Key Role of the Kuroshio Current in the Formation of Frontal Structure of an Extratropical Cyclone Associated with Heavy Precipitation

In this study, the authors examined how the Kuroshio Current affected the frontal structure of an extratropical cyclone associated with heavy precipitation. We focused on an extratropical cyclone and related heavy precipitation on Miyake Island, which is situated near the Kuroshio Current, in January 2017. We first investigated the frontal structure and the attendant precipitation using observational data and objective analysis data. As the cyclone grew around the Kuroshio Current, a nonclassic front (outer front) intensified to the north of a warm front. A line-shaped precipitation band developed along the outer front, contributing to the heavy precipitation on Miyake Island. The outer front lay along a sea surface temperature (SST) front associated with the Kuroshio Current in its early life stage. To clarify the impact of the SST front on the formation of the incipient outer front, we next conducted cloud-resolving numerical experiments using SST distributions with and without the oceanic frontal structure. In the real SST experiment, sensible heat supply from the Kuroshio Current was large, while the amount from the ocean on the northern side of the warm current was small. This meridional difference in sensible heating caused frontogenesis along the SST front, which created the incipient front. Consequently, the real SST experiment reproduced the formation of the front. In contrast, the SST sensitivity experiments did not simulate such a sensible heating pattern or front formation. Thus, the Kuroshio Current played a key role in forming the incipient outer front and contributed to the heavy precipitation.