El Ni?o发展年和La Ni?a年东亚夏季风季节内变化的比较

Comparison of Intraseasonal Variation of the East Asian Summer Monsoon between El Ni?o Developing Years and La Ni?a Years

基于1979~2013年多种再分析资料,合成分析了El Ni?o发展年和La Ni?a年东亚夏季风的季节内变化。结果表明,东亚夏季风在两种情况下呈现出不同的季节内变化特征。在El Ni?o发展年,初夏期间高纬度地区出现偏北风异常,造成东亚地区位势高度场偏低,西太平洋副热带高压偏东,但均不显著。盛夏期间,El Ni?o强迫造成中太平洋对流增强,副热带西太平洋出现气旋异常,位势高度显著降低,副热带高压明显偏东。与此不同的是,La Ni?a年春季暖池海温偏高,造成夏季对流偏强,西太平洋地区位势高度场偏低,副热带高压减弱东退。此外,La Ni?a年东亚夏季风的季节内变化较为复杂,6月异常较弱,7月达到最强,8月又开始减弱。因此,虽然El Ni?o发展年和La Ni?a年夏季平均副高异常有一定的相似性,但季节内变化则有很大差异,其成因也完全不同。

Based on various reanalysis datasets during 1979-2013, we compare the intraseasonal variation of the East Asian summer monsoon（EASM） between El Nino developing years and La Nina years. It is shown that the EASM exhibits different features in the intraseasonal variation under the two situations. During the early summer of El Nino developing years, there exist weak northerly anomalies in the high latitudes, which induce negative geopotential height anomalies over East Asian and a slightly eastward retreat of the western Pacific subtropical high（WPSH）. During the late summer, a cyclonic anomaly is induced in the subtropical western Pacific with enhanced convection in the central Pacific due to El Nino forcing. As a result, geopotential height reduces significantly and the WPSH tends to retreat eastward remarkably. In La Nina years, however, the warm pool convection in the summer clearly enhances due to warmer sea surface temperature in the western Pacific in the spring. Accordingly, geopotential height reduces and the WPSH tends to retreat eastward. Besides, the intraseasonal variation of the East Asian summer monsoon exhibits complicated characters in La Nina years with the strongest anomaly occurring in July and relatively weak anomaly occurring in June and August. Although the summer mean WPSH in El Nino developing years is somewhat similar to that in La Nina years, there is a significant discrepancy in the intraseasonal variation. More importantly, the physical mechanisms for the intraseasonal variation are completely different.