Distribution of the tropical Pacific surface zonal wind anomaly and its relation with two types of El Nio

E1 Nino events with an eastern Pacific pattern （EP） and central Pacific pattern （CP） were first separated using rotated empirical orthogonal functions （REOF）. Lead/lag regression and rotated singular value decomposition （RSVD） analyses were then carried out to study the relation between the surface zonal wind （SZW） anomalies and sea surface temperature （SST） anomalies in the tropical Pacific. A possible physical process for the CP E1 Nifio was proposed. For the EP E1 Nino, strong westerly anomalies that spread eastward continuously produce an anomalous ocean zonal convergence zone （ZCZ） centered on about 165°W. This SZW anomaly pattern favors poleward and eastward Sverdrup transport at the equator. For the CP E1Nino, westerly anomalies and the ZCZ are mainly confined to the western Pacific, and easterly anomalies blow in the eastern Pacific. This SZW anomaly pattern restrains poleward and eastward Sverdrup transport at the equator; however, there is an eastward Sverdrup transport at about 5°N, which favors the wanning of the north-eastern tropical Pacific. It is found that the slowness of eastward propagation of subsurface warm water （partly from the downwelling caused by Ekman convergence and the ZCZ） is due to the slowdown of the undercurrent in the central basin, and vertical advection in the central Pacific may be important in the formation and disappearance of the CP E1 Nifio.

不同类型El Nio事件对黑潮流域影响的统计分析

利用全球海表面温度、海面流速、海平面异常、降雨和海面风场融合及再分析资料,采用相关分析和合成分析统计诊断方法,对1979-2014年发生的东太平洋和中太平洋El Nio事件对黑潮流域的影响进行分析,为黑潮流域及我国东南部气候变化预测提供参考。研究结果表明:前者对海表面温度的影响要比后者强烈,并经历了从降温到升温的效应转换;两者对黑潮流域海流流速的影响整体上呈相反状态,前者对东海PN断面海流流速起减缓作用,而后者对其流速起加速作用;对于黑潮流域的海平面异常,两者均没有明显的规律性影响;前者会引起黑潮流域降水增多,而后者则会引起黑潮流域降水减少。因此,东太平洋和中太平洋El Nio事件对黑潮流域的影响具有明显的差异。

Asymmetry of Atmospheric Responses to Two-Type El Nino and La Nina over Northwest Pacific

The mechanism for asymmetric atmospheric responses to the central Pacific(CP) El Ni?o and La Ni?a over the western North Pacific(WNP) is studied in this paper. The negative anomalies of rainfall over the key region of WNP are explained by diagnosing the column-integrated equations of moisture and moist static energy(MSE). It is revealed that the nonlinear advection of moist enthalpy is critical to introduce negative rainfall anomalies over the region. The anomalous easterly(westerly) in La Ni?a(CP El Ni?o) causes negative advection of anomalous moist enthalpy, inducing negative heating anomaly and an anticyclone anomaly in the WNP, which weakens(strengthens) the cyclone(anticyclone) in La Ni?a(CP El Ni?o). The MSE budget analysis shows a larger nonlinear term in CP El Ni?o than in eastern Pacific(EP) El Ni?o, inconsistent with the amplitudes of sea surface temperature anomalies. The reason is that the nonlinear term transforms to positive above 700 h Pa in EP El Ni?o, offsetting the negative advection below 700 h Pa and thus making the nonlinear term smaller. The nonlinear term is negative at low levels in CP El Ni?o, resulting in a larger nonlinear term. The stronger precipitation anomalies in the WNP during EP El Ni?o can be attributed to the linear moist enthalpy advection. The mean easterly wind at mid levels causes a larger(smaller) positive moist enthalpy advection in CP(EP) El Ni?o, due to a larger(smaller) moist enthalpy gradient, resulting in a positive(negative) linear moist enthalpy advection, which weakens(strengthens) the negative precipitation anomalies in the key region.

厄尔尼诺事件对中国夏季水汽输送和降水分布影响的新研究

基于1961—2016年中国地面台站降水观测资料和多种再分析资料,分析了东部型和中部型两类厄尔尼诺事件对中国夏季水汽输送和降水的不同影响。结果表明:(1)厄尔尼诺事件对中国夏季降水的影响在发生当年和次年有明显的不同,主要影响是在其发生的次年,中国大部分地区的夏季降水明显偏多。(2)东部型厄尔尼诺事件当年夏季,西北太平洋副热带高压(副高)偏东偏弱,水汽输送条件较弱,不利于中国大范围降水的发生;中部型事件当年夏季,低纬度印度洋和西太平洋蒸发异常偏强,来自阿拉伯海、孟加拉湾和西北太平洋向华南地区的水汽输送和净水汽收支增加,有利于华南地区降水的异常增多。(3)东部型厄尔尼诺事件次年夏季,副热带太平洋蒸发异常偏强,副高西伸,由于东亚-太平洋(EAP)遥相关型的建立,副高西侧的强西南气流将来自太平洋蒸发的大量水汽持续输送至中国中东部地区。此外,在东亚-太平洋遥相关型影响下中高纬度地区建立了亚洲双阻型环流,其间的低槽冷涡与上游阻高之间的强偏北气流有利于北冰洋的水汽持续输送到西北和华北北部地区,中国大部分地区净水汽收支均增加,中国北方和南方地区的降水均产生了明显的同步性增多响应,形成了南北两条异常雨带。中部型厄尔尼诺事件次年夏季,副高较常年偏西且偏北,来自太平洋蒸发的大量水汽输送到江淮地区,使其净水汽收支增加和降水偏多。因此,厄尔尼诺事件的发生不仅对长江流域和淮河流域等南方地区的降水有重要影响,对华北、东北和西北地区的降水异常也有相当的作用。