of Guinea (GG), which peaks in June, by two months. The implication is that, the onshore moisture transport by the monsoon is confined to the surface with off shore transport aloft for most of the summer, except for a summer months. During the August, the on shore transport extends far up in the troposphere supplying moisture and instability for heaviest rainfall in the Sahel. The anomalous wet season in 1999 is caused by the positive (eastward) anomalies of IMT from the Atlantic and the anomalous dry summer of 2005 is associated with negative (westward) anomalies of IMT over the Atlantic. The socioeconomic vulnerability of the agricultural societies in the Sahel region of West Africa to the vagary of summer rainfall has received world recognizance. The recently initiated international endeavor of the African Monsoon Multidicispline Analysis underscores the fundamental gaps in our knowledge of the coupled atmosphere-ocean-land system in this region and the large systematic errors in dynamical models used for prediction. The results of this study should made timely fundamental advances in our understanding the factors controlling rainfall in the west Africa.

2019 International OVWST Meeting
May 29-31 in Portland, Maine, USA
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