An 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 the ET system 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 of ET. This amplified trough, however, exhibits pronounced anticyclonic breaking and thus, in contrast to many previous ET studies, it is not clear that the amplification of the upper-level wave propagates into the farther downstream region. Subsequently, the ET system merges with the upstream cyclone. The upstream trough undergoes strong deformation and cyclonic breaking associated with straining due to the cyclonic circulation of the ET system. With the decay of this trough, the ET system 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 of ET exhibits 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-ET regime and two ET regimes reminiscent of the northwest and northeast patterns, respectively, introduced previously by Harr et al.. A stagnation point located on the axis of the upstream trough governs the bifurcation into no-ET and ET regimes. A stagnation point located on the axis of the downstream ridge governs the bifurcation into northwest and northeast patterns.
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