West African Monsoon Case Study

WEST AFRICAN MONSOON REGIMES

Introduction

The West African monsoon (WAM) is a very important climatic event in the West African region because it produces most of the precipitation for the region which still depends mostly on rain-fed agriculture for their livelihood (Nicholson and Grist, 2003; and Sultan et al, 2008). This is weather system is most important in the Sahel area of the region, where rainfall is highly variable and any anomaly in the rainfall regime could mean untold hardship to the millions of people that depend on it (Baron et al, 2005). Therefore, understanding the mechanisms and the parameters that most significantly contribute to the variability of the WAM would be greatly beneficial to the region’s agricultural development

These variations are reflected in weather atlases provided by JMA and ECMWF using 25 years and 49 years reanalysis data respectively. The section will try to highlight the different features on these atlases that help to identify, and where possible, define processes and mechanisms driving the WAM. However, emphasis will be placed on the eastern tropical Atlantic Ocean, the continental West Africa and where necessary, references will be made to the other parts of the world. Some surface climatologies such as mean sea level pressure level, 10m wind vectors and isotachs, sea surface temperature and ice, total precipitation and monthly maximum of daily precipitation. The use of the two atlases will also offer us the opportunity to compare both atlases for consistency, as well as ensure that all the elements that we want to compare are available. The analysis will be done

They show changes in the surface wind system with speeds slowing down and most of the wind not flowing inland, while some components flow outwards from the continent back to the coast. This change in wind system has effects on the other elements being discussed.

The mean sea level pressure charts show a decay of the low pressure center that was initially over West Africa and a shift of the center to the east Africa. This shift is in line with the observations in the wind system, indicating a change in the rainfall regime. Lebel et al (2003) and Thorncroft et al (2011) observed an abrupt change in the weather system at this point, which they said signals the withdrawal of the WAM (see fig.)

The SST over the region increases slightly during this season (higher than the previous), which in turn increases rainfall inland after the brief break experienced in August. The total precipitation charts indicate intense precipitation during this period. The two maxima observed during the previous season are intensified and the northern extent of the rainfall regime moves further northward into the savanna. However, Thorncroft et al (2011) described this period as marking the end of the rainy season, and that the withdrawal of rainfall from WAM is abrupt and