A new historical database describing major floods and associated weather systems that occurred in coastal catchments, from Brisbane in southeastern Australia to Eden approximately 1500 km further south, is described. In order to produce a homogeneous record of major flood and weather-type frequency we restrict attention to the period 1860-2012, when the region (i) is extensively populated, (ii) has an extensive coverage of meteorological stations, (iii) is extensively connected by telecommunication, and (iv) when there is busy coastal shipping offshore. A total of 253 major floods over this period are identified. A flood is considered here to be 'major' if it causes inundation of a river within approximately 50 km of the coast or if there is non-riverine flooding over land near the coast, extending 20 km or more along the coast. All major floods are associated with either (a) East Coast Lows (ECLs) or (b) Tropical Interactions (TIs). Three types of TIs are identified and described. ECLs triggered more major floods than TIs (57 per cent versus 43 per cent), but TIs caused more deaths from freshwater flooding (62 per cent versus 38 per cent) and they tended to cause over twice as many deaths per event (3.6 versus 1.7 deaths/event on average). Some of the most extreme events identified occurred in the 19th century and early-to-mid 20th century. If such events were to occur today they would have catastrophic impacts due to the massive increase in urban development in the study region since that time. IntroductionMajor floods frequently isolate towns, and can cause death, major disruptions to road and rail links, the evacuation of many houses and business premises, and the widespread flooding of farmland. Major flooding can lead to high death tolls. For example, there were at least 90 known deaths caused by floods in June 1852, and 25 deaths during the Brisbane River catchment floods in January 2011.Given these severe impacts, it is of interest to know how frequently major floods have occurred in the past and what weather systems were associated with such floods. We therefore examine a large number of historical records describing floods, and associated weather and impacts. We restrict attention to major floods that have occurred in coastal catchments in eastern Australia during 1860-2012 in the north to Eden (New South Wales (NSW)) 1500 km further south. The study region is depicted in Fig. 1. Over six million people live in this rapidly developing zone.As we are specifically concerned with major flooding near the coast, we consider a flood to be major if (i) it causes inundation of a river within approximately 50 km of the coast or (ii) if there is non-riverine flooding overland near the coast, from the active part of a weather system, that extends at least 20 km along the coast. In (i) extreme rainfall extends well into the hinterland and the upper reaches of the river catchments, causing a flood that drains down the river systems to coastal areas. In (ii) extreme rainfall is confined to the coast and floods form ...

Tropical cyclones moving inland over northern Australia are occasionally observed to reintensify, even in the absence of well-defined extratropical systems. Unlike cases of classical extratropical rejuvenation, such reintensifying storms retain their warm-core structure, often redeveloping such features as eyes. It is here hypothesized that the intensification or reintensification of these systems, christened agukabams, is made possible by large vertical heat fluxes from a deep layer of very hot, sandy soil that has been wetted by the first rains of the approaching systems, significantly increasing its thermal diffusivity. To test this hypothesis, simulations are performed with a simple tropical cyclone model coupled to a one-dimensional soil model. These simulations suggest that warm-core cyclones can indeed intensify when the underlying soil is sufficiently warm and wet and are maintained by heat transfer from the soil. The simulations also suggest that when the storms are sufficiently isolated from their oceanic source of moisture, the rainfall they produce is insufficient to keep the soil wet enough to transfer significant quantities of heat, and the storms then decay rapidly.

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The variability in the number of severe floods that occurred in coastal catchments in southeastern Australia since the mid–nineteenth century, along with the variability in both the frequency of the weather types that triggered the floods and the associated death tolls, is analyzed. Previous research has shown that all of the severe floods identified were associated with one of two major weather types: east coast lows (ECLs) and tropical interactions (TIs). El Niño–Southern Oscillation (ENSO) is shown to strongly modulate the frequency of severe coastal flooding, weather types, and the number of associated deaths. The analysis presented herein, which examines links over more than a century, provides one of very few known statistically significant links between ENSO and death tolls anywhere in the world. Over the period 1876/77–2013/14 the average numbers of coastal floods, ECLs, TIs, and deaths associated with freshwater drowning in La Niña years are 92%, 55%, 150%, and 220% higher than the corresponding averages in El Niño years. The average number of deaths per flood in La Niña years is 3.2, which is 66% higher than the average in El Niño years. Death tolls of 10 or more occurred in only 5% of El Niño years, but in 27% of La Niña years. The interdecadal Pacific oscillation also modulates the frequency of severe floods, weather types, and death tolls. The results of this study are consistent with earlier research over shorter periods and broader regions, using less-complete datasets.

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