Comparison of Intraseasonal Variation of the East Asian Summer Monsoon between El Niño Developing Years and La Niña Years
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摘要: 基于1979~2013年多种再分析资料,合成分析了El Niño发展年和La Niña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 Niño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 Niño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 Niña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 Niña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 Niño发展年和La Niña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。
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关键词:
- 东亚夏季风 /
- 西太平洋副热带高压 /
- 季节内变化 /
- El Niño发展年 /
- La Niña年
Abstract: Based on various reanalysis datasets during 1979-2013, we compare the intraseasonal variation of the East Asian summer monsoon (EASM) between El Niño developing years and La Niña years. It is shown that the EASM exhibits different features in the intraseasonal variation under the two situations. During the early summer of El Niño 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 Niño forcing. As a result, geopotential height reduces significantly and the WPSH tends to retreat eastward remarkably. In La Niña 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 Niña years with the strongest anomaly occurring in July and relatively weak anomaly occurring in June and August. Although the summer mean WPSH in El Niño developing years is somewhat similar to that in La Niña years, there is a significant discrepancy in the intraseasonal variation. More importantly, the physical mechanisms for the intraseasonal variation are completely different. -
图 3 (a)向外长波辐射(单位:W/m2)、(b)850 hPa风场、(c)500 hPa位势高度(单位:gpm)盛夏(7月20日至8月31日)与初夏(6月1日至7月19日)的差值场;(d)夏季平均(实线)、初夏(长虚线)和盛夏(短虚线)西太平洋副热带高压(单位:gpm)
Figure 3. Differences between the late summer (20 July to 31 August) and early summer (1 June to 19 July): (a) Outgoing longwave radiation (W/m2); (b) 850-hPa wind (m/s); (c) 500-hPa geopotential height (gpm). (d) The western Pacific subtropical high (gpm) of the summer mean (solid), the early summer (long-dashed contours), and late summer (short-dashed contours)
图 4 (a)El Niño发展年春季、(b)La Niña年春季、(c)El Niño发展年夏季、(d)La Niña夏季海表温度异常(单位:℃)。阴影区为超过95%显著性检验的区域
Figure 4. Sea surface temperature anomalies (℃) in (a) the spring of El Niño developing years, (b) the spring of La Niña years, (c) the summer of El Niño developing years, and (d) the summer of La Niña years. Shaded areas indicate significance at/above the 95% confidence level
图 5 El Niño发展年初夏东亚夏季风异常场(阴影区为超过95%显著性检验的区域):(a)向外长波辐射(单位:W/m2);(b)850 hPa风场;(c)500 hPa位势高度(单位:gpm);(d)气候平均(实线)和El Niño发展年初夏(虚线)西太平洋副热带高压(单位:gpm)
Figure 5. East Asian summer monsoon anomalies in the early summer of El Niño developing years (shaded areas indicate significance at/above the 95% confidence level): (a) Outgoing longwave radiation (W/m2); (b) 850-hPa wind (m/s); (c) 500-hPa geopotential height (gpm). (d) The western Pacific subtropical high (gpm) of the climatology (solid contours) and in the early summer of El Niño developing years (dashed contours)
图 7 La Niña年6月东亚夏季风异常场(阴影区为超过95%显著性检验的区域):(a)向外长波辐射(单位:W/m2);(b)850 hPa风场;(c)500 hPa位势高度(单位:gpm)。(d)气候平均(实线)和La Niña年6月(虚线)西太平洋副热带高压(单位:gpm)
Figure 7. East Asian summer monsoon anomaly in June of La Niña years (shaded areas indicate significance at/above the 95% confidence level): (a) Outgoing longwave radiation (W/m2); (b) 850-hPa wind; (c) 500-hPa geopotential height (gpm). (d) The western Pacific subtropical high (gpm) of the climatology and in June of La Niña years
图 10 夏季平均西太平洋副热带高压(单位:gpm):(a)El Niño发展年;(b)La Niña年。图 10a和图 10b中的长虚线分别为El Niño发展年和La Niña年,实线为气候平均
Figure 10. Summer mean western Pacific subtropical high (gpm): (a) El Niño developing years; (b) La Niña years. The dashed contours in Fig. 10a and Fig. 10b represent El Niño developing years and La Niña years, respectively. The solid contours represent the climatology
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