Large-scale Regions of Negative Potential Vorticity Along the Subtropical Jet During the Extratropical Transition of Hurricane Fiona

Coherent regions of inertial instability are generally thought to be relatively rare at large-scales in middle latitudes, however, analysis of isentropic potential vorticity (PV) in the upper-troposphere reveals they occur relatively frequently on the equatorward side of the subtropical jet. For example, such features were abundant during the vigorous extratropical transition of hurricane Fiona. It is hypothesized that conditionally unstable parcels within eye-wall convection conserve negative values of equivalent PV while ascending along a core, moist isentrope. In the case of Fiona, the 350K moist isentrope appears to link a boundary-layer resevoir of negative equivalent-PV air to the like-signed region of the subtropical jet. This resulted in a rapid intensification of the upper-level horizontal PV gradient with direct impacts on the extratropical transtion process. It is hypothesized that this phenomena occurs preferentially during the extratropical transition process with important implications on dynamical processes across scales.