Contribution of Tropical and Subtropical Circulation Anomalies to the Intensity of East Asian Winter Monsoon over Lower-Latitude Region
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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。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.
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图 1 1960/1961～2017/2018年冬季东亚1000 hPa经向风（简称v1000）EOF（a）第1、（b）2模态及其各自对应的时间系数，其中（c）和（d）分别为EOF1和EOF2的时间系数。黑色阴影表示1500 m以上地形，下同
Figure 1. The (a) first and (b) second empirical orthogonal function (EOF) modes of 1000 hPa meridional winds (v1000) in East Asia from the winter of 1960/1961 to the winter of 2017/2018 and corresponding time series of the principal component (PC), in which (c) PC1 is for EOF1 and (d) PC2 for EOF2. The black shading denotes topography higher than 1500 m, the same below
图 2 冬季EOF2（a）正位相和（b）负位相合成的1000 hPa风距平（单位：m s−1）以及（c）二者差值（正位相减负位相）。正、负位相分别有14年和12年，是以标准化PC2超过±0.75标准差为标准选择的；黄色阴影表示经向风差值通过95%置信度检验
Figure 2. Composites of winter 1000hPa wind anomalies for (a) positive and (b) negative phases of EOF2 and (c) the difference between the two phases (units: m s−1). There are 14 (12) years with positive (negative) phases, which are selected when the normalized PC2 is higher (lower) than 0.75(−0.75) standard deviation. The yellow shading indicates the differences of meridional winds significant at the 95% confidence level
图 3 冬季EOF2（a）正位相和（b）负位相合成的海平面气压（SLP，等值线，）及其距平（阴影）以及（c）二者距平差值（正位相减负位相）。正、负位相分别有14年和12年，是以标准化的PC2超过±0.75标准差为标准选择的；单位：hPa。图中黑色圆点表示（a，b）距平和（c）差值通过95%置信度检验
Figure 3. Composites of winter SLPs (Sea Level Pressure, contour) and their anomalies (shaded) for (a) positive and (b) negative phases of EOF2; (c) the difference between the SLP anomalies in two phases (units: hPa). There are 14 (12) years with positive (negative) phases, which are selected when the normalized PC2 is higher (lower) than 0.75(−0.75) standard deviation. The black dots indicate anomalies in (a) and (b) and differences in (c) that are significant at the 95% confidence level
图 4 （a）1979/1980～2017/2018年冬季PC2与同期向外长波辐射（OLR）的相关系数分布；（b）和（c）分别是15个EOF2正位相年和11个负位相年合成的OLR距平（单位：W m−2）。黑色圆点表示（a）相关系数和（b，c）EOF2正负位相合成OLR差值通过95%置信度检验，(a) 中红色方框表示热带西太平洋对流关键区
Figure 4. (a) Distribution of correlation coefficients between the winter PC2 and simultaneous outgoing longwave radiation (OLR) from the winter of 1979/1980 to the winter of 2017/2018; (b) composite anomalies of winter OLR (units: W m−2) in 15 positive-phase years of EOF2; (c) as in (b), but for 11 negative-phase years. The black dots indicate correlation coefficients in (a), and the differences between the composite OLR anomalies in positive-phase and negative-phase years in (b) and (c) are significant above the 95% confidence level. The red square in (a) denotes the key area of convection over the tropical western Pacific
图 5 1960/1961～2017/2018年冬季（a）PC2与同期200 hPa 纬向风的相关系数分布，其中黑色圆点表示相关系数通过95%置信度检验，红色方框为副热带西风急流关键区；（b）气候平均200 hPa 纬向风（单位：m s−1）
Figure 5. (a) Distribution of correlation coefficients between PC2 and 200 hPa zonal winds, the black dots indicate the correlation coefficients significant at the 95% confidence level from the winter of 1960/1961 to the winter of 2017/2018, the red square denotes the key area of subtropical westerly jet; (b) climate mean value of zonal wind at 200 hPa (units: m s−1)
图 6 1960/1961～2017/2018年冬季PC2回归的105°～135°E平均垂直环流距平（黑色矢量v, ω，v单位：m s−1，ω单位: hPa h−1）和纬向风距平（等值线和阴影，单位：m s−1）。黑色圆点表示回归纬向风距平通过95%置信度检验
Figure 6. Anomalies of vertical circulation (black vectors, v, ω, v units:m s−1, ω: hPa h−1) and zonal winds (contours and colorful shadings, units: m s−1) averaged along 105°–135°E regressed upon PC2 from the winter of 1960/1961 to the winter of 2017/2018. The black dots indicate the regressed zonal wind anomalies that are significant at the 95% confidence level
图 9 1995/1996年冬季（a）1000 hPa风距平（单位：m s−1）和（b）OLR距平（单位：W m−2）和（c）105°～135°E平均的垂直环流距平（矢量）和纬向风距平（等值线和阴影）。红线表示正距平，蓝线表示负距平
Figure 9. Anomalies of (a) 1000 hPa winds (units: m s−1), (b) OLR (units: W m−2), and (c) vertical circulation (vectors) and zonal winds (contours and shadings) averaged along 105°–135°E during the winter of 1995/1996. The red lines are for positive anomalies, and the blue lines for negative anomalies
图 7 1960/1961～2017/2018年冬季（a）西风急流独立指数和（b）西太平洋对流独立指数与105°～135°E平均垂直环流距平（黑色矢量，v，ω，v单位：m s−1，ω单位: hPa h−1）和纬向风距平（等值线和阴影，单位：m s−1）偏回归系数分布。黑色圆点表示回归纬向风距平通过95%置信度检验
Figure 7. Distributions of the partial regression coefficients between the anomalies of vertical circulation (black vectors, v, ω, v units: m s−1, ω: hPa h−1) and zonal winds (contours and colorful shadings, units: m s−1) averaged along 105°E–135°E and the independent indexes of (a) westerly jet and (b) convection over the tropical western Pacific Ocean from the winter of 1960/1961 to the winter of 2017/2018, respectively. The black dots indicate the regressed zonal wind anomalies that are significant at the 95% confidence level
图 8 1960/1961～2017/2018年冬季（a）西风急流独立指数、（b）西太平洋对流独立指数和（c）西风急流—热带西太平洋对流上升运动合成指数回归得到的1000 hPa风距平（单位：m s−1）。黄色阴影表示回归的经向风距平通过95%置信度检验
Figure 8. Anomalous 1000 hPa winds obtained by the regression against the independent indexes of (a) westerly jet and (b) convection over the tropical western Pacific Ocean, and (c) the index reflecting the joint variation of westerly jet and convection over the tropical western Pacific Ocean from the winter of 1960/1961 to the winter of 2017/2018. The yellow shaded areas indicate the regressed meridional wind anomalies that are significant at the 95% confidence level
图 10 1995年12月21～26日OLR（左）、500 hPa 垂直速度（ω，中）和v1000（右）标准化距平的逐日变化
Figure 10. The daily variations of standardized anomalies of OLR (left column) and vertical velocity (ω) at 500 hPa (middle column) and v1000 (right column) from 21 to 26 December 1995
图 11 1995/1996年冬季逐日标准化热带西太平洋500 hPa垂直速度指数序列（蓝线）和我国南方地区v1000指数序列（红线）。蓝色箭头表示5次的热带西太平洋垂直上升距平加强过程，红色箭头表示4次中国南方地区偏北风距平加强
Figure 11. The index series of daily standardized vertical velocity over the tropical western Pacific Ocean (blue line) and the v1000 over southern China (red line) in the winter of 1995/1996. The blue arrows denote five processes of the enhancement of vertical ascending movement anomalies over the tropical western Pacific Ocean. The red arrows denote four processes of the enhancement of northerly anomalies over southern China
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