Cut-off low–induced extreme rainfall in the Western Cape, South Africa: The roles of the Agulhas and Benguela current systems

Cut-Off Lows (COLs) are often associated with extreme rainfall over the Western Cape, yet the role of adjacent ocean currents in modulating COL-related precipitation remains poorly understood. This study investigates the influence of the Agulhas Current System (ACS) and the Benguela Upwelling System (BUS) on COL dynamics and rainfall using ensemble simulations with the Model for Prediction Across Scales–Atmosphere (MPAS-A). The control run (MPAS_CTR) reproduces the COL trajectory with a consistent southwestward bias and a six-hour lag relative to ERA5 and CFSR, while successfully capturing key structural features (cold-core center, geopotential minimum, cyclonic winds) and rainfall distribution. MPAS_CTR achieves the lowest RMSE (28.17 mm) and a correlation of 0.69 with reanalysis datasets, realistically simulating peak intensity (22.24 mm) and the southeastward rainfall maximum consistent with observed moisture transport. Sensitivity experiments highlight the asymmetric ocean–atmosphere coupling. Removing the ACS thermal signature reduces Western Cape rainfall by 18.08%, linked to a 10.66 W m −2 decline in latent-heat flux, weaker boundary-layer buoyancy, suppressed convection, and diminished COL penetration (−0.4 Pa s −1 uplift, −0.1 PVU anomaly). In contrast, BUS warming increases latent-heat flux (+3.89 W m −2 ) and strengthens PV anomalies (>0.1 PVU), but rainfall change is negligible (1.93%) as added moisture remains confined to the western flank and circulation constraints prevent uplift into the COL core. Overall, COL rainfall intensity reflects the combined modulation of Ertel PV anomalies, jet streak–induced vertical uplift, and localized SST forcing, with ACS exerting dominant control while BUS effects remain secondary and circulation-limited. These findings underscore the critical role of ACS thermal forcing in driving COL-induced rainfall extremes over the Western Cape.