Synoptic mechanisms behind historical rainfall records in Australasia: Cyclone Jasper and Auckland low

Two recent events in far north-east Australia and Auckland, New Zealand, produced rainfall of a seemingly unprecedented nature, with the Australian event involving Tropical Cyclone Jasper and the New Zealand case involving a tropical low affecting Auckland. The aim of this paper is to identify mechanisms that produced the extreme rainfall. The analyses of the wind and thermal structure found that multiple factors occurring simultaneously were responsible for the extreme rainfall for both events. The problem is to deduce if this was result of climate change and global warming or long, expected intervals between such unique synoptic events arriving at a particular location on the globe. In the Jasper case, a search is conducted for similar extreme events dating back to the 19th Century and weather systems producing daily record rainfall. These record rainfall events are more common in the past two centuries than in recent decades over much of Australia. With the Auckland case, the extreme rainfall was unprecedented although four events on a larger scale involving tropical cyclones had huge impacts on New Zealand. Severe Tropical Cyclone Gabrielle occurred soon after the Auckland event, preceded by three others in 1988, 1968 and 1936. The structure of the Auckland low is shown to be unique when compared with Gabrielle. The latter was more intense and closer to Auckland than the former; however, it produced much less rain in the study area. These results will hopefully assist climate researchers in determining whether the events involving Jasper and the Auckland low were rare occurrences triggered by the simultaneous combination of several factors, or if they were influenced by the effects of a warming world.