The extratropical transition of a tropical cyclone has been described as a two-stage process by Klein et al. (2000): transformation, which includes the initial structural changes that the tropical cyclone undergoes as it begins to interact with a drier, colder midlatitude environment to its poleward side; and re-intensification, which occurs as part of an interaction with a midlatitude upper-level trough. While there appears to be many similarities in the structural changes of tropical cyclones during the transformation stage as observed in satellite imagery, more diversity apparently exists among cases during the re-intensification stage of extratropical transition (Klein et al. 2001). This has led to the hypothesis (Harr and Elsberry 2000a) that the details of the transformed structure of the tropical cyclone have less bearing on the re-intensification process than does the structure of the midlatitude environment.

This presentation uses the U. S. Navy’s Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPS) to explore how the characteristics of extratropical transition change as the structure of the midlatitude circulation is varied. Two sets of simulations will be presented. The first set will compare the in situ development from an isolated upper-level trough with that of an identical trough that interacts with a poleward-moving tropical cyclone. Various initial displacements of the tropical cyclone relative to the midlatitude trough are tested to find the most favorable arrangement for re-intensification. The second set will examine the effect of varying the structure of the midlatitude circulation on the resulting interaction (or non-interaction) with the tropical cyclone. A counter-intuitive result is that a larger amplitude midlatitude trough does not necessarily lead to a more vigorous re-intensification. Particular emphasis will be placed on model diagnostics that examine the physical processes that result in different intensification paths for each transition of the tropical cyclone.