Synoptic climatology of hybrid cyclones in the Australian region

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.