秋季西北太平洋热带气旋累积能量的年际变化及其预报

Interannual variations and prediction of accumulated cyclone energy of tropical cyclones in autumn over western North Pacific

秋季是西北太平洋热带气旋平均强度最强的季节,热带气旋累积能量(accumulated cyclone energy,ACE)是热带气旋平均强度的表征指标,基于1979—2015年日本气象厅最佳路径热带气旋数据集,以及美国冰雪中心海冰数据和哈得来环流中心海温数据,利用回归分析和多元逐步回归等方法,对秋季西北太平洋ACE指数进行了分析和预报。研究表明:秋季西北太平洋ACE指数具有显著的年际变化特征,与厄尔尼诺-南方涛动(ENSO)有关,最大和最小值分别出现在1991年的厄尔尼诺年和1999年的拉尼娜年,在厄尔尼诺发展年的秋季ACE一般较强,而在拉尼娜衰减年的秋季热带气旋强度则较弱; ACE指数变化受来自北极海冰变化强迫中纬度异常波列的影响及其受到厄尔尼诺海温模态的调制;由于海冰在波弗特海的异常增多,强迫对流层高层夏季出现类似北半球环球遥相关型异常波列,波列正压下传,使得夏秋季西北太平洋副热带高压东退北移;副热带高压活动的变化和太平洋海温的异常分布影响了局地的环流,热带气旋生成源地弱的垂直风切变区域偏东和涡度显著增大有利于热带气旋在暖海洋上发展强盛。最后进行建模预报,预报效果为0.69。若单独使用海温或海冰作为唯一要素来预报,预报效果将大大降低。

The autumn identifies the highest mean intensity of tropical cyclones over western North Pacific and accumulated cyclone energy(ACE)serves as the characteristic index for the mean intensity of tropical cyclones.Based on the tropical cyclone best-track dataset from Japan Meteorological Agency,the sea ice data from National Snow and Ice Data Center,and the SST data from Met Office Hadley Centre from 1979 to 2015,the ACE index in autumn over western North Pacific is analyzed and predicted using regression analysis and multiple stepwise regression.The results show that the ACE index in autumn over western North Pacific is of obvious interannual variations and is closely related with El Nino-Southern Oscillation(ENSO),and the maximum(minimum)value appeared in 1991(1999)when El Nio(La Nia)occurred.ACE is generally stronger in autumn of the El Nino developing years and the tropical cyclones are weaker in autumn of the La Nina declining years.ACE anomalies in autumn over western North Pacific are influenced by the forcing of the mid-latitude anomalous wave trains resulting from the change of the Arctic sea ice and by the SST pattern in El Nino.Due to the abnormal increase of sea ice in the Beaufort Sea,anomalous wave trains(similar with Circum-Global Teleconnection in the Northern Hemisphere)appear at the upper troposphere in summer,whose positive pressure makes the subtropical high over western North Pacific move eastward and northward in summer and autumn.The variations of the subtropical high and the abnormal distribution of SST over the Pacific Ocean affected the Walker Circulation.To the east of Philippine Sea,the eastward vertical wind shear is weaker,which is favorable for developing tropical cyclones.At the place where tropical cyclones are generated,the positive vorticity anomalies contribute to strengthen their intensities.Finally,the forecast skill score of the model is 0.69.If SST or the sea ice was used as the only factor,the forecast skill score would be greatly reduced.