Abstract
This study investigates why an extreme hot midsummer occurred in Central and South China (CSC) during 2017. It is shown that the western North Pacific subtropical high (WNPSH) was abnormally intensified and westward-extending, resulting in anomalous high pressure and consequent extreme heat over CSC. The abnormal WNPSH was favored by the warming of the western tropical Pacific (WTP), which was unrelated to ENSO and manifested its own individual effect. The WTP warming enhanced the convection in-situ and led to anomalous high pressure over CSC via a local meridional circulation. The influence of the WTP was confirmed by CAM4 model experiments. A comparison between the 2017 midsummer and 2010 midsummer (with a stronger WNPSH but weaker extreme heat) indicated that the influence of the WNPSH on extreme heat can be modulated by the associated precipitation in the northwestern flank.
The role of the WTP was verified by regression analyses on the interannual variation of the WTP sea surface temperature anomaly (SSTA). On the other hand, the WTP has undergone prominent warming during the past few decades, resulting from decadal to long-term changes and favoring extreme warm conditions. Through a mechanism similar to the interannual variation, the decadal to long-term changes have reinforced the influence of WTP warming on the temperature over CSC, contributing to the more frequent hot midsummers recently. It is estimated that more than 50% of the temperature anomaly over CSC in the 2017 midsummer was due to the WTP warming, and 40% was related to the decadal to long-term changes of the WTP SSTA.
概要
2017年盛夏7-8月中国南方地区出现了大范围高温. 同时, 热带西太平洋海温异常偏暖, 而热带中东太平洋海温异常信号弱, 因此, 2017年暖夏可以体现热带西太平洋暖海温对高温的单独影响. 热带西太平洋暖海温有利于加强局地对流, 并进一步激发异常的局地经圈环流, 其下沉支使西太副高西伸加强, 在中国南方上空形成反气旋式异常, 从而有利于高温发生. 热带西太平洋暖海温的作用在CAM4模式数值试验中也得到了验证. 事实上, 西太副高的强度在2010年达到最强, 但2010盛夏的高温频次不如2017年多, 这是因为西太副高对高温的影响还受到副高西北侧降水的调节.
基于热带西太平洋海温的年际变化序列进行回归分析, 结果也表明热带西太平洋海温对中国南方盛夏温度有显著影响. 另一方面, 热带西太平洋在过去几十年呈现明显的增温, 其中包含年代际变化和长期变化趋势. 热带西太平洋在年代际和长期变化时间尺度上的增暖有利于极端暖位相的出现, 并且通过与年际分量类似的物理过程对中国南方温度产生影响, 因而有利于近年来高温的频繁发生. 根据回归方程估算, 2017年盛夏中国南方的温度异常有50%来自热带西太平洋暖海温的贡献, 其中40%与热带西太平洋海温的年代际和长期增暖趋势有关.
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Acknowledgements
We thank the two reviewers for their helpful comments and suggestions. This work was jointly supported by National Key R&D Program of China (Grant No. 2016YFA0600601), the National Natural Science Foundation of China (Grant Nos. 41605027, 41721004, 41530530 and 41731173), the Leading Talents of Guangdong Province Program, the Pioneer Hundred Talents Program of the Chinese Academy of Sciences, and the Fundamental Research Funds for the Central Universities.
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• The abnormally intensified WNPSH that caused the extreme heat over CSC in 2017 was favored by the WTP warming.
• The influence of the WNPSH on such extreme heat can be modulated by the associated precipitation in the northwestern flank.
• The decadal to long-term changes of the WTP SSTA have contributed prominently to the extreme heat over CSC.
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Chen, R., Wen, Z., Lu, R. et al. Causes of the Extreme Hot Midsummer in Central and South China during 2017: Role of the Western Tropical Pacific Warming. Adv. Atmos. Sci. 36, 465–478 (2019). https://doi.org/10.1007/s00376-018-8177-4
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DOI: https://doi.org/10.1007/s00376-018-8177-4