Interaction of tropical cyclones with midlatitude baroclinic waves in idealised scenarios of extra-tropical transition

Tropical cyclones that move into higher latitudes frequently undergo extra-tropical transition (ET). In addition to the hazardous weather conditions associated with the ET system itself, there is also a larger scale impact on the midlatitude flow in the downstream region. In the presence of an ET system the predictability of the downstream flow can be decreased significantly. The characteristics of the flow modification in the midlatitudes are largely unknown. Open questions include which flow patterns are most susceptible to modification by ET and allow for strongest propagation further downstream.

In previous work we have used idealized moist numerical experiments to investigate the mechanisms by which an ET system excites downstream baroclinic development on a straight jet. We found that the tropical cyclone modifies the upper-level midlatitude flow over an extended period of time initiating baroclinic development in the downstream region from top to bottom. The structure of the ET system may influence the characteristics of the midlatitude development. Building on these results we now explore the impact of an ET system on the development of idealized baroclinic life cycles, again focusing on the downstream region. We use the same model setup as in our previous study but add an initial perturbation into the initially straight jet to excite a baroclinic wave. By varying the initial perturbation we have some control on the characteristics of the baroclinic development. A baroclinic wave constitutes a more realistic, albeit more complex representation of the midlatitude flow than a straight jet.

We present two contrasting ET-scenarios. In one scenario ET takes place in the early stage of baroclinic wave development and in the other the interaction occurs in the mature stage. The modification of the upper-level flow - as compared to a benchmark run without the ET system - is taken as a measure of the impact of ET on the midlatitudes. In both scenarios significant modification of the midlatitude upper-level flow is found in the vicinity of the tropical cyclone during ET. In the subsequent evolution these modifications propagate into the downstream region, exhibiting wave train characteristics. In the early development scenario ET leads to the amplification of the meridional component of the upper-level flow, whereas in the second scenario the mature systems tend to decay faster in the presence of the ET system. In general, the impact on the early development is stronger than on the mature systems.

It has been shown in earlier studies that the development of the ET system itself is very sensitive to the relative location of the midlatitude flow features and the tropical cyclone. In our idealized experiments, we observe that this high sensitivity may carry over into the downstream region. We conclude the presentation by discussing the characteristics of these high sensitivity regions.