Downstream development associated with the extratropical transition of tropical cyclones over the western North Pacific

Extratropical transition (ET) has been described as an evolutionary process by which a tropical cyclone (TC) loses tropical characteristics and becomes more extratropical in nature. One conceptual model of ET involves two stages, a transformation stage and a re-intensification stage. The transformation stage is defined by an interaction between a recurving TC and a pre-existing, lower-tropospheric baroclinic zone and the associated vertical wind shear. The deepening of the ex-TC as a baroclinic cyclone is defined as the re-intensification stage of ET. This stage is marked by the evolution of extratropical features such as fronts and extratropical-type wind, cloud and precipitation patterns.

Downstream from an ET event, the midlatitude circulation often becomes perturbed due to interaction between the decaying TC and the midlatitude circulation into which it is moving. These perturbations may impact downstream development of the extratropical circulation. Furthermore, the predictability of the midlatitude circulation downstream of an ET event, which is defined by the spread among members of an ensemble prediction system, is often reduced during the ET event.

Downstream baroclinic development has been defined as process of dispersion and spreading of eddy kinetic energy (Ke) in a growing unstable system that is characteristic of high-frequency baroclinic waves. In this framework, downstream baroclinic development may be involved in the generation of observed disturbances that may not be anticipated based solely on an examination of local available baroclinity. Ageostrophic geopotential fluxes from an upstream circulation comprised the major source of a downstream cyclone Ke during its developmental stage, and the radiation of ageostrophic geopotential fluxes downstream was the major sink of the upstream cyclone Ke during its decay stage.

In this study, a local analysis of Ke is used to examine the role of ET on downstream development over the North Pacific. Four cases of ET are used to examine the sensitivity of the downstream development process to varying TC and midlatitude circulation characteristics. These include cases of large and small TCs and cases during midsummer and autumn seasons. Results indicate that under favorable phasing between the decaying TC and the midlatitude circulation the process of ET maintains downstream centers of Ke and delays weakening due to the radiation of Ke to the next downstream center. In cases when the decaying TC does not properly phase with the midlatitude circulation, which results in no re-intensification as an extratropical cyclone, the absorption of the TC into the midlatitude circulation provides a local increases in Ke that results in a short-term increase in the downstream development process. Therefore, the impact on downstream development is only slightly sensitive to the re-intensification of the decaying TC as an extratropical cyclone.