Atmospheric Sensitivity to SST near the Kuroshio Extension during the Extratropical Transition of Typhoon Tokage*

Abstract

Abstract It is hypothesized that the tropical-to-extratropical transition of a cyclone in the western North Pacific can be sensitive to the underlying sea surface temperature (SST) distribution. This hypothesis was tested through a case study of Typhoon Tokage using a series of high-resolution simulations by the Weather Research Forecast (WRF) numerical weather prediction model. Simulations were carried out for a control SST distribution and for SST distributions with imposed warm and cold perturbations of 1.5°C maximum amplitude in the vicinity of the Kuroshio Extension. The simulations with the warm SST perturbation yielded a cyclone slightly weaker than in the control SST case about 2 days after transition. In contrast, the cold SST perturbation case yielded a cyclone with a central pressure 10 hPa lower than in the control case at the same point in the storm’s life cycle, apparently due to its more northward track and hence closer proximity to an approaching upper-level trough and perhaps in association with a stronger warm front. The effects of the regional SST on the simulated storms are manifested not just locally, but also cause substantial impacts on 500-hPa geopotential heights over much of the North Pacific basin. Retrospective analysis of meridional heat fluxes associated with these events using the NCEP–NCAR reanalysis was carried out for early fall (September–November) seasons with relatively warm and cool SST in the region of the imposed SST perturbations. Differences in the patterns of these fluxes between the warm and cool years are broadly consistent with the results from the warm versus cool SST simulations for Typhoon Tokage.