Interaction of a tropical cyclone with a high‐amplitude, midlatitude wave pattern: Waviness analysis, trough deformation and track bifurcation

Abstract

AbstractAn idealized scenario of extratropical transition (ET) is investigated, in which a tropical cyclone interacts with a high‐amplitude, upper‐level wave pattern and well‐developed surface cyclones. Early during the interaction, the external forcing of the upper‐level wave by theETsystem is quantified based on a metric for the waviness of the midlatitude flow. Local amplification of the wave pattern is diagnosed, associated prominently with the trough downstream ofET. This amplified trough, however, exhibits pronounced anticyclonic breaking and thus, in contrast to many previousETstudies, it is not clear that the amplification of the upper‐level wave propagates into the farther downstream region. Subsequently, theETsystem merges with the upstream cyclone. The upstream trough undergoes strong deformation and cyclonic breaking associated with straining due to the cyclonic circulation of theETsystem. With the decay of this trough, theETsystem weakens considerably and the upper‐level wave pattern changes locally to a zonal flow orientation. This zonal flow pattern then extends into the downstream region and promotes the decay of the downstream baroclinic systems.As in previous studies, the evolution ofETexhibits large sensitivity to the initial location of the tropical cyclone. Examining the steering flow's topology, i.e. identifying the stagnation points and the streamlines emanating from these points, helps to identify three different regimes: a no‐ETregime and twoETregimes reminiscent of the northwest and northeast patterns, respectively, introduced previously by Harret al.. A stagnation point located on the axis of the upstream trough governs the bifurcation into no‐ETandETregimes. A stagnation point located on the axis of the downstream ridge governs the bifurcation into northwest and northeast patterns.