2015/2016厄尔尼诺事件的衰减位相及其对应的西北太平洋环流异常

Decaying phase of the 2015/2016 El Nio event and its associated western North Pacific atmospheric circulation anomaly

利用季节平均Had ISST海温、CMAP降水及NCEP风场数据,分析了2015/2016超级厄尔尼诺衰减期的特征及其对应的西北太平洋大气环流异常。结果表明:2015/2016厄尔尼诺事件除了成熟位相冬季强度大以外,还具有在随后春季衰减快,到夏季就消亡的特征。伴随着厄尔尼诺的迅速衰减,西北太平洋有较强的反气旋环流异常维持。厄尔尼诺衰减位相与西太反气旋异常存在相互作用。一方面,由于此次厄尔尼诺事件强度强,衰减期热带印度洋有显著的暖海温异常从冬季一直维持到夏季,有利于西太反气旋的增强和维持。另一方面,西太反气旋环流异常的维持及其南侧东风异常的发展使得中东太平洋正海温异常减弱,令厄尔尼诺事件快速衰退。此外,通过与1982/1983和1997/1998年比较发现,这三次超强厄尔尼诺事件虽强度相当,但衰减位相及与之相联系的西太反气旋异常都不尽相同。1982/1983事件衰减慢,维持时间长,对应的西太反气旋强度弱。而1997/1998事件衰减快,维持时间短,对应的西太反气旋强度在春季和夏季都强盛维持。2015/2016事件的衰减速度明显快于1982/1983事件,对应的西太反气旋也强于1982/1983事件,由于2015/2016事件增暖中心偏向于中东太平洋,而这里是厄尔尼诺衰减过程中负海温异常最先出现的区域,因此尽管2015/2016事件中西太反气旋异常的强度弱于1997/1998事件,但衰减速度及衰减位相维持时间与1997/1998相当。本文研究结果表明厄尔尼诺衰减与西太反气旋异常之间的关系较为复杂,需进一步研究。

Based on the seasonal mean sea surface temperature（ SST） in Had ISST,precipitation in CMAP and wind field in the NCEP/ NCAR reanalysis dataset,the decaying phase of the 2015/2016 super El Nino event and its associated western North Pacific atmospheric circulation anomaly are investigated in this study. The 2015/2016 El Nino event not only exhibits strong intensity in the mature winter,but also declines rapidly in the subsequent spring and terminates in the summer. During the 2015/2016 winter,the SST anomaly over the central and eastern tropical Pacific is greater than two standard deviations,and is 60% more than the intensity in the composite observations of other El Nino events. This case is recognized as a super El Nino event,but it rapidly declines in the subsequent spring and transforms into a La Nia event in the summer. Additionally,an anticyclonic circulation anomaly persists over the western North Pacific（ WNPAC）,accompanied by the rapid decline of El Nino. On the one hand,the strong El Nino intensity and its accompanying warming over the Indian Ocean contribute to the steady persistence of the WNPAC associated with the 2015/2016 super El Nino event. On the other hand,the a-nomalous easterlies over the central and western equatorial Pacific,induced by the strong WNPAC,favor the rapid decline of the El Nino decaying phase. Furthermore,a thorough comparison of the 2015/2016 El Nino event to the cases of 1982/1983 and1997/1998 suggests that their decaying phases are quite different,despite all being super El Ninos with similar wintertime intensity. The 1982/1983 El Nino declines slowly and persists into the following autumn,whereas the1997/1998 case declines rapidly and has already terminated in the following summer. The length of the El Nino decaying phase is associated with the strength of the springtime WNPAC. The 1982/1983（ 1997/1998） El Nino is related to a weak（ strong） WNPAC in the decaying phase. On the one hand,the decaying rate of the 2015/2016 El Nino is faster than the 1982/1983 case,due to the stronger WNPAC in the 2015/2016 El Nino than that in the1982/1983 event. On the other hand,the warmest center in the 2015/2016 case is close to the central equatorial Pacific,where the negative SST anomaly first emerges. Thus,although the WNPAC is stronger in 1997/1998 case,the decaying length is similar to the 2015/2016 event. The results imply a complicated interaction between the WNPAC and the El Nino decaying phase,which needs to be further studied.