Synoptic-Scale Diagnosis of the Extratropical Transition of a Southwest Pacific Tropical Cyclone

Abstract

This is the first of two case studies on the transformation of tropical storms in the southwest Pacific. This study uses ECMWF analyses and observations to examine the behavior of Tropical Cyclone Patsy in December 1986 prior to and during its transit to midlatitudes. These analyses adequately depicted synoptic-scale features during the extratropical phase but underestimated the inner core of strong wind during the hurricane phase. Interactions with troughs in the subtropical westerlies provided the impetus for the major motion and structure changes during Patsy's life. During the hurricane phase, Patsy was steered by the flow above 400 hPa. The passage of a first trough to poleward led to disintegration of the characteristic tropical cyclone structure. Decay was due to shearing effects, loss of upper-level outflow, and the transient incorporation of cooler air. into the low-level circulation. A few days later, Patsy commenced moving rapidly south as a single center as ascent redeveloped in the poleward sector ahead of the next approaching trough. During this extratropical transition stage, cyclone motion was controlled by the mean flow below 600 hPa and was directed slightly to the riot of the ascent maximum. This ascent was triggered by cyclonic vorticity advection in the equatorward entrance region of a propagating subtropical jet that was able to tap Patsy's moisture supply. It was prolonged by a long period of coupling with this jet, which was in turn sustained by outflow from the heating within the baroclinic zone beneath it.