Active Role of Sea Surface Temperature Changes Over the Kuroshio in the Development of Distant Tropical Cyclones in Boreal Fall

This study examined the active role of sea surface temperature (SST) changes over the Kuroshio in the development of distant tropical cyclones (TCs) approaching Japan in October, using a cloud-resolving model. We designed a real SST experiment and SST sensitivity runs by adding and subtracting SST anomalies equivalent to a spread of year-to-year October-mean Kuroshio's SST changes. Under a synoptic environment that low-level northeasterlies prevailed over the Kuroshio, systematic differences in TC intensity between the real and modified SST runs were simulated despite the developing TCs being far away from the modified SST areas. In the warm (cool) SST runs, active (inactive) moisture import from the Kuroshio increased (decreased) the equivalent potential temperature within the planetary boundary layer (PBL) in the inner core, thereby enhancing (weakening) the latent eyewall heating and the associated secondary circulation. The differences in the secondary circulation led to those in the inward advection of absolute angular momentum in the PBL, affecting a vortex spin-up in the inner core. We also found that the surface heat and moisture supply from the Kuroshio facilitated the warming and moistening processes of midlatitude air parcels traveling over the underlying current, causing further reinforcement of the moisture import into the distant TC. These thermodynamic processes were more and less apparent in the warm and cool SST runs, respectively. Since the synoptic environment in this study often arises in boreal fall, the Kuroshio's SST warming in recent decades may influence the intensities of TCs approaching Japan in fall.