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

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this recordHybrid 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.Helmholtz-AssociationAustralian Research Council Centre of Excellence for Climate ExtremesAustralian Research Counci

Synoptic climatology of hybrid cyclones in the Australian region

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record In May and September 2016, two intense hybrid cyclones (HCs) developed over the Great Australian Bight damaging infrastructure and causing a state‐wide power outage in South Australia. These two cyclones motivate the compilation of the first synoptic climatology of HCs in the Australian region, including an analysis of their importance for wind and precipitation extremes, and a composite view of the large‐scale flow in which they develop. HCs are identified in ERA‐Interim data from 1979 to 2010 using an objective feature tracking method and a cyclone phase space diagnostic. HCs exhibit a pronounced seasonal cycle with most of them occurring from May to September. During these months, HCs are most frequent over the Tasman Sea and the Great Australian Bight where they account for 50% of all cyclones. A common characteristic of all HCs is that the strongest precipitation, which is locally extreme in 91% of all HCs, falls in the warm‐sector and along a bent‐back warm front on the poleward side of the cyclones. Moreover, the area affected by extreme precipitation and the maximum precipitation in HCs are no different from non‐hybrid cyclones (NHCs). In contrast, the area affected by extreme wind gusts is significantly larger in HCs than for NHCs. In both HCs and NHCs the strongest near‐surface wind gusts typically occur in the cold air mass in the wake of the cyclones, especially in those over the Great Australian Bight. The upper‐tropospheric structure of HCs is characterised by an elongated cyclonic potential vorticity anomaly embedded between two ridges that eventually cuts off. In contrast, NHCs are characterised by a zonal flow upstream and upper‐tropospheric cyclonic wave breaking.Helmholtz-AssociationAustralian Research Council Centre of Excellence for Climate ExtremesAustralian Research Counci

The future of midlatitude cyclones

This is the final version. Available from the publisher via the DOI in this record.Purpose of Review This review brings together recent research on the structure, characteristics, dynamics, and impacts of extratropical cyclones in the future. It draws on research using idealized models and complex climate simulations, to evaluate what is known and unknown about these future changes. Recent Findings There are interacting processes that contribute to the uncertainties in future extratropical cyclone changes, e.g., changes in the horizontal and vertical structure of the atmosphere and increasing moisture content due to rising temperatures. Summary While precipitation intensity will most likely increase, along with associated increased latent heating, it is unclear to what extent and for which particular climate conditions this will feedback to increase the intensity of the cyclones. Future research could focus on bridging the gap between idealized models and complex climate models, as well as better understanding of the regional impacts of future changes in extratropical cyclones.Natural Environment Research Council (NERC