摘要:基于多种再分析资料和观测资料，对比分析了两次超长La Ni?a事件中东亚夏季风的季节内变化。选取的两次事件分别发生在1984～1985年和1999～2000年，但强度有明显差异，其中前者为中等强度事件，而后者则为强事件。在两次事件过程中，暖池对流偏强，西太平洋副热带高压（副高）偏东偏弱，但季节内变化有很大差异。对强事件而言，6月对流开始发展，异常值在7月达到最大，8月稍弱，这与La Ni?a年合成结果一致，表明La Ni?a信号主导了东亚夏季风的季节内变化。与此不同的是，在1984～1985年事件中，6月和8月对流偏强，7月偏弱，呈双峰型异常变化。分析表明，当前一个月海温偏高时，后一个月对流偏强，减弱了太阳辐射，造成局地海温降低，偏低的海温又反过来抑制了后一个月的对流发展，暖池地区局地海气相互作用在中等强度La Ni?a事件中起到关键作用。因此，在两次超长La Ni?a事件中，东亚夏季风的季节内变化过程和影响因子有很大差异。此外，由于副高偏东，中国东部夏季降水总体上偏少。
关键词:超长La Ni?a事件/
东亚夏季风/
季节内变化/
暖池对流/
局地海气相互作用
Abstract:Based on multi-reanalysis data and observational data, this study focuses on the intraseasonal variation of the East Asian summer monsoon (EASM) during two multiyear La Ni?a events, occurred in 1984?1985 and 1999?2000, respectively. There is a significant discrepancy in intensity between two events, with moderate intensity for the former and strong intensity for the latter. The warm pool convection is enhanced during two events, inducing eastward retreat of the western Pacific subtropical high (WPSH) with weak intensity, but the intraseasonal variation is different between two events. During the strong event, convection begins to develop in June and reaches its maximum in July, then decays in August. This feature is consistent with the composite result in La Ni?a years, indicating that a La Ni?a signal dominates the EASM intraseasonal variation. By contrast, convection is enhanced in June and August with two peaks, but it is suppressed in July during 1984?1985 La Ni?a event. The result also shows that, when the sea surface temperature (SST) is higher in the preceding month, convection in the next month is enhanced along with a reduced solar radiation and local SST. Consequently, convection is suppressed due to the reduced SST in the preceding month. During the moderate La Ni?a event, the local air?sea interaction in the warm pool plays a key role. Therefore, the EASM exhibits a remarkable difference in the intraseasonal variation and impacting factors between two events. Due to eastward retreat of the WPSH, in general, there is less rainfall in the eastern part of China.
Key words:Multiyear La Ni?a event/
East Asian summer monsoon/
Intraseasonal variation/
Warm pool convection/
Local air?sea interaction
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