Seasonality and Trends of Drivers of Mesoscale Convective Systems in Southern West Africa

Abstract

Mesoscale convective systems (MCSs) are the major source of extreme rainfall over land in the tropics and are expected to intensify with global warming. In the Sahel, changes in surface temperature gradients and associated changes in wind shear have been found to be important for MCS intensification in recent decades. Here we extend that analysis to southern West Africa (SWA) by combining 34 years of cloud-top temperatures with rainfall and reanalysis data. We identify clear trends in intense MCSs since 1983 and their associated atmospheric drivers. We also find a marked annual cycle in the drivers, linked to changes in the convective regime during the progression of the West African monsoon. Before the peak of the first rainy season, we identify a shear regime where increased temperature gradients play a crucial role for MCS intensity trends. From June onward, SWA moves into a less unstable, moist regime during which MCS trends are mainly linked to frequency increase and may be more influenced by total column water vapor. However, during both seasons we find that MCSs with the most intense convection occur in an environment with stronger wind shear, increased low-level humidity, and drier midlevels. Comparing the sensitivity of MCS intensity and peak rainfall to low-level moisture and wind shear conditions preceding events, we find a dominant role for wind shear. We conclude that MCS trends are directly linked to a strengthening of two distinct convective regimes that cause the seasonal change of SWA MCS characteristics. However, the convective environment that ultimately produces the most intense MCSs remains the same.

“Late-stage” deforestation enhances storm trends in coastal West Africa

“…These trends are largely positive across SWA ( Fig. 1 B and C ), consistent with earlier large-scale studies ( 27 , 29 ). There are, however, strong within-region variations, with a tendency for areas with weak LST trends to have weaker trends in convective activity ( Fig.…”

“… 26 ), which are typically triggered in the afternoon and propagate westwards. There are important, large-scale, positive trends in intense MCSs in SWA ( 27 ). More intense African MCSs have been linked to increasing meridional temperature gradients ( 27 – 29 ), while recent decades have seen more frequent MCSs across SWA in the second rainy season ( 27 ).…”

“…There are important, large-scale, positive trends in intense MCSs in SWA ( 27 ). More intense African MCSs have been linked to increasing meridional temperature gradients ( 27 – 29 ), while recent decades have seen more frequent MCSs across SWA in the second rainy season ( 27 ). Positive trends are also evident for extreme, daily rainfall totals in gauge-based, regional analyses over SWA ( 30 , 31 ).…”

Understanding mechanisms for trends in Sahelian squall lines: Roles of thermodynamics and shear

“…(2017), who used 35 years of satellite observations and rain gauge readings from the West African Sahel to reveal a persistent increase in the frequency of the most intense MCSs over this period: this was inferred from a trend of C/decade in cloud top temperatures. Intensification of MCSs was restricted to a narrow Sahelian belt in the months of June to September but extends to the Guinea coast in the spring (Klein et al ., 2021). This increase in the frequency of MCSs is only weakly related to the multi‐decadal recovery of the Sahel annual rainfall but exhibits a similar trend to global land temperatures.…”

“…(2020) found that rain rates scaled with the product of time‐of‐storm total column water and in‐storm vertical velocity. However, despite pre‐storm wind shear modulating in‐storm vertical velocity and the resulting cloud‐top temperatures, they found no direct correlation between wind shear and precipitation rates in CP4A, in contrast to observations for the Gulf of Guinea (Klein et al ., 2021).…”