The extratropical transition of Hurricane Erin (2001): a potential vorticity perspective

Abstract

The Ertel potential vorticity (PV) is a powerful tool that provides insight into the development and evolution of weather systems. In this study the extratropical transition of Hurricane Erin (2001) is investigated from a PV perspective using operational analyses along with best track data and satellite imagery. Insight is gained from dynamic tropopause maps, on which potential temperature and wind are plotted on a constant PV surface - denoted an isertelic surface in deference to Ertel. The addition of low-level PV to the dynamic tropopause maps gives a clear overview of extratropical transition. On dynamic tropopause maps an upper-level trough appears as a region of low isertelic potential temperature, i.e. a tropopause depression. Two tropopause depressions interacted with Hurricane Erin. During the first interaction the hurricane recurved, but the tropopause depression passed to the north of the hurricane. At this time the circulation of Erin was impinging on the low-level baroclinic zone. During the interaction with the second tropopause depression the hurricane accelerated northeastwards and reintensified rapidly as an extratropical cyclone. The reintensification began when Erin moved into a favourable location for cyclogenesis both from a PV perspective, since Erin was located directly to the east of the tropopause depression, and from quasigeostrophic dynamics, since Erin was located between the entry region of a downstream jet and the exit region of an upstream jet streak. Subsequently, the tropopause depression wrapped up cyclonically and the low-level PV remnants of Hurricane Erin moved into the centre of the extratropical cyclone, thus enhancing the surface winds. Vertical cross sections through the tropical and midlatitude PV anomalies illustrate their approach and interaction. Both PV diagnostics and trajectory calculations indicate that the outflow of a second tropical cyclone, Gabrielle, steepened the tropopause in the region of the upstream jet streak, thus strengthening the upstream jet streak and contributing to Erin's reintensification. Time series of dynamic tropopause maps and trajectory calculations suggest that the pronounced ridging downstream of Erin can be attributed to the outflow of Erin. The downstream impact of the outflow extended across the Atlantic basin and influenced western Europe.