Supertyphoon Dale (1996): An impact from the deep tropics to the arctic

Dale was born as a tropical depression in the tropical Pacific Ocean on 2 November 1996 and dissipated three weeks later as an extratropical cyclone in northwestern Russia. Dale became a typhoon on 6 November and a supertyphoon on 9 November (140 kts on 10 November). Dale moved to the northwest and then recurved to the northeast as it entered the influence of a strong subtropical westerly jet south of Japan and underwent extratropical transition (ET). Dale rapidly reintensified into a powerful extratropical cyclone as it turned back to the northwest over Siberia and then slowly weakened as it moved westward along the north coast of Russia. Northward-moving modified tropical air associated with Dale flowed across the North Pole, then back south to Greenland and North America. The purpose of this presentation is to document Dale's unique life cycle to better understand the reasons for its behavior.

ECMWF ERA-40 gridded reanalyses (1.125 deg) were used to diagnose the synoptic aspects of Dale's lifecycle. Track/intensity information was obtained from the Joint Typhoon Warning Center (JTWC) and the ERA-40 analyses in the tropics and midlatitudes, respectively. COADS marine observations were used to refine Dale's track.

The necessary condition for barotropic instability was satisfied along a mid-Pacific shear line under which Dale formed. The source of this instability came from an upper-tropospheric potential vorticity (PV) tail that extended into the tropics, leading to a reversal of the meridional PV gradient. The PV tail moved slowly downward and equatorward, resulting in the formation of a cyclonic disturbance in the unstable tropical air. Dale's ET phase began when it interacted with an upper-tropospheric PV anomaly embedded within a 100 m/s jet on 13 November. A strong poleward flux of modified tropical air occurred during the ET phase of Dale and resulted in the further amplification of a downstream 200 hPa ridge that had been reinforced by an earlier cyclogenesis event. Dale explosively reintensified as it crossed the upper-level jet axis (storm central pressure dropped ~38 hPa in 48 hours on 13-14 November before bottoming out near 943 hPa). The warm air ahead of Dale moved over the North Pole, as attested by the 500-1000 hPa thickness values that increased ~30 dam to 532 dam in 24 hours on 16 November, eventually reaching Greenland from the north on 17 November.