摘要:东亚冬季风具有南北一致变化和南北反相变化两种主要模态。与第一模态反映的南北贯穿的冬季风整体强弱变化不同，第二模态体现了低纬度（中国南方地区）冬季风强弱变化不依赖于中高纬度（中国北方地区）冬季风强弱、甚至与之相反的变化状态。本文利用经验正交函数分析、相关分析、偏相关分析等方法重点研究了在第二模态背景下，低纬度（中国南方地区）冬季风强弱变化对应的热带和副热带环流异常特征。研究发现：热带辐合带是影响低纬度冬季风的一个重要系统。当热带辐合带加强并向北推进时，热带西太平洋及南海地区对流上升运动相应加强。这一上升支可能强迫出低层偏北风异常，从而引起低纬度冬季风加强。此外，副热带高空急流是影响低纬度冬季风的另一个重要系统。急流轴上风速加强会造成入口区准地转偏北风的异常，它强迫出的正次级环流也会相应加强，对应急流北侧的异常下沉和南侧的异常上升，并促使低层产生偏北风异常，也即促进了低纬度冬季风加强。进一步考察热带辐合带对流活动和副热带急流风速异常对低纬度冬季风的独立和协同影响发现，前者的影响相对更为重要。而在二者同时增强的综合作用下，可引起中国南部35°N以南地区的偏北风异常显著增强，反之亦然。上述结果揭示，冬季低纬度风场的变化不仅受到北方冷空气爆发的影响，它还受制于热带、副热带环流系统异常的共同调控作用。
关键词:东亚冬季风/
热带环流/
副热带环流/
低纬度冬季风
Abstract:The East Asian winter monsoon (EAWM) has two dominant modes: the in-phase and out-of-phase variations of wind anomalies over northern and southern China. Different from the first mode reflecting a uniform strong/weak situation of the EAWM throughout eastern China, the second mode indicates a situation that the intensity of low-latitude EAWM over southern China is independent of and even opposite to that of mid–high-latitude EAWM over northern China. The present study focuses on the characteristics of tropical and subtropical circulation anomalies associated with the variability of low-latitude EAWM under the background of the second mode by using empirical orthogonal function analysis, correlation analysis, and partial correlation analysis. The results reveal that the inter-tropical convergence zone (ITCZ) can be considered as an important circulation system that contributes to the variation of low-latitude EAWM. Corresponding to a stronger and northward-extended ITCZ, convective ascensions over the tropical western Pacific and South China Sea are strengthened. This anomalous ascension may induce low-level anomalous northerly wind, thus resulting in a stronger low-latitude EAWM. In addition, the subtropical upper-level jet can be regarded as another important circulation system affecting the low-latitude EAWM. An increase in wind speed along the axis of the jet may cause anomalous northerly quasi-geostrophic winds near the jet entrance. Associated with the forced positive secondary circulation anomaly with anomalous descent (ascension) to the north (south) of the jet, the low-level anomalous northerly wind appears under the jet, which in turn, facilitates a stronger low-latitude EAWM. Finally, both the individual and joint effects of tropical convective activities and upper-level subtropical jet on the low-latitude EAWM are further investigated. Relatively, the influence of the ITCZ seems more important. When the two circulation anomalies simultaneously increase (i.e., more active convective activity of the ITCZ and stronger wind speed along the subtropical upper-level jet), their joint effect can significantly reinforce northerly winds to the south of 35°N over southern China and vice versa. The abovementioned results imply that the variability of low-latitude EAWM is not only affected by cold air surges from northern China but also modulated by the joint effects of tropical and subtropical circulation anomalies.
Key words:East Asian winter monsoon/
Tropical circulation/
Subtropical circulation/
Low-latitude EAWM
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