2015年西北太平洋台风季提早展开：2015/2016超级厄尔尼诺的影响

Early onset of the typhoon season over the western North Pacific in 2015:Influence of the 2015/2016 super El Nio event

2015年1—5月西北太平洋上异常出现了7个热带风暴（Tropical Storms,TS）,其中有5个发展成台风（Typhoons,TY）,分别为气候平均态（1979—2015年）的2.5和3.6倍,亦即2015年的台风季提前展开。利用ERA-Interim再分析资料、JTWC热带气旋最佳化路径数据等资料,通过计算台风生成指数（Genesis Potential Index,GPI）和比较天气尺度和季节内振荡分量,探讨2015年台风季提前的原因。结果表明：1）2015年初异常活跃的台风活动与2015/2016年超级El Nino事件于西北太平洋上引发的海气状态异常有关。2）通过对大尺度环境场和台风潜在生成指数（GPI）的诊断分析发现,动力因子（低层涡度、垂直速度）和热力因子（与海表面温度、大气温度有关的潜在强度、中低层大气相对湿度）均对2015年1—5月台风的发生有正贡献。其中,涡度项的贡献最大,相对湿度的贡献次之。3）3-10 d天气尺度扰动和10-90 d季节内振荡在2015年1—5月也异常活跃,有利于TS和TY的生成与发展。

El Nino is one of the most important climate variations at interannual scales. It can influence the global climate by changing the character of the sea-air system every 3—7 years,including the activity of tropical storms（ TSs） and typhoons over the western North Pacific（ WNP）. Composite results of El Nino events showthat there are more TS systems with longer lifetime and greater strength occurring more to the southeast over the WNP in the summer of El Nino developing years because of the changed mean position of Walker circulation. However,this is not the case for the 2015/2016 super El Nino year. The typhoon season over the WNP is generally from June to October. Climatologically（ 1979—2015）,there are only 2. 8 TSs generated during January to May before the onset of the typhoon season. Among them,about 1. 4 TS systems may develop into a typhoon system. However,seven（ five） TS（ typhoon） systems occurred over the WNP in January to May in 2015,based on the JTWC best-track dataset. This is 2. 5（ 3. 6） times larger than the climate mean,suggesting that the typhoon season in2015 started earlier than usual. To investigate what mechanisms induced the earlier onset of the typhoon season in 2015,we diagnosed the large-scale environments from ERA-Interim and NOAA ERSST data. The results showthat the unusual growth of TS activity in early 2015 was related to the anomalous air-sea conditions of the 2015/2016 super El Nino event.The anomalous warming over the central equatorial Pacific associated with the abortion of the 2014/2015 El Nino was accompanied by a cyclonic anomaly,low-level convergence and increased atmospheric moisture content over the WNP,where tropical cyclones were generated. We further analyzed the genesis potential index（ GPI） to understand the relative roles of different dynamic and thermodynamic effects in the increased number of typhoons during January to May 2015,as compared to the climatological state. The results of the GPI diagnosis indicated that the dynamic（ low-level vorticity and vertical velocity） and thermodynamic（ potential intensity related to sea surface and air temperature and atmospheric relative humidity in the lower to middle levels） factors both contributed positively to the increases in typhoon genesis during January to May in 2015. The low-level vorticity anomaly showed the largest contribution,while the enhanced relative humidity was the secondary contributor. The former may have been related to the Rossby wave response to the warm sea surface temperature in the central equatorial Pacific. The low-level westerly wind associated with the cyclonic anomaly may have weakened the trade winds and vertical wind shear. The processes related to increased moisture content were examined in detail through moisture budget diagnosis. The leading term was related to the advection of low-level seasonal mean moisture in the warm pool via the anomalous ascending motion of the Walker circulation. The strengthened vertical moisture advection rapidly increased from November 2014 to January 2015 and reached its maximum value in February2015,resulting in mid-level moistening and atmospheric diabatic heating. Apart from the influence of large-scale air-sea conditions,3—10-day synoptic-scale disturbances and 10—90-day intraseasonal oscillation were also enhanced during January to May 2015,favoring the genesis and development of tropical depressions/ typhoon systems.