Climate and impact attribution of compound flooding induced by tropical cyclone Idai in Mozambique

Abstract. In this study, we investigate the effect of climate change on tropical cyclone (TC) induced compound flooding and impacts for TC Idai, making landfall in Mozambique in 2019. TCs are one of the most damaging extreme events and are challenging to attribute using conventional, probabilistic methods. We develop a storyline attribution framework including a state-of-the-art modelling chain that combines hydrological, coastal, flood and impact models to simulate the changes in flooding and its impact under factual and counterfactual scenarios, with the climate trend removed. For the case of TC Idai, we find that sea level rise and change in wind-driven storm surge lead to the largest increase in flood damage (27 % compared to the counterfactual), while causing a less than 1 % increase in flood volume and flood extent. Climate trends in rainfall lead to the largest increase in flood volume and flood extent (9 % and 2 %, respectively, compared to the counterfactual) but account for a smaller increase in flood damage (4 %). Changes in all drivers combined lead to the same increase in flood volume and flood extent as the rain-only scenario (9 % and 2 %, respectively) but the largest increase in flood damage (31 %). A non-linear relationship between flood hazard and flood damage results in a stronger climate footprint on TC impacts than hazards. Assessing the combination of all climate change-affected flood drivers is crucial for obtaining a comprehensive view on the effect of climate change. The attribution framework presented in this paper is applicable for TC-prone regions across the globe and can be applied in data-poor, yet often highly impacted and vulnerable areas which are currently underrepresented in attribution studies.