Tropical storm Chedza and associated floods over south-eastern Africa

Abstract

Widespread flooding over parts of Malawi, Mozambique, and Madagascar occurred in January 2015. An impact assessment by the World Bank indicated huge damage to property, infrastructure, and agriculture over several regions in south-eastern Africa as well as significant loss of life. The flooding was associated with tropical storm Chedza that developed in the Mozambique Channel on 11 January 2015. This study investigates the atmospheric circulation and potential mechanisms responsible for the heavy rainfall event that occurred between 11 and 17 January over Mozambique and Malawi using the Weather Research and Forecasting (WRF) model, the Global Forecast System atmospheric reanalysis, satellite-derived rainfall and wind data, and station rainfall data. Tropical Rainfall Measuring Mission rainfall estimates and rainfall station data indicated that southern Malawi and northern Mozambique experienced the majority of rainfall during the early stages of tropical storm Chedza, while Madagascar experienced heavy falls when tropical storm Chedza tracked over the island on 17 January 2015. Furthermore, an analysis of the station data revealed that the heavy rainfall over Mozambique occurred between 11 and 13 January with some stations recording about 80% of their total January 2015 rainfall as resulting from this event. The WRF simulation of the event indicated a low-level easterly to south-easterly onshore flow over southern Mozambique that interacted with a north-westerly monsoonal flow along the northern flanks (periphery) of the storm in the northern Mozambique Channel that led to moisture flux convergence in the regions of heavy rainfall. Furthermore, moisture from the south-west Indian Ocean was advected into the region during the heavy rainfall. This moisture convergence along with strong uplift and convective instability over the region acted together to create favourable conditions for the development of tropical storm Chedza and the associated heavy rainfall.