Analysis and Prediction of the West African Monsoon Onset

The West African Monsoon onset marks a vital point in the seasonal monsoon cycle over the region with direct implications for local farmers and other stakeholders. In this work, valuable insight into the exact definition of the monsoon onset, its level of spatial consistency and cause of inter-annual variability of onsets has been presented. Criteria are presented to determine the value of monsoon onset definitions. There exist over seventeen unique onset definitions in publication. In this work, a representative sub-set of definitions have been compared to assess the relative value and suitability of onset definitions. It is found that the length scale over which a definition is defined determines the relevance to certain users. Local farmers require knowledge on local onset definitions which often have no similarity to regional onset definitions. Local onset dates are shown to have a pragmatic level of spatial homogeneity. Local Onset Regions (LORs) are presented over which local onset variability can be studied using a representative time series of onset dates. Using LORs, it is found that the seasonal progression of the Inter-Tropical Front and the phase of the Madden-Julian Oscillation drive the inter-annual variability of local onsets. The late passage of the Inter-Tropical Front past a LOR is linked to later onset in that region. Furthermore, when the Madden-Julian Oscillation inhibits convection across the Guinea Coast, local onset dates tend to be earlier than climatology. Further research into predicting drivers of local onset variability are suggested. Finally, seasonal forecast models tend to under-predict the variability of onset dates across West Africa. There is little significant correlation between observed onset dates (regional or local) and forecasts. It is concluded that seasonal onset forecasts are currently of little value to forecast users in West Africa. Suggestions as to the cause of this limitation are discussed.