Downstream development of the extratropical transition of tropical cyclones in the Southern Hemisphere

Tropical cyclones (TCs) that move into the midlatitudes undergo changes in their structure and transition into extratropical cyclones. The process is known as extratropical transition (ET), which can affect the weather further downstream.

The current study conducts a comprehensive synoptic-climatological analysis of the downstream development of the midlatitude flow associated with ET over the Southern Hemisphere. We use a state-of-the-art low-pressure system detection and classification scheme to objectively track tropical cyclones and detect those that undergo ET based on ERA5 data. Case-to-case variability of the TC structural changes and downstream influence during ET is examined by clustering ET events into four clusters.

We found that the transitioning cyclones in clusters 2 and 3 lead to a pronounced downstream ridge development. Mechanisms of the interaction between the cyclone and midlatitude flow are investigated using potential vorticity and eddy kinetic energy diagnostics. In the potential vorticity framework, the diabatically-driven divergent TC outflow anchors the eastward-propagating upstream trough and contributes substantially to downstream ridge amplification. The nonlinear interaction between the cyclone and midlatitude flow serves as a secondary important factor for the ridge building. From the eddy kinetic energy viewpoint, the downstream development occurs because the transitioning cyclone injects additional energy into the midlatitude flow, which is redistributed by the ageostrophic wind and thus enhances downstream energy.

Clusters 2 and 3 highlight two pathways of the interaction between the cyclone and midlatitude flow. In Cluster 2, the transitioning cyclone deforms an initially zonally-oriented jet anticyclonically and excites Rossby wave development downstream. This is characterised by the development of a notable downstream trough and associated surface cyclone development. Conversely, in Cluster 3, a preexisting upstream Rossby wave captures the cyclone during ET, and while the downstream ridge amplifies, no downstream trough development is observed.