Forecasting Challenges Associated with Tropical Cyclones within Subtropical Gyres

Abstract

his case study examines the complex history of a tropical storm that formed southeast of a large subtropical gyre. In real time the tropical storm was incorrectly identified as being two separate storms, and at one time was mislocated by 465 km. The unique forecast problems associated with tropical cyclones within a subtropical gyre are described. The tropical storm propagated around the gyre and encountered a midlevel temperature gradient to the north. The interaction of the storm with this gradient produced a strong midtropospheric temperature dipole. Temperature advection within this feature produced a change in structure to a subtropical storm corotating with an upper low. The subtropical storm turned equatorward and nearly came to a halt as the upper low became aligned with the storm. As convection increased over warm water, the upper low shifted away from the center and the storm reversed direction and moved poleward. These sudden track changes have frequently been observed in the northwest Pacific, but the role of midtropospheric temperature gradients has not previously been addressed. Clear air at the gyre center coincided with a region of cold advection. A fishhook structure in the gyre cloudiness developed as a result of warm advection east and north of the gyre. The subtropical structure of the storm evolved within the fishhook. It is recommended that the Joint Typhoon Warning Center (JTWC) provide formal warnings on subtropical storms, because their baroclinic nature can produce dramatic track changes associated with the presence of upper lows near the center.