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

Abstract

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 through a k‐means clustering of 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 greatest 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.