Typhoon Dale originated out of an unusually deep near-equatorial surface trough in the western Pacific in early November 1996. The trough was noteworthy because sea-level pressures marking the trough axis decreased 5-10 hPa from late October to early November (~1 hPa/day) to values as low as 1002 hPa, stayed "low" for 5-10 days, and then abruptly increased again. The sea-level pressure decrease could not be readily traced to an eastward-moving disturbance from the Indian Ocean, but rather appeared to be associated with in situ pressure falls over a 40-60 longitude expanse of the western Pacific Ocean. Subsequently, Dale achieved Supertyphoon status on 10-12 November, recurved poleward, regenerated into a very intense extratropical cyclone, moved boldly across the Arctic circle, and advected remnant modified tropical air across the North Pole on its forward side.
This poster will focus on large-scale circulation changes associated with the prominent decrease in sea-level pressure in the near-equatorial western Pacific trough. Emphasis will be placed on understanding how the large-scale circulation pattern interacted with smaller-scale disturbances and evolved into a highly favorable configuration for tropical cyclogenesis.