Abstract
Abstract
Anomalously high sea surface temperatures (SSTs) are becoming more frequent under the ongoing ocean warming, but their effects on midlatitude extreme rainfall remain poorly understood. This study aims to clarify how anomalously high North Atlantic SSTs during a prominent marine heatwave in 2023 influenced record-breaking torrential rainfall over Scotland in early October. Using a convection-permitting atmospheric model, we compare a control experiment forced by the observed SST with a sensitivity experiment in which the basin-scale warm SST anomaly in October 2023 has been artificially removed. Relative to this cooler-SST experiment, a synoptic-scale cyclone over the central North Atlantic is stronger under the observed SST, and the associated poleward moisture transport toward a quasi-stationary front across Scotland is modestly enhanced. Frontogenesis is also stronger over Scotland, and the frontal zone thus becomes more favorable for inducing slantwise ascent of warm, moist air. Even under weak convective instability, these coherent large-scale and mesoscale changes can lead to enhanced precipitation, producing locally up to 30% more rainfall than under the cooler SST. These results suggest that anomalously high SSTs can amplify midlatitude rainfall extremes not only by increasing moisture availability but also by modulating cyclone and frontal dynamics so as to favor substantially heavier precipitation.