Synoptic climatology of hybrid cyclones in the Australian region

Abstract

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 characterized by an elongated cyclonic potential vorticity anomaly embedded between two ridges that eventually cuts off. In contrast, NHCs are characterized by a zonal flow upstream and upper‐tropospheric cyclonic wave breaking.

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

“…In this study, HCs are defined as cyclones for which B < 10, and , meaning they are characterized by a lower‐tropospheric symmetric warm core and an upper‐tropospheric cold core. For the period May–September 1979–2010, Quinting et al () identified 573 HCs, and these form the basis of the present study.…”

“…All analyses in this study are based on 6‐hourly ERA‐Interim reanalyses (Dee et al ) regridded to a uniform 0.75° latitude–longitude grid. For a detailed description of the cyclone tracking and of the identification of the HCs, the reader is referred to the companion study by Quinting et al ().…”

“…The interior layer includes all perturbation PV between 850–550 hPa. A composite vertical cross‐section of all HCs reveals that anomalies of cyclonic PV that are connected to the stratospheric PV reservoir do not extend below 500 hPa (figure 8 in Quinting et al ). Hence, cyclonic PV perturbations in the interior layer are likely to be attributable to non‐PV‐conserving processes such as diabatic heating.…”

“…Mills and Wu, ; Griffiths et al ; Mills, ; Mills et al ). From a climatological perspective, a companion study by Quinting et al () reveals that up to 90% of HCs are associated with locally extreme precipitation, which is comparable to other non‐hybrid cyclones. The study also shows that HCs frequently reach their maximum intensity over the Great Australian Bight and the Tasman Sea.…”

“…HCs in the Australian region typically reach their maximum intensity in an amplified flow comprising upper‐tropospheric ridges upstream and downstream of the cyclone and a north–south elongated trough associated with the cyclone itself. However, Quinting et al () noted considerable case‐to‐case variability of the upper‐tropospheric potential vorticity (PV) structure related to the amplitude of the ridges upstream and downstream of the cyclones as revealed by the three leading empirical orthogonal functions (EOFs) of the 315 K PV anomaly (figure 10 in Quinting et al ). This case‐to‐case variability motivates the present study, which explicitly addresses: the identification of typical upper‐tropospheric PV structures characterizing the observed case‐to‐case variability, the contribution of circulation anomalies to the overall cyclone intensity in a quantitative composite PV framework, the dependence of the cyclone motion on the identified upper‐tropospheric PV structures, and the mechanisms explaining the dependence of the cyclone motion on the upper‐tropospheric PV structures. …”