Modeling the cooling influence of the Agulhas Current System on the heritage day floods at Theewaterskloof Dam, Cape Town, South Africa

Abstract The heritage day floods of 23–26 September 2023 were among the most severe hydrological events to impact the Western Cape, South Africa, causing widespread inundation, infrastructure damage, and loss of life. Understanding how ocean–atmosphere interactions influence flood severity is critical for hazard assessment. This study examines the role of the Agulhas Current System (ACS) in shaping the severity of the event. We employed the Hydrologic Engineering Center River Analysis System to simulate the flood dynamics at Theewaterskloof Dam and validated the results against Copernicus satellite observations. Simulations were conducted using streamflow and precipitation from the South Africa Flood and Drought Monitor and observed precipitation from the Department of Water and Sanitation, where the precipitation forcing was applied using the rain-on-grid method. We also used precipitation from the Model for Prediction Across Scales-Atmosphere (MPAS) in ensemble runs to quantify atmospheric uncertainty. A sensitivity experiment reduced sea‑surface temperature over the ACS in MPAS simulations to assess the influence of current cooling on flood behaviour. Overall, the model performance was similar across all forcing approaches (R ≈ 0.78–0.81). Sensitivity experiments revealed that ACS cooling reduced rainfall intensity, flood extent, and flood depth, particularly in floodplain regions. This study demonstrates a physically consistent pathway linking ACS cooling to reduced flood severity. Findings highlight the importance of ocean‑driven moisture supply in shaping inland flooding, with implications for flood hazard assessment, early warning systems, and climate adaptation strategies in regulated catchments.