Classification of Large-Scale Environments that drive the formation of Mesoscale Convective Systems over Southern West Africa

Abstract. Mesoscale convective systems (MCSs) are frequently observed over southern West Africa (SWA) throughout most of the year. However, it has not yet been identified what variations in typical large-scale environments of the West African monsoon seasonal cycle may favour MCS occurrence in this region. Here, six distinct synoptic states are identified and are further associated with being either a dry season, pre-, post-, or peak-monsoon synoptic circulation type using self organizing maps (SOMs) with inputs from reanalysis data. We identified a pronounced annual cycle of MCS numbers with frequency peaks in June and September which can be associated with peak rainfall during the major and minor rainy seasons respectively across SWA. Comparing daily MCS frequencies, MCSs are most likely to develop during post-monsoon conditions featuring a northward-displaced moisture anomaly (0.42 MCSs per day), which can be linked to strengthened low-level westerlies. Considering that these post-monsoon conditions occur predominantly from September and into November, these patterns may in some cases be representative of a delayed monsoon retreat. On the other hand, under peak monsoon conditions, we observe easterly wind anomalies during MCS days, which reduce moisture content over the Sahel but introduce more moisture over the coast. Finally, we find all MCS-day synoptic states to exhibit positive shear anomalies. Seasons with the strongest shear anomalies are associated with the strongest low-level temperature anomalies to the north of SWA, highlighting that a warmer Sahel can promote MCS-favourable conditions in SWA. These significant positive zonal shear anomalies for MCS days illustrate the importance of shear for MCS development in SWA throughout the year.