Hydroclimatic extremes during the past 1400 years in the monsoonal ‘Top End’, Australia

Abstract Estimates of the frequency of hydroclimatic extremes from stream gauging, meteorological observations and satellite imagery are not sufficiently robust for estimation of event frequencies because of the shortness of the records. Accordingly, a 1400-year record of extreme hydroclimatic events, both floods and coast-crossing cyclones (TCs), is presented for the Top End of monsoonal Australia. From this record, event frequencies have been estimated, along with peak flows for some floods, and the relationships of the events to climate states explored. The paleorecords documented in this paper are based on radiometrically dated sedimentary archives from beach ridges and chénier ridges for TCs and mainly from paleoflood deposits for large floods. Large floods in the region are a result of either TCs or intense tropical lows that commonly begin as TCs. This hydrological connection is apparent in episodes of co-occurring TCs and floods, both in the recent past and over the entire 1400-year record. Both the El Niño–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO) have influenced the occurrence of the extreme events. Flood frequency has varied through time with evidence for long periods with few events. The Modern Warm Period from ca 1850 CE is a time of high-flood and TC frequency, consistent with an increase in high stream flows from an independent study of the Katherine/Daly River and rainfall in monsoonal Australia more widely. The results presented here should be part of flood mitigation strategies, and assessments of the role of climate change. KEY POINTS Extreme floods and tropical cyclones are a generative feature of the landscape and environment of the Top End of the Northern Territory, Australia, and often cause significant damage to human interests. Traditional monitoring methods for calculating the frequency of such events are too short to provide robust results. The combination of traditional records with reconstructions of events from dated sedimentary archives has produced a 1400-year record of extreme events, most of which were a result of conditions produced by ENSO and the IPO. The increased frequency of TCs and large floods since about 1850 CE in a 1400-year record is important information for assessments of the role of climate change in monsoonal Australia and one input to disaster mitigation.