April 2022 Floods over East Coast South Africa: Interactions between a Mesoscale Convective System and a Coastal Meso-Low

Extreme rainfall occurred over the South African east coast during 10-12 April 2022, leading to over 500 deaths and massive damage. This study seeks to understand a key feature, namely the progression of the rainfall maxima from the northern KwaZulu-Natal (KZN) coast during the night of 10-11 April by similar to 550 km to the Eastern Cape coast about 48 h later. The large-scale circulation was dominated by a cut-off low over the South African interior with a strong ridging anticyclone southeast of the country in the South Indian Ocean. Satellite, rain gauge, and ERA5 reanalysis are used to show that the timing and location of the heaviest rainfall are closely tied to, firstly, the formation of a Mesoscale Convective System (MCS) at similar to 0300UTC April 11 on the northern KZN coast which tracked south and then offshore, and secondly, a meso-trough which formed a coastal meso-low by similar to 2100 UTC April 11. The meso-low tracked south over the warm waters of the Agulhas Current before strengthening into Subtropical Depression Issa. Mesoscale interaction between the MCS and coastal meso-low determined the positioning and timing of the strongest onshore low-level jet, moisture convergence and uplift, and hence the progression of the heavy rainfall down the coast. Such mesoscale interaction has not previously been documented in southern Africa or the Southern Hemisphere.