The Influence of the Downstream State on Extratropical Transition: Hurricane Earl (1998) Case Study

Abstract

Abstract The multitude of tropical–extratropical interactions that occur during an extratropical transition (ET) complicate the prediction and diagnosis of these extreme events. This study focuses on the analysis of a double ET and reintensification event that took place between 5 and 7 September 1998. Ex-Hurricanes Earl and Danielle reintensified rapidly over the western and eastern North Atlantic, respectively. A set of simulations designed to test the sensitivity of Earl's ET to features in the downstream state was run using a set of idealizations for a numerical model's initial and boundary conditions downstream of ex-Hurricane Earl. Dynamic tropopause analyses and the “PV thinking” paradigm applied under the Eady model highlight important developmental and structural differences between the tests. In fact, two distinct solution modes are diagnosed both in the control and in the sensitivity tests. Earl's ET proceeds according to a “baroclinic mode” of redevelopment, whereas Danielle displays distinct “tropical mode” ET signatures throughout the period of investigation. The presence of a strong zonal jet immediately downstream of the transitioning cyclone is found to be sufficient to induce a baroclinic mode of redevelopment characterized by cyclonic potential vorticity rollup and strong near-surface frontogenesis. Under the influence of an upstream jet in isolation, the reintensification takes on distinctly tropical characteristics as the enhanced northward intrusion of warm, moist air ahead of the system creates a local environment favorable for a tropical mode of redevelopment. A description of the dynamics associated with these two distinct redevelopment modes may aid in the understanding and prediction of these events.