2013年夏季高温归因分析——热带及热带外海温影响的模拟对比

High Temperature Attribution Analysis in Summer 2013—A Comparison of the Effects of Tropical and Extratropical Sea Temperature

摘要: 西太平洋副热带高压(简称副高)整体强度偏弱、特征线控制范围面积偏小，但西段异常西伸、北抬，是2013年夏季我国东部高温事件发生期间大气环流的一个重要特征。本文利用ECHAM5大气环流模式，分别以月气候态海温、2013年夏季(5月~8月)全球、热带以及热带外海温异常作为下边界强迫场，进行模拟试验，研究了海温异常对副高的影响，进而分析高温的形成机制。结果显示：1) 全球海温异常有利于副高整体面积偏小、强度偏弱，这一影响主要来自热带海温异常的贡献，热带外海温的作用相反且是次要的，它部分抵消热带海温的影响。机制上，自印度洋至中东太平洋“负”–“正”–“负”的热带海温分布，激发Walker环流异常，西太平洋低层的纬向东风及其北侧的反气旋偏弱、偏东；同时西太平洋强的上升运动又向北下传，进而通过Hadley环流，影响副高前部环流，导致副热带地区虽有下沉运动，但主要位于对流层高层，不利于副高的加强、西伸。2) 尽管全球及热带海温强迫能够解释副高的整体行为，但模拟的副高异常偏东，不能解释该年副高西段的西伸、北抬。于是，我们从大气内部过程的角度，分析了不同海温背景下，副高西段西伸、北抬发生概率的差异，发现在2013年全球海温(特别是热带海温)背景下，发生极端偏西偏北的概率显著增加。表明：该年的高温事件可以解释为海温异常背景下发生的一个极端事件。

Abstract: In summer 2013, on the interannual timescale, the anomalous western Pacific Subtropical high (WPSH) is featured by the weaker intensity, the smaller controlling zone, the westward extension and northward shift, which is the main cause for the heat wave event. This study investigated the influence of sea surface temperature (SST) anomalies by one atmospheric general circulation model, and analyzed the possible mechanisms of the extreme event. One control experiment and three sensitive experiments were separately conducted by using different boundary forcings: the monthly climatological SST, the global SST anomaly, the tropical SST anomaly and the extratropical SST anomaly in summer of 2013 (from May to August). The results showed that: 1) The global SST anomaly is in favor of an anomalous weak WPSH. The tropical SST anomaly dominated this effect. The extratropical SST anomaly would enhance the whole WPSH, playing a secondary role. Mechanically, the SST anomaly in summer 2013, showing a negative-positive-negative pattern from the tropical Indian Ocean to the equatorial eastern Pacific, would excite anomalous Walker circulation, accompanied by (enhanced) weakened zonal westerly (easterly) anomalies at the low-level. The anomalous zonal winds could induce an anomalous weak western North Pacific anticyclone over its northern side region. On the other hand, along with the simulated anomalous strong convection, the anomalous ascending branch over the tropical western Pacific shifted northward and occurred over the entire troposphere. It could induce the anomalous descending activities over the subtropical region through the Hadley circulation, which were mainly located over the upper troposphere and shifted more northward, which went against the increase intensity and westward extension of WPSH. 2) In spite of the well-simulated weaker intensity of the whole WPSH, this model could not capture the anomalous westward extension and northward shift of the westernmost ridge line point, with an anomalous eastward retreat. Then, in view of the roles of atmospheric internal variability, we analyzed the probabilistic differences of the WPSH indices on different SST forcings. It was found that the global SST anomaly, especially the tropical SST anomaly, would significantly increase the probability of the extreme event that with anomalous westward and northward extension of WPSH, which means that the heat wave in summer 2013 might be an extreme event on the background of tropical SST anomaly.