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 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 140 kt) just before it turned to the northwest three days later. Dale recurved to the northeast around the west side of the subtropical high as it entered the influence of a strong sub-tropical westerly jet and underwent extratropical transition (ET) south of Japan. Dale rapidly reintensified into a powerful extratropical cyclone (EC) as it turned northward toward Siberia and slowly died as it moved westward along the north coast of Russia. Noteworthy aspects of Dale include: 1) a transport of modified tropical air across the North Pole toward Greenland, and 2) environmental sea level pressure perturbations of ~ 10 hPa in 10 days over the tropical western Pacific Ocean. The focus of this talk will be on the synoptic-scale aspects of Dale's life cycle to better understand the reasons for this noteworthy behavior.

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 the mid-Pacific shear line under which tropical depression Dale was born was barotropically unstable. Dale interacted with a strong westerly jet and an upper-tropospheric potential vorticity (PV) anomaly embedded within the jet on 13 November. This interaction led to the ET of Dale within a highly baroclinic environment (winds reached 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. The 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 the low 940s (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 82oN before it turned west to along the north coast of Russia before finally dissipating on 22 November.