The intensity and motion of hybrid cyclones in the Australian region in a composite potential vorticity framework

Hybrid cyclones (HCs) in the Australian region typically reach their peak intensity in an amplified flow comprising upper-tropospheric ridges upstream and downstream of the cyclone and a north–south elongated trough. Nonetheless, there is considerable case-to-case variability. Taking a composite viewpoint, the present study investigates how such variations in the upper-tropospheric potential vorticity (PV) anomalies affect the subsequent intensity and motion of HCs in the Australian region. First, cyclones are grouped into four clusters with structurally-similar environments by applying a k-means clustering to the 315-K PV anomaly. The clusters reveal that HCs can be associated with a north–south elongated trough (Cluster 1), a PV cut- off (Cluster 2), and cyclonically breaking troughs (Clusters 3 and 4). Second, the effect of these features on the intensity and tracks is quantified using piecewise PV inversion. The maximum intensity of cyclones in Cluster 1 is largely determined by their upper-tropospheric cyclonic PV anomaly. Conversely, diabatically generated lower- tropospheric PV anomalies dominate the intensity of cyclones in Clusters 3 and 4. In these two clusters, the cyclonically breaking trough and a downstream ridge induce an anomalous northeasterly low-level flow across the cyclone centre. The downstream ridge is most pronounced in Cluster 4, leading to the furthest poleward cyclone displacement compared to the other clusters. In Clusters 1 and 2, the upper-level PV anomaly primarily slows the eastward motion of the cyclones. In agreement with recent idealised studies, the analysis suggests that the effect of upper-tropospheric PV anomalies on the poleward motion of HCs is analogous to the beta-gyres that influence the motion of tropical cyclones.