ASYMMETRIC EFFECTS OF ENSO ON PRECIPITATION IN THE WESTERN NORTH PACIFIC DURING THE FOLLOWING SUMMER
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摘要: 通过对1979—2018年Niño3.4指数和第二年夏季西北太平洋（Northwest Pacific, WNP）降水异常的分析发现，厄尔尼诺强度与第二年夏季WNP降水异常存在显著的负相关关系，而与拉尼娜并没有显著的相关关系。将厄尔尼诺和拉尼娜分为东部型和中部型以后发现，东部型厄尔尼诺和拉尼娜对次年夏季WNP降水的影响不对称，东部型厄尔尼诺强度与次年夏季WNP降水异常为负相关关系，而东部型拉尼娜与次年夏季WNP降水异常关系并不显著。相反，中部型厄尔尼诺和拉尼娜对WNP次年夏季降水的影响较为对称。不管是厄尔尼诺还是拉尼娜，其强度与次年夏季WNP降水异常相关性均非常显著。分析速度势和流函数场以及它们所对应的海表面温度场发现，强东部型厄尔尼诺、强中部型厄尔尼诺、强/弱东部型拉尼娜和强中部型拉尼娜事件中WNP降水主要受风场的辐合辐散控制，其降水异常区与辐合辐散中心对应得很好。而弱东部型厄尔尼诺和弱中部型厄尔尼诺事件中的WNP降水主要与中太平洋海温Gill型对称加热有关，在辐合中心西边产生一对气旋，导致了西北太平洋降水的正异常。而对于弱中部型拉尼娜，由于副热带中太平洋加热的作用，在西北太平洋区东部产生一个反气旋，导致了该区域的降水负异常。从而导致了强中部型拉尼娜年WNP区的降水显著大于弱中部型拉尼娜年WNP区的降水。可以看出，虽然中部型厄尔尼诺和中部型拉尼娜年降水异常在数值上是对称的，并且有显著的线性关系，但是他们对降水的影响机制是不同的。Abstract: Based on the analysis of Niño3.4 index from 1979 to 2018 and the precipitation anomaly of the next summer in the Western North Pacific(WNP), it is found that there was a significant negative correlation between the intensity of El Niño and the WNP precipitation anomaly in the following summer, while there was no significant correlation between La Niña and WNP precipitation anomaly in the following summer. After dividing El Niño and La Niña into EP type and CP type, it is found that the influence of EP El Niño and La Niña on the WNP summer precipitation is asymmetrical, and there is a negative correlation between the intensity of EP El Niño and the following summer WNP precipitation anomaly, while the correlation between EP La Niña and the following summer WNP precipitation anomaly is not significant. The effects of CP El Niño and La Niña on the following summer precipitation of WNP are symmetrical. The intensity of both CP El Niño and La Niña was significantly correlated with the WNP precipitation anomaly in the following summer. The analysis of velocity potential and stream function field shows that the precipitation of strong EP El Niño, strong CP El Niño, strong EP La Niña, weak EP La Niña and strong CP La Niña is mainly controlled by the convergence and divergence of wind field, and the precipitation anomaly area corresponds well to the convergence and divergence center. The weak EP El Niño and the weak CP El Niño are mainly influenced by the Gill' s symmetrical heating of the central Pacific Ocean, and a pair of cyclones are generated in the west of the convergence center, which leads to the positive anomaly of precipitation in the northwest Pacific Ocean. The weak CP La Niña, influenced by the heating of the subtropical central Pacific Ocean, generated an anticyclone in the eastern part of the WNP, which resulted in the negative precipitation anomaly in this region. As a result, the precipitation of strong CP La Niña in the WNP region is significantly greater than that of weak CP La Niña in the WNP region. From this, we can see that although the CP El Niño and CP La Niña precipitation are symmetrical in value, and there is a significant linear relationship, the mechanisms of their influence on precipitation are different.
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Key words:
- ENSO /
- Western North Pacific /
- precipitation /
- asymmetry
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图 1 WNP降水与常用ENSO指数的散点图
a.1979—2018年冬季Niño3.4指数与次年夏季WNP降水异常散点图；b.东部型ENSO年冬季Niño3指数与次年夏季WNP降水异常散点图；c.中部型ENSO年冬季Niño4指数与次年夏季WNP降水异常散点图；d.中部型ENSO年冬季EMI指数与次年夏季WNP降水异常散点图。降水和指数均经过标准化，R2为方差解释率，数值为相关系数r的平方，WNP区域范围为120 °E~180 °，0 °~20 °N。
图 2 东部型厄尔尼诺（a）、东部型拉尼娜（b）、中部型厄尔尼诺（c）、中部型拉尼娜（d）次年夏季WNP降水距平场
其中阴影部分通过了0.1显著性检验。红框区域为代表了WNP降水的区域。单位：mm/d。
图 3 强东部型厄尔尼诺(a)、强东部型拉尼娜(b)、弱东部型厄尔尼诺(c)、弱东部型拉尼娜(d)次年夏季降水异常合成场以及弱东部型厄尔尼诺减强东部型厄尔尼诺的差值场（e）和强东部型拉尼娜减弱东部型拉尼娜的差值场（f）
其中阴影部分通过了0.1的显著性检验。红框区域为代表了WNP降水的区域。单位：mm/d。
图 5 东部型强弱厄尔尼诺和拉尼娜次年夏季850 hPa速度势场和降水异常场
a.强东部型厄尔尼诺；b.强东部型拉尼娜；c.弱东部型厄尔尼诺；d.弱东部型拉尼娜。填色为降水场，单位：mm/d，等值线为速度势，实线为正，虚线为负，单位：105 m2/s，打点区为降水通过0.1显著性检验的地区。红框区域为WNP降水的区域。
图 7 东部型各强弱厄尔尼诺和拉尼娜的冬季到次年夏季的海表面温度合成图
从第一行到第三行依次为冬季、春季、夏季，从第一列到第四列依次为强厄尔尼诺、弱厄尔尼诺、强拉尼娜、弱拉尼娜。其中打点区通过了0.1的显著性检验，红框区域为WNP降水的区域。
图 9 东部型强弱厄尔尼诺和拉尼娜次年夏季850 hPa速度势场和海表面温度异常场
a.强东部型厄尔尼诺；b.强东部型拉尼娜；c.弱东部型厄尔尼诺；d.弱东部型拉尼娜。填色为SST场，单位：℃，等值线为速度势，实线为正，虚线为负，单位：105 m2/s。红框区域代表WNP降水的区域。打点区为SST通过了0.1的显著性检验。
图 11 东部型强弱厄尔尼诺和拉尼娜次年夏季850 hPa流函数场和降水异常场
a.强东部型厄尔尼诺；b.强东部型拉尼娜；c.弱东部型厄尔尼诺；d.弱东部型拉尼娜。填色为SST场，单位：℃。等值线为流函数，实线为正，虚线为负，单位：105 m2/s。红框区域代表WNP降水的区域。打点区为SST通过了0.1的显著性检验。
表 1 1979—2018年东部型和中部型厄尔尼诺和拉尼娜年份
类型 东部型 中部型 厄尔尼诺 1981、1982、1986、1991、1997、2006 1979、1987、1989、1990、1992、1993 2015 1994、2002、2003、2004、2009、2014 拉尼娜 1980、1984、1985、1995、1996、1999 1983、1988、1998、2000、2008、2010、 2001、2005、2007、2012、2013、2017 2011、2016 
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表 2 各类型ENSO强弱年份
类型 强 弱 东部厄尔尼诺 1982、1997、2015 1981、1986、1991、2006 东部拉尼娜 1984、1999、2005、2007、2017 1980、1985、1995、1996、2001、2012、2013 中部厄尔尼诺 1987、1990、1994、2002、2004、2009、2014 1979、1989、1992、1993、2003 中部拉尼娜 1988、1998、2010、2011 1983、2000、2008、2016
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