Downstream impact of tropical cyclones: mature baroclinic wave scenario

The interaction of tropical cyclones (TCs) with the midlatitude flow can have a significant detrimental impact on the predictability of midlatitude weather systems. The same interaction may trigger severe midlatitude weather events. The potential of severe weather in regions of reduced predictability highlights the importance to better understand the physical processes that govern the interaction of TCs with the midlatitude flow.

Continuing our study of idealized scenarios we here analyze the interaction of a TC with a mature baroclinic wave. A classic life-cycle experiment of a baroclinic wave constitutes our benchmark that is compared to experiments with a TC initially located equatorwards of the baroclinic wave. The TCs in this experimental setup undergo extratropical transition (ET). The differences in the life-cycle experiment and the ET experiments are then considered to be the impact of ET on the midlatitude flow. This impact is discussed in a potential vorticity framework, including piecewise PV inversion.

The midlatitude flow in our scenario is characterized by 5 mature cyclones underneath a high-amplitude, wavy upper-level flow. During ET, the remnant tropical cyclone vortex imposes a pronounced deformation field on one of the upper-level troughs. Deformation of this trough and associated wave breaking leads to a significant local decrease of upper-level wave amplitude. Diabatic contributions to the decay of the trough will be assessed also.

The decay of the trough apparently initiates a successive breaking of troughs in the downstream region. This downstream dispersion of the direct impact of ET exhibits wave packet characteristics. The evolution in the downstream region is sensitive to the initial location and intensity of the TC. Thus, uncertainties associated with the evolution of the ET system itself transfer into the downstream region contributing to the reduced predictability in this region.