Climate change, typhoons, and floods on the Ara River, central Japan

Portions of the flood history of the Ara River in central Japan are documented using data from the 20th century, the historical past, and the geological past (the Holocene). In the 20 th century, annual maximum floods were found to have been larger from 1910–1950 and 1982–1998, and smaller from 1952–1981 (no data for 1951). The dates of shifts in flood magnitudes are close to those found for shifts in flood magnitudes in the Upper Mississippi Valley of the U.S., and for major shifts in the dominance of differing phases of the PDO (Pacific Decadal Oscillation) and the ENSO. During the Little Ice Age, historical documentary records indicate that floods were frequent but showed decadal-scale variations with higher (lower) frequencies during cooler (warmer) periods and during periods dominated by El Niño (La Niña) phases of the ENSO and positive (negative) phases of the PDO. For the Holocene, reconstruction of a part of the flood record based on the analysis of terrace and floodplain gravels indicates that floods were generally larger during the early to mid-Holocene but variable during the mid- to late Holocene with generally larger (smaller) floods during colder (warmer) periods. Flood magnitudes also showed a weak relationship with El Niño activity after the mid-Holocene. The Ara River Holocene flood record showed similarities in the timing of periods of larger (smaller) floods with significant periods of fluvial activity in Japan, Europe, and the U.S. As typhoons are the main cause of large floods in the 20th century and the historical past, trends in the magnitudes and frequencies of floods on the Ara River at various time scales are linked to similar-scale trends in the behavior of typhoons in the Northwestern Pacific Ocean. This typhoon and flood behavior can be related to climate change through shifts in the persistence of atmospheric circulation patterns and phenomena such as the ENSO and PDO. It is therefore suggested that shifts in the global climate at various time-scales throughout the Holocene have been the main driving force for changes in the behavior of typhoons and, consequently, in the Ara River flood regime.