Marine heat waves and tropical cyclones - Two devastating types of coastal hazard in South-eastern Africa

Abstract

The coast of southeastern Africa is the only part of the continent to experience landfalling tropical cyclones (TCs), the most destructive and hazardous of weather systems. It is also a region which frequently experiences long-lasting marine heat waves (MHWs). Three coastal regions in southeastern Africa with high biodiversity and relatively large populations dependent on local coral reefs and artisanal fishing are analysed in terms of the characteristics of TCs and MHWs. Significantly increasing trends in MHW duration and frequency, TC heat potential and coral bleaching events are found during austral summer in all three coastal areas, with northeastern Madagascar experiencing the highest rate. The co-occurrence of TCs and MHWs in each region is also considered using a parameter-space plot of MHW intensity versus TC wind speed. A high percentage of pre-existing MHWs help strengthen TCs as they pass over the MHW region but an even higher percentage of the TCs tend to weaken or end MHWs after they both co-occur. Of the three coastal zones considered, northeastern Madagascar is more prone to landfalling severe cyclones and long-lasting MHWs. It is suggested that as the TC approaches the MHW region, the rapid increase in surface wind friction velocity, intense heat loss to the atmosphere and entrainment cooling are all important processes that determine the rate at which the MHW weakens towards its demise. TC-MHWs co-occurring together may be considered a compound extreme event. The magnitude of their impacts on the southeast African coast in recent decades highlights the need for increased monitoring and improved real-time forecasting of these devastating coastal hazards.