Austral summer droughts and their driving mechanisms in observations and present-day climate simulations over Malawi

Droughts are a key feature of weather systems over Malawi and southern Africa. Their societal relevance in Malawi underscores the need for improved understanding of drought characteristics and atmospheric processes that drive them. We use the Standardized Precipitation and Evapotranspiration Index with the run theory to identify and characterize droughts in observations (1961-2017) and CMIP5 models across Malawi. We find no major differences in drought duration, severity, and intensity between the northern and southern parts of Malawi. However, circulation patterns associated with droughts in the two regions are different, and typically organized in such a way that droughts in one region coincide with wetter conditions in the other. Anomalous circulation patterns diminishing moisture convergence and convection over the affected region are a typical feature of summer droughts. We show that precipitation variability is principally governed by advected moisture, transported via three main tracks of northwesterly, northeasterly, and southeasterly moisture fluxes. The three tracks interact to form a convergence zone with a peak situated over Malawi. Variability in the respective moisture flux tracks influences the variability in the location and intensity of the convergence zone, and thus the location of the ensuing drought. We note links between variability in the moisture advection tracks and El Nino Southern Oscillations and other modes of variability including the Indian Ocean Dipole and Subtropical Indian Ocean Dipole. Both negative and positive biases in drought frequency are apparent in CMIP5 models but the majority overestimate drought duration and severity. The relationship between precipitation and net total moisture flux is consistently simulated. However, we note significant model inconsistencies in the relationship between precipitation and moisture flux from the southeasterly track, which potentially undermines the confidence in model simulation of drought processes over Malawi.