Coupling Mode of Westerly–Monsoonal Flow over the Tibetan Plateau and its Seasonal Variation
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摘要: 青藏高原是我国的水塔,西风与季风及其相互作用是导致亚洲天气和气候变化最重要的环流系统。本文基于1981~2020年大气再分析资料,采用经验正交函数分解方法(Empirical Orthogonal Function,EOF)提取了西风与季风季节循环分量在青藏高原的耦合模态,并对其季节变化特征进行分析。研究发现,第一主模态方差贡献率高达78.39%,主要反映的是东亚季风、南亚季风和对流层高层中纬度西风的季节循环特征及各个季节的年际变化特征。夏季在对流层高层高原及其南侧主要为东风气流,范围从北纬5°至35°,对流层低层则表现为典型的绕高原气旋式季风环流系统,热带和副热带地区为西南季风控制,冬季的环流结构刚好相反。耦合模态的冬、夏季节转换节点与东亚季风和南亚季风的季节转换时间基本一致。从年际变化的角度来看,各个季节耦合模态的强度偏强时,东亚季风和南亚季风均偏强,西风带位置偏北;反之,季风偏弱,西风带位置偏南。厄尔尼诺—南方涛动(El Niño–Southern Oscillation,ENSO)是影响西风与季风耦合模态年际变化的关键外强迫,拉尼娜(La Niña)事件发生的前夏、前秋和次年夏季耦合模态的强度均增强,冬季至次年春季耦合模态的强度均减弱。西风与季风耦合的第二主模态主要表现为对流层高层高原上的东风及其南侧西风,以及低层南亚季风区的西南季风和西北太平洋反气旋的协同变化特征。该模态的方差贡献率为4.68%,表现出明显年际差异的同时还呈现显著减弱的长期趋势,尤其是在冬季。Abstract: Tibetan Plateau (TP) is regarded as the “Chinese Water Tower.” Interaction between the westerly and monsoon flows around the TP has an important impact on the Asian climate. Based on the atmospheric reanalysis dataset during 1981–2020, we extracted the coupling modes of the seasonal cycle component of westerly wind and monsoon flow over the TP via the empirical orthogonal function (EOF) method, and discussed their seasonal variation characteristics. The first mode accounts for 78.39% of the total variances, mainly reflecting the seasonal cycle characteristics of the East Asian monsoon, South Asian monsoon, and midlatitude westerly wind, as well as their interannual variation in each season. In summers, easterly winds prevail on the TP and the southern side of TP at the upper troposphere, ranging from 5° to 35°N. Simultaneously, the lower troposphere is characterized by a typical cyclonic monsoon circulation around the TP, and the tropical and subtropical areas are controlled by the southwest monsoon. Notably, the circulation structure is opposite in the winter. The transit timing of this mode from winter (summer) to summer (winter) is consistent with the onset (ending) of East Asian and South Asian summer monsoon. On the interannual timescale, the enhancement of the coupling mode is correlated with the intensity of East Asian and South Asian monsoons, as well as the northward movement of westerly in each season. For a weak coupling mode, the monsoon and westerly display opposite characteristics. El Niño–Southern Oscillation is the key external force affecting the interannual variation of the westerly monsoon coupling mode. Its impact is strengthened in summer, autumn, and the following summer, while weakened in winter and the following spring of the La Niña event. The second coupling mode of westerly monsoon is characterized by the coordinated variation of easterly wind over the TP and westerly wind in the south of the TP in the upper troposphere, and the southwesterly in the South Asian monsoon region and anticyclone in the Northwest Pacific at the lower troposphere. The variance contribution rate of this mode is 4.68%, showing the interannual variation with a significant long-term weakening trend, especially in winter.
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图 1 (a)春、(d)夏、(g)秋、(j)冬季气候平均的850 hPa风场相对于其年平均值的距平(矢量箭头,m s−1);(b)春、(e)夏、(h)秋、(k)冬季气候平均的200 hPa风场相对于其年平均值的距平(矢量箭头,单位:m s−1),以及纬向风速相对于其年平均值的距平(U_speed,填色,单位:m s−1);(c)春、(f)夏、(i)秋、(l)冬季沿75°~100°E平均的气候平均风场 [由垂直速度(单位:−102 m s−1)和经向速度(单位:m s−1)合成] 相对于其年平均值的距平,以及垂直风速相对于其年平均值的距平(填色,单位:−102 m s−1)的经向—垂直剖面
Figure 1. Seasonal characteristics of the climatic winds relative to their annual mean. (a), (d), (g), and (j) are the wind anomalies at 850 hPa in spring, summer, autumn, and winter, respectively, relative to their annual mean (vectors, units: m s−1); (b), (e), (h), and (k) are the wind anomalies (vectors, units: m s−1) and zonal wind speed anomalies (U_speed, coloring, units: m s−1) at 200 hPa in spring, summer, autumn, and winter, respectively, relative to its annual mean; (c), (f), (i), and (l) are the meridional–vertical cross-sections (75°~100°E mean) of wind anomalies [vector, the combination of meridional winds (units: m s−1) and vertical winds (units: −102 m s−1)] and vertical wind speed (coloring, units: −102 m s−1) in spring, summer, autumn, and winter, respectively, relative to its annual mean
图 2 1981~2020年(0°~60°N,30°~160°E)区域内对流层高、低层环流逐日季节循环分量的EOF分析结果,其中(a)和(b)分别为西风—季风耦合第一主模态的高层(200 hPa)和低层(850 hPa)环流,(c)和(d)分别为西风—季风耦合第二主模态的高层(200 hPa)和低层(850 hPa)环流,矢量为风场,填色为200 hPa纬向风速(U_speed);(e)为西风—季风耦合模态第一主成分(PC1,红色实线)和第二主成分(PC2,蓝色实线)的气候平均值(气候态为1981~2010年)及其年际变率(红色和蓝色阴影分别为第一主成分和第二主成分的年际变率)
Figure 2. Coupling modes of the westerly monsoon flow. EOF analysis results of the daily annual cycle components of the upper and lower troposphere circulation in the region of (0°–60°N, 30°–160°E) from 1981 to 2020. (a) and (b) are the circulation of the first mode of the westerly monsoon at the upper level (200 hPa) and lower level (850 hPa), respectively. (c) and (d) represent the circulation of the second mode of westerly monsoon in the upper level (200 hPa) and lower level (850 hPa), respectively. The vectors represent the winds, while the coloring represents the zonal wind speed (U_speed). (e) depicts the climate mean of the first (PC1, solid red line) and second (PC2, solid blue line) principal components of the westerly monsoon coupled mode and their interannual variability (red and blue shades denote the interannual variability of the first and second principal components, respectively)
图 3 西风—季风耦合模态PC1的振幅和位相回归的夏季低层风场。(a)振幅(A)与夏季平均850 hPa风场的回归系数;(b)由冬至夏季节转换时间(P1)和(c)由夏至冬季节转换时间(P2)早晚与夏季平均850 hPa风场的回归系数。蓝色矢量箭头表示回归系数通过90%信度检验的区域
Figure 3. The lower level (850 hPa) winds in summer regressed by the amplitude and phases of PC1. Regression coefficients of (a) amplitude (A), (b) the transit time from winter to summer (P1), and (c) the transit time from summer to winter (P2) with winds averaged at 850 hPa in summer. The blue vector arrows represent the area where the regression coefficient is statistically significant at the 90% confidence level
图 4 西风—季风耦合的第一(红色实线)和第二(蓝色虚线)主模态所对应的主成分在(a)春、(b)夏、(c)秋、(d)冬季季节平均的年际变化时间序列;(e)春、(f)夏、(g)秋、(h)冬季西风—季风耦合模态的PC1与同期南亚季风(浅蓝色)和东亚季风(深蓝色)指数的相关系数,自下而上的两条红色虚线分别表示相关系数通过90%和99%信度检验的阈值,南亚季风指数(WY)和东亚季风指数(ZHW)分别取自Webster and Yang(1992)和Zhu et al.(2005)
Figure 4. Interannual variation time series of the first (red solid line) and second (blue dotted line) principal components (PC1 and PC2) corresponding to the coupling mode of westerly monsoon averaging in (a) spring, (b) summer, (c) autumn, and (d) winter, respectively. Correlation coefficients between the PC1 of the westerly monsoon coupling mode with South Asian monsoon (sky blue bar) and East Asian monsoon (blue bar) in (e) spring, (f) summer, (g) autumn, and (h) winter, respectively. The two red dashed lines from bottom to top in (e), (f), (g) and (h) represent the threshold values of correlation coefficients passing the 90% and 99% reliability tests, respectively. The South Asian monsoon and East Asian monsoon indices are calculated according to Webster & Yang (1992) and Zhu et al. (2005), respectively
图 5 不同季节西风—季风耦合模态PC1与同期环流异常的年际变化关系。(a)春、(c)夏、(e)秋、(g)冬季PC1与异常降水(PREC,填色)及850 hPa水汽通量(uvq,矢量箭头)相关系数的分布;(b)春、(d)夏、(f)秋、(h)冬季PC1与250 hPa异常辐散风场(DV_winds,矢量箭头)及纬向风速(U_speed,填色)相关系数的分布,黑色等值线为气候态的纬向西风。图中的蓝色箭头仅绘制了PC1与850 hPa水汽通量距平和250 hPa辐散风场距平之间相关系数分别通过90%信度检验的区域
Figure 5. The interannual relationship between the PC1 of the westerly monsoon coupling mode in different seasons and the circulation anomalies over the same period. Distributions of the correlation coefficients between PC1 and precipitation anomalies (PREC, coloring), as well as PC1 and water vapor flux anomalies at 850 hPa (uvq, vector) in (a) spring, (c) summer, (e) autumn, and (g) winter, respectively. Distributions of the correlation coefficients between PC1 and abnormal zonal wind speed (U_speed, coloring), as well as PC1 and divergent winds anomalies at 200 hPa (DV_winds, vector) in (b) spring, (d) summer, (f) autumn, and (h) winter, respectively. The black contours denote the climatic zonal westerly wind. The blue arrow only shows the areas where the correlation coefficients between PC1 and water vapor flux anomalies at 850 hPa, as well as PC1 and divergent winds anomalies at 200 hPa pass the test at 90% confidence level, respectively
图 6 不同季节西风—季风耦合模态PC1与海温异常的关系。(a)冬、(b)春、(c)夏、(d)秋季PC1异常与同期海温异常(SSTA)的关系,其中自西向东的两个红色矩形分别表示热带洋全区海温一致模态和Nño3.4指数关键区所在位置,打点区域为相关系数通过90%信度检验的区域;(e)前夏至次年夏季PC1异常与冬季Niño3.4指数和IOBM指数的相关系数;(f)前秋至次年秋季PC1异常与春季Niño3.4指数和IOBM指数的相关系数;(g)前冬至次年冬季PC1异常与夏季Niño3.4指数和IOBM指数的相关系数;(h)前春至次年春季PC1异常与秋季Niño3.4指数和IOBM指数的相关系数
Figure 6. Relationship between the PC1 of westerly monsoon coupling mode and SST anomaly (SSTA) in different seasons. Distribution of the correlation coefficient between PC1 and SSTA in (a) winter, (b) spring, (c) summer, and (d) autumn, respectively. The two red rectangles from west to east represent the location of the Indian Ocean basin mode and Niño3.4 index, respectively. Th area where the correlation coefficient passes the 90% reliability test are indicated by dots. (e) Correlation coefficients between PC1 anomaly from summer to the following summer and Niño3.4 index, as well as IOBM index in winter. (f) Correlation coefficients between PC1 anomaly from autumn to the following autumn and Niño3.4 index, as well as IOBM index in spring. (g) Correlation coefficients between PC1 anomaly from winter to the following winter and Niño3.4 index, as well as IOBM index in summer. (e) Correlation coefficients between PC1 anomaly from spring to the following spring and Niño3.4 index, as well as IOBM index in autumn
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