Since the 1980s, Sahel rainfall totals, extreme rainfall, and the share of rainfall from extreme events have all trended upward. In observational and reanalysis datasets, these increases are linked to trends in mesoscale convective systems (MCSs) and extreme deep convection (cold clusters). Throughout this period, precipitation metrics have increased first via increases in MCS frequency and the relative increase in cold clusters, and later via an increase in storm precipitation intensity. Until the late 2000s, increases in the frequency of strong storms were supported by increased vertical shear of the zonal wind, as the African easterly jet intensified in response to the strengthening meridional temperature gradient over the Sahel. Afterwards, the storm frequency and vertical wind shear stopped increasing. Yet, extreme precipitation continued to increase, as the storms' precipitation intensity increased. We link the higher precipitation intensity to an increase in atmospheric moisture in both the boundary layer and aloft.

We propose the lag-1 autocorrelation of daily precipitation as a simple diagnostic of tropical precipitation variability in climate models. This metric generally has a relatively uniform distribution of positive values across the tropics. However, selected land regions are characterized by exceptionally low autocorrelation values. Low values correspond to the dominance of high frequency variance in precipitation, and specifically of high frequency convectively coupled equatorial waves. Consistent with previous work, we show that CMIP6 climate models overestimate the autocorrelation. Global kilometer-scale models capture the observed autocorrelation when deep convection is explicitly simulated. When a deep convection parameterization is used, though, the autocorrelation increases over land and ocean, suggesting that land surface-atmosphere interactions are not responsible for the changes in autocorrelation. Furthermore, the metric also tracks the accuracy of the representation of the relative importance of high frequency and low frequency convectively coupled equatorial waves in the models.