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Inter-decadal Shift of East Asian Summer Monsoon in the Early 1990s
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摘要: 利用1979—2009年JRA-25和NCEP/NCAR再分析资料，通过复矢量经验正交方法揭示了东亚地区夏季850 hPa风场变率的优势模态。结果表明：两套再分析资料所揭示的东亚夏季风在20世纪90年代初均发生了年代际转型，与我国夏季降水的年代际转型时间一致。伴随着东亚夏季风的年代际转型，我国北方大部分地区夏季降水减少，尤其是我国东北北部和长江、黄河之间105°E附近区域显著减少，而华南地区和淮河流域降水显著增加。从动力上解释我国夏季降水年代际转型特征，夏季500 hPa高度场两个时段 (1993—2009年和1979—1992年) 的差值分布显示为欧亚大陆北部准纬向遥相关波列，夏季850 hPa风场差值分布表现为贝加尔湖东南侧和日本以南地区存在两个异常反气旋式环流，而我国南方地区和鄂霍次克海附近均为异常气旋式环流。夏季西北太平洋、北印度洋以及部分中高纬度海洋的海温和春季欧亚大陆积雪在20世纪90年代初出现显著变化，春季北极海冰的年代际转型发生在20世纪90年代初，都可能成为东亚夏季风年代际转型的原因。Abstract: Using JRA-25 and NCEP/NCAR reanalysis data from 1979 to 2009, dominant modes of summer season (June—August) 850 hPa wind field variability over East Asia is revealed by means of the complex vector empirical orthogonal function method. The two reanalysis data are consistent with the description of the first East Asian summer monsoon (EASM) mode, whereas the first mode had been studied, showing that the first mode could not reflect the inter-decadal shift of Chinese summer precipitation in the early 1990s. Consequently, the inter-decadal shift feature of the second EASM mode is deeply analyzed, as well as its effect on summer precipitation in China. Moreover, the possible external forcing factors exerting effects on the inter-decadal shift of EASM are discussed.Results show that, EASM which is revealed by two sets of reanalysis data to have undergone one inter-decadal shift in the early 1990s. The inter-decadal shift time of EASM is consistent with the inter-decadal shift time of summer precipitation in China. EASM is closely related to the mid-high latitude atmospheric circulation anomalies. Corresponding anomalous 500 hPa geopotential height fields show an anomalous quasi-zonal teleconnection pattern in northern Eurasia, whereas the distribution of summer precipitation anomalies show a meridional dipole pattern. Accompanied by the inter-decadal shift of EASM, after the early 1990s, summer precipitation decreases in the majority of northern China, especially in north of the northeast and the area between the Yangtze River and the Yellow River in the vicinity of 105°E. While summer precipitation increases significantly in South China and the Huaihe River Basin. From the perspective of dynamic, the characteristics of inter-decadal shift of summer precipitation in China are described. The difference distribution of summer 500 hPa geopotential height fields between two periods (1993—2009 and 1979—1992) show northern Eurasian quasi-zonal teleconnection pattern, then the difference distribution of summer 850 hPa wind fields show the structure that there are two anomalous anti-cyclonic circulations in southeast of Lake Baikal and south of Japan, while there are two anomalous cyclonic circulations in southern China and Okhotsk Sea.The possible external forcing for the inter-decadal shift of EASM are various, including summer sea surface temperature (SST) in the northwestern Pacific, north Indian Ocean and the part of high latitude ocean (North Atlantic and North Pacific), as well as changes of spring Eurasia snow water equivalent in the early 1990s, inter-decadal shift of spring Arctic sea ice in the early 1990s, especially the high-latitude forcing factor. The role of these external forcing in inter-decadal shift of EASM is unclear and further study is essential.
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图 1 夏季平均850 hPa风场回归 (a) M21, (b) M22，(c) M21和M22标准化时间序列
Fig. 1 Regression map of the summer mean 850 hPa wind (a) M21, (b) M22, (c) normalized time series of M21 and M22
图 2 夏季平均500 hPa高度场对M21(a) 和M22(b) 的回归 (单位: gpm)
(浅色和深色阴影区分别表示高度场异常达到0.05和0.01的显著性水平)
Fig. 2 Regression of summer mean 500 hPa heights derived from a linear regression on M21(a) and M22(b)(unit: gpm)(the light and dark shaded areas denote that height anomalies are significant at 0.05 and 0.01 levels, respectively)
图 3 我国夏季降水对M21(a) 和M22(b) 的回归
(蓝色等值线表示夏季降水异常达到0.05的显著性水平)
Fig. 3 Regression of Chinese summer rainfall derived from a linear regression on M21(a) and M22(b)
(blue contours denote summer rainfall anomalies significant at 0.05 level)
图 4 M21和M22(a) 及J21和J22(b) 标准化时间序列的7年滑动平均以及M21(c) 和J21(d) 的M-K统计量曲线
(两条细实线表示0.05显著性水平)
Fig. 4 7-year running means of the normalized time series of M21 and M22(a) and with J21 and J22(b) besides M-K statistic curve line of M21(c) and J21(d)
(two thin solid lines denote 0.05 significant level)
图 5 1993—2009年和1979—1992年两个时段我国夏季降水量差值和t检验结果 (蓝色等值线表示降水异常达到0.05显著性水平)
Fig. 5 The difference distribution of summer (JJA) rainfalls in China between 1993—2009 and1979—1992 with t-test
(blue contours denote summer rainfall anomalies significant at 0.05 level)
图 6 1993—2009年和1979—1992年两个时段夏季850 hPa风场差值分布 (阴影区表示夏季850 hPa经向风异常达到0.05显著性水平) 及500 hPa高度场的差值分布 (单位：gpm)(浅色和深阴影区表示高度场异常分别达到0.05和0.01显著性水平) (a) NCEP/NCAR再分析资料风场差值, (b) JRA-25再分析资料风场差值，(c) NCEP/NCAR再分析资料高度场差值，(d) JRA-25再分析资料高度场差值
Fig. 6 The difference distribution of summer 850 hPa wind between the two periods (1993—2009 and 1979—1992) (shaded areas denote that meridional wind anomalies are significant at 0.05 level) with mean 500 hPa height (unit: gpm)(the light and dark shaded areas denote height anomalies are significant at 0.05 and 0.01 levels, respectively) (a) the difference of wind derived from NCEP/NCAR reanalysis data, (b) the difference of wind derived from JRA-25 reanalysis data, (c) the difference of height derived from NCEP/NCAR reanalysis data, (d) the difference of height derived from JRA-25 reanalysis data
图 7 1993—2009年和1979—1992年两个时段夏季海表面温度合成差以及t检验 (单位：℃)(浅色、深色阴影区域分别表示海温异常达到0.05和0.01显著性水平)
Fig. 7 The difference distribution of summer sea surface temperature between the two periods (1993—2009 and 1979—1992) with t-test (unit:℃)
(light and dark shaded areas denote the summer sea surface temperature anomalies are significant at 0.05 and 0.01 levels, respectively)
图 8 1993—2007年和1979—1992年两个时段春季欧亚大陆平均积雪水当量合成差 (a) 与积雪水当量差值的t检验 (b)
(红、蓝色区域分别表示积雪水当量正异常和负异常达到0.05显著性水平)
Fig. 8 The difference distribution of spring mean snow-water equivalent between the two periods (1993—2007 and 1979—1992)(a) and the t-test of the difference distribution of spring mean snow-water equivalent (b)(the red and blue areas denote positive and negative anomalies of spring mean snow-water equivalent are significant at 0.05 level)
图 9 春季北极海冰密集度 (SIC) 差值分布和春季北极海冰密集度的EOF第1模态分布
(a)1993—2009年和1979—1992年两个时段春季SIC差值的t检验 (红色和蓝色区域分别表示海冰正异常和负异常达到0.05显著性水平)，(b) EOF第1模态的空间分布, (c) EOF第1模态时间序列的M-K统计量曲线 (两条细实线表示0.05显著性水平)
Fig. 9 The difference distribution of Arctic sea ice concentration (SIC) in spring and the distribution of EOF1 of spring SIC (a) the distribution of difference of spring Arctic SIC between the two periods
(1993—2009 and 1979—1992)(red and blue areas denote that positive and negative anomalies of the spring Arctic SIC are significant at 0.05 level), (b) spatial distribution of EOF1, (c) M-K statistic line of EOF1 time series (two thin solid lines denote 0.05 significant level)
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