LAUSR

The anomalous behavior of the western Pacific subtropical high (WPSH) in El Niño developing summer is studied based on the composite results of eight major El Niño events during 1979–2013. It is shown that the WPSH tends to retreat eastwards with weak intensity during the developing summer. The anomaly exhibits an intraseasonal variation with a weaker anomaly in June and July and a stronger anomaly in August, indicating that different underlying physical mechanisms may be responsible for the anomalous WPSH during early and late summer periods. In June and July, owing to the cold advection anomaly characterized as a weak northerly anomaly from high latitudes, geopotential height in East Asia is reduced and the WPSH tends to retreat eastwards slightly. By contrast, enhanced convection over the warm pool in August makes the atmosphere more sensitive to El Niño forcing. Consequently, a cyclonic anomaly in the western Pacific is induced, which is consistent with the seasonal march of atmospheric circulation from July to August. Accordingly, geopotential height in the western Pacific is reduced significantly, and the WPSH tends to retreat eastwards remarkably in August. Different from the developing summer, geopotential height in the decaying summer over East Asia and the western Pacific tends to enhance and extend northwards from June to August consistently, reaching the maximum anomaly in August. Therefore, the seasonal march plays an important role in the WPSH anomaly for both the developing and decaying summer.摘要基于 1979––2013 年间 8 个主要 El Niño 事件, 合成分析了发展年夏季西太平洋副热带高压 (副高) 的异常变化. 结果表明, 发展年夏季副高偏东, 强度偏弱. 副高异常呈现显著的季节内变化, 6––7 月异常较弱而 8 月异常较强, 说明初夏和盛夏副高异常的物理机制是不同的. 在 6––7 月, 由于高纬度环流变化造成北风和冷平流异常, 东亚地区位势高度降低, 副高略为偏东. 但在 8 月, 由于暖池对流增强造成大气对 El Niño 强迫更为敏感. 西太平洋地区出现气旋异常, 这与 7––8 月大气环流的季节进程相合. 在此情况下, 西太平洋地区位势高度显著降低, 副高亦随之大幅度东退. 与发展年夏季不同的是, 东亚和西太平洋地区位势高度在衰减年夏季增强, 并在 6––8 月一致向北推进, 在 8 月异常达到最强. 因此, 季节进程对发展年和衰减年副高异常变化均起到重要作用.