Supertyphoon Dale (1996): A remarkable storm from birth through extratropical transition to explosive reintensification that impacted the tropics, midlatitudes and the Arctic

Dale was born as a tropical depression (TD) in the tropical northwestern Pacific Ocean on 2 November 1996 and dissipated three weeks later as an extratropical cyclone (EC) in northern Russia just south of the Kara Sea. Dale tracked westward around a subtropical high, became a typhoon on 6 November and then a supertyphoon (estimated peak at 140 kt) just before it turned to the northwest three days later. Dale recurved to the northeast around the east side of a strong polar vortex as it entered the influence of a strong subtropical westerly jet and underwent extratropical transition (ET) over the Bering Sea. Dale rapidly reintensified into a powerful EC as it turned northwestward toward Siberia and slowly died as it moved westward along the north coast of Russia. This presentation will focus on the synoptic scale aspects of Dale's life cycle with emphasis on noteworthy aspects that include: 1) environmental sea level pressure perturbations of 12 hPa in 10 days over the tropical western Pacific during genesis, 2) the relationship between Dale and a powerful 200 hPa East Asian jet during ET, and 3) a surge of modified tropical air ahead of Dale that crossed the North Pole and reached northern Greenland during the EC phase.

ECMWF ERA-40 2.5 gridded reanalyses were used for diagnostic calculations. Joint Typhoon Warning Center (JTWC) best track and intensity estimates were obtained for Dale during its tropical phase. Ship reports from the Comprehensive Ocean-Atmosphere Data Set were used to refine Dale analyses during its lifetime.

Results indicate that two mesoscale convective vortices (MCVs) were initiated in the tropical northwestern Pacfic by two upper-tropospheric potential vorticity (PV) anomalies. The entrance regions of two tropical thermal jets combined to create strong upper-level divergence that helped lead to the organization of the MCVs into TD Dale. Dale interacted with a strong westerly jet and an upper-tropospheric PV anomaly embedded within the jet on 13 November. This interaction led to the ET of Dale within a highly baroclinic environment (winds exceeded 100 m/s in a 200 hPa jet streak ahead of Dale) and resulted in a strong poleward flux of modified tropical air ahead of the storm. This poleward flux of modified tropical air ahead of Dale led to a further amplification of a preexisting downstream ridge toward the North Pole. As Dale crossed the strong jet axis explosive reintensification occurred as attested by a central pressure decrease of 38 hPa in 48 hours on 13 and 14 November to about 943 hPa. The 500-1000 hPa thickness over the North Pole increased 30 dam to 532 dam in 24 hours on 16 November as the modified tropical air crossed the Pole. While still a strong storm, Dale tracked north to 82°N then turned west along the north coast of Russia before finally dissipating on 22 November.