Investigating A Potential Pathway for Gulf Stream Influence on the Extratropical Transition of North Atlantic Tropical Cyclones

As tropical cyclones (TCs) undergo extratropical transition (ET), they develop distinct frontal boundaries across the resulting extratropical cyclone. In the North Atlantic Ocean (NATL), this can often happen near the Gulf Stream (GS). Previous work has demonstrated that the GS can influence the development of fronts in midlatitude winter cyclones. The mechanisms of air-sea interactions associated with WBCs occur at multiple spatiotemporal scales, with the extent and exact nature of those interactions debated within the literature. Could the influence of the GS on frontal development in midlatitude winter storms also apply to storms undergoing ET? Here, we present both an observational-based statistical analysis, as well as results from case-study simulations, of a possible pathway for the GS to influence TCs undergoing ET via local small-scale SST gradient changes.

Composites of NATL TCs indicate that the magnitude of the GS sea surface temperature (SST) gradient in the time prior to the TC passing is significantly weaker for TCs that begin the ET process but ultimately do not complete it, compared with TCs that do complete ET. Using a simple index of the GS SST gradient strength, both the sensible heat flux gradient and, to a lesser degree, lower-tropospheric diabatic frontogenesis are shown to scale with the local SST gradient used in this index. Our results suggest that there is some support for a mechanism in which the GS SST gradient influences the sensible heat flux gradient and subsequent surface diabatic frontogenesis in the region, impacting the favorability of the environment for a passing TC to complete ET.

To investigate this possible mechanism more closely and establish causality, we use the Weather Research and Forecasting (WRF) model to test case study simulations of Hurricane Teddy as it undergoes ET near the GS. We analyze this by modifying the magnitude and strength of the local grid point-scale SST gradient strength associated with the GS in the North Atlantic in the days prior to Teddy passing over the GS. These different simulations are then compared to determine impacts in terms of the track, intensity, frontal development, strength of both the adiabatic and diabatic frontogenesis, during Teddy’s ET. These results provide insight into the dynamical mechanisms by which surface forcing could exert an influence on ET.