37年ぶりの大渇水

─小笠原諸島父島における2016～2017年の少雨について─

抄録

 From May 2016 to April 2017, Chichi-jima in the Ogasawara (Bonin) Islands suffered a severe drought. After an absence of 37 years from 1980, the ratio of effective water storage at four dams supplying drinking water on Chichi-jima dropped below 20%. Atmospheric conditions during the drought are studied and compared to those of severe drought years in 2011 and 1980. In comparison with the long-term mean from 1981 to 2010, a negative precipitation anomaly was apparent from May to July 2016, and from September to December 2016. It was also apparent in March and April 2017. From May to July 2016, the Indo–western Pacific ocean capacitor mode was responsible for less precipitation. From September to October 2016, the P–J (Pacific–Japan) pattern brought about less precipitation. After November 2016, similar atmospheric conditions did not continue for more than one month; however, Chichi-jima was covered with an anti-cyclonic circulation anomaly, except in March 2017 when a winter pressure pattern intensified. Less precipitation was also observed from January to July 2011 and from May to October in 1980. The former was attributable to an intensification of convective activity in the tropical western Pacific, accompanied by a sea surface temperature anomaly greater than +0.5°C. In the tropical western Pacific, a cyclonic circulation anomaly was found, while an anti-cyclonic circulation anomaly prevailed around Chichi-jima. In 1980, precipitation from the Bai-u frontal zone was rarely observed from May to June. From July to September, the Okhotsk High was extremely strong, so the Pacific High remained in the southern part rather than in the usual summer position. Fine weather continued until October, which together brought about the severe drought in 1980.