Increasing rain intensity over Okinawa, 1982-2005, and the link to changes in characteristics of northwest Pacific typhoons

Abstract

Daily rainfall variation over the strategically situated western Pacific island of Okinawa is investigated for 1982-2005. Although no trend in the annual rainfall was observed, analyses using a probability distribution function for each year revealed that the frequency of light (0-3 mm d -1) and heavy (26-50 mm d -1) rainfall events have a statistically significant decreasing trend, while extreme (>75 mm d -1) rainfall events have been increasing. Such events are associated with typhoons. An analysis of typhoons passing near Okinawa during 1982-2005 revealed that a cause of the increasing trend in heavy rainfall was increased rainfall amounts per typhoon rather than a rising number of strong typhoons approaching the island. In particular, typhoons which passed to the west of Okinawa, or which approached the island within 100 km, during the peak typhoon month of September showed a statistically significant increase in total rainfall of 6.5 mm y -1 over this time period. This is partially due to a slowing of the typhoon speeds in the latter half of the record. This work supports other recent studies that suggest changes to characteristics of stronger tropical storms are not restricted to the North Atlantic although, owing to the limited length of the record, we are not able to determine at this time whether the observed trends are unusual compared with natural multidecadal variability. Copyright 2010 by the American Geophysical Union.