Extratropical Transition of Tropical Cyclones over the Western North Pacific. Part II: The Impact of Midlatitude Circulation Characteristics

Abstract

Two characteristic midlatitude circulation patterns (labeled northwest and northeast) are found to be associated with extratropical transition (ET) of tropical cyclones over the western North Pacific Ocean. Although in both cases the tropical cyclone moves poleward ahead of a midlatitude trough, the primary midlatitude circulation is either that trough or is a large quasi-stationary cyclone to the northeast of the poleward-moving tropical cyclone. Transition into a northwest pattern typically results in the development within 36 h of an intense extratropical cyclone that moves north?northeast. A tropical cyclone that moves into a northeast pattern enters into strong zonal flow between the primary midlatitude circulation and the subtropical ridge to the southeast. These systems move rapidly eastward and do not intensify significantly during the 36 h following transition. In Part I of this study, the ET of Typhoon (TY) David (1997) and the ET of TY Opal (1997) were investigated in terms of the formation of extratropical cyclone features. In this study, the same cases of ET are examined to define interactions between the decaying tropical cyclone and these two general synoptic environments in terms of the distributions of heat and momentum fluxes and generation of kinetic energy between the cyclone and the environment. During transition into either circulation pattern, the tropical cyclone is initially impacted by upper-tropospheric eddy angular momentum fluxes associated with the juxtaposition of the midlatitude circulation. During transition into a northwest pattern, the tropical cyclone couples with the midlatitude baroclinic zone such that low-level eddy heat fluxes contribute to the extratropical cyclone development. During transition into a northeast pattern, the strong zonal flow seems to prevent a direct interaction between the decaying tropical cyclone and the primary midlatitude circulation. Eddy heat fluxes do not increase and minimal baroclinic development occurs. The two types of extratropical transition also have significant differences in the generation of kinetic energy during the reintensification as an extratropical cyclone. Extratropical transition into a northwest pattern results in barotropic and baroclinic production of kinetic energy through direct solenoidal circulations that result from the coupling of the tropical cyclone and midlatitude trough. The movement of a tropical cyclone toward the large, quasi-stationary extratropical cyclone in the northeast pattern results in barotropic destruction of kinetic energy that inhibits significant reintensification.