Extratropical transitions: Large-scale aspects

The process whereby a tropical storm transitions into an extratropical cyclone is not fully understood. It is common to find weakening tropical storms that gradually transition into weak extratropical cyclones while continuing to dissipate slowly. However, a significant subset of transitioning tropical storms may intensify rapidly after the preliminary weakening in response to middle latitude trough interactions. At issue is how to understand these interactions and anticipate which tropical cyclones will become likely candidates for significant reintensification after transition considering these storms carry the potential to be destructive as to life and property.

We will present results from several interesting cases of recent extratropical transitions from the Pacific (Dale, 1996) and the Atlantic (Danny 1997; Floyd 1999, and Irene 1999) to compare and contrast the evolution of the large-scale flow prior to, during, and subsequent to transition. Dennis (1999) will be examined as an example of a null case of a storm that never transitioned after landfall. Critical to determining whether a storm will reintensify after undergoing extratropical transition appears to be the extent to which low-level warm air advection develops poleward of the storm, the extent to which the storm becomes coupled to the ascending branch of a secondary circulation in the equatorward-entrance region of a downstream jet streak, and how both processes feedback and influence each other. This issue will be discussed from a potential vorticity perspective to help understand the dynamics of the extratropical transition process.