小笠原諸島父島における降水量の季節変化に及ぼす台風の影響

—エルニーニョラニーニャ現象に着目して—

抄録

 Precipitation on the Ogasawara (Bonin) Islands from summer to autumn depends the intensity, frequency and tracks of tropical cyclones (TCs), which are affected by El Niño/La Niña events (EN/LN). This study is the first to investigate the seasonal variability of the ratio of TCs approaching the Ogasawara Islands to the total number of TCs generated, and to calculate the ratio of TC-induced precipitation to total precipitation during EN/LN TCs extracted from within 300 km of Chichi-jima by QGIS, when they are defined as “TCs approaching Chichi-jima”. Using precipitation at Chichi-jima Observation Station, TC-induced precipitation is calculated when a TC is within 500 km of Chichi-jima. From August to November, the ratio of TCs approaching the Ogasawara Islands to the total number of TCs generated over the Western North Pacific is highest in October. For the same period, the number of TCs approaching the Ogasawara Islands per year during EN is more than that during LN. Reflecting the anomaly of sea surface temperature, the genesis position of TCs during LN shifts westward. TC-induced precipitation on Chichi-jima from August to October during EN is larger than that during LN. in particular, in September, TC-induced precipitation during EN is 40 mm more than that during LN. The former accounts for 61% of the total precipitation in September. These phenomena are explained by the fact that the genesis position of TCs shifts eastward or south-eastward during EN, keeping the central pressure of TCs approaching Chichi-jima lower than that during LN. Also, the presence time of a typhoon from its genesis until it enters the 500 km range of Chichi-jima is longer in EN than in LN. Within the 500 km range of Chichi-jima, the central pressure of a TC is lower in EN than in LN. All these contribute to a large volume of TC-induced precipitation on Chichi-jima during EN.