Response Characteristics of the East Asian Winter Monsoon to ENSO Events
Response Characteristics of the East Asian Winter Monsoon to ENSO Events
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发表时间：
2008
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通讯作者：
He Xi
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作者：
He Xi
ENSO现象是海气相互作用中最突出的信号，在东亚冬季风的年际变化中起着重要作用。本文利用海洋Nio指数（ONI）和NCEP/NCAR再分析资料，采用复合方法分析了东亚冬季风对ENSO事件的响应，并将1950—2005年冬季分为强El Nio、强La Nia、弱El Nio、弱La Nia和非ENSO事件5类。结果表明，东亚低纬度环流对ENSO事件的响应具有准线性特征。热带大气对暖事件的响应与冷事件的响应相反，对弱ENSO事件的响应与强ENSO事件相似，只是强度减弱。而中高纬环流异常在冷暖ENSO事件中的分布不表现出明显的相位反转，说明大气对海温异常的响应是复杂的。热带外大气对强El Nio事件的显著响应信号主要表现为东亚中高纬度西风气流增强、北亚正距温、长江中下游异常风切变向东扩展。菲律宾海异常反气旋与西风带南支槽增强共同作用，有利于南海及中国东南沿海的偏南气流。而在强拉尼娜a事件期间，长江中下游脊不明显，中国南部和南海被偏北异常覆盖，面积缩小，通过显著性检验。弱El Nio事件下，菲律宾海反气旋异常西北方向的西南气流东移，与西风带南支槽前的西南气流分离，导致南海和南海偏南异常不如强El Nio事件下明显。非常有趣的是，在弱拉尼娜a的情况下，反常系统与强厄尔尼诺相似，但相位相反。如图所示ONI的零星地块指标和NTI指标代表温暖状态在亚洲北部,NTI积极在8与强大的厄尔倪o 9冬天,冬天1的7 El倪o较弱,冬天在4 - 8和强大的La倪11 12冬天拉倪较弱,也没有明确的证据表明,在亚洲出现的极端的温暖和寒冷的冬天与ENSO事件密切相关。
The ENSO phenomenon,which is known to be the most prominent signal in air-sea interactions,plays an important role in the interannual variability of East Asian winter monsoon.In this paper,the responses of East Asian winter monsoon to ENSO events are analyzed using the composite method based on the Oceanic Nio Index(ONI) and NCEP/NCAR reanalysis data with classification of the winters from 1950-2005 into 5 categories of strong El Nio,strong La Nia,weak El Nio,weak La Nia and non-ENSO event.The results show that response of the East Asian circulation in low latitudes to ENSO events has quasi-linear characteristics,i.e.response of the tropical atmosphere to the warm events is reverse to that of cold events,and that to weak ENSO events is similar to strong ENSO events except for weakened intensity.However,distribution of circulation anomalies in middle and high latitudes does not show obvious phase reversal for warm and cold ENSO events,indicating that response of the atmosphere to SST anomaly is complex.The signals of significant response of the extratropical atmosphere to strong El Nio events are represented mainly by enhancement of the westerly flow in middle and high latitudes in East Asia,positive anomalous temperature in northern Asia,eastward extension of anomalous wind shear in the middle and lower reaches of the Yangtze River.The anomalous Philippine Sea anticyclone,working together with intensified trough in south branch of westerlies,is favorable for the southerly flow in the South China Sea and along the southeast coast of China.But,during the strong La Nia events,the ridge in the middle and lower reaches of the Yangtze River is not evident,and the southern part of China and the South China Sea are covered by northerly anomaly,with shrinked area that passes significance test.Under weak El Nio events,the southwesterly flow in the northwest of the anticyclonic anomaly in the Philippine Sea is shifted eastward,and separated with southwesterly flow ahead of the trough in south branch of westerlies,resulting in the southerly anomaly in South China and the South China Sea less notable than that under strong El Nio events.It is very interesting to see anomalous systems in weak La Nia cases to be similar to those with strong El Nio but with inverse phase.As shown by scattered plots of ONI index and NTI index that represents warmth status in northern Asia,the NTI is positive in 8 out of 9 winters with strong El Nio and 1 out of 7 winters with weak El Nio,and negative in 4 out of 8 winters with strong La Nia and 11 out of 12 winters with weak La Nia,and there is no clear evidence that occurrences of extreme warm and cold winters in Asia are closely linked with ENSO events.