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南海夏季风强度年代际变化基本特征
Interdecadal Variabilities of SCS Summer Monsoon Intensity
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摘要: 利用美国国家环境预报中心/美国国家大气研究中心 (NCEP/NCAR) 的再分析资料, 通过合成分析、对比分析等方法, 对南海夏季风的年代际变化的划分、不同年代际阶段平均场的差异及其成因进行研究。分析表明:南海夏季风强度具有显著的年代际变化特征, 在20世纪70年代末出现了年代际时间尺度的转折, 可将其分成两个阶段, 1960—1976年 (简称第一阶段) 和1980—1998年 (简称第二阶段), 第二阶段与第一阶段相比, 南海地区夏季西南风强度显著减弱, 年际变化的方差显著变大, 变化周期变短, 但是南海中南部地区的上升运动却有所加强。夏季, 从低层到对流层高层, 中国大陆上空的气温显著降低, 海洋上空的气温有所升高, 在热力作用下, 导致大陆中低层位势高度增加比海洋上大, 形成大陆地区反气旋性环流加强, 从而减弱了南海中北部地区的西南风。从辐散风场来看, 赤道东太平洋地区海温显著增加可能对南海中南部地区上升运动的加强起着重要的作用。Abstract: In order to diagnose the interdecadal variabilities of South China Sea (SCS) summer monsoon activities and reveal the inner relationships of the sea surface temperature and the general circulation anomalies to the interdecadal variabilities of SCS summer monsoon activity, by using NCEP/NCAR reanalysis grid data, the interdecadal phases of SCS summer monsoon are divided, the differences of the average fields in the different interdecadal phases and their causes are studied with composite and comparison analysis. The results show that the intensity of SCS summer monsoon is characterized by intedecadal variability that has an abrupt jump occurring around 1976. It can be divided into two phases. The first phase is from 1960 to 1976 and the second phase is from 1980 to 1998. Comparing the southwesterly over SCS in the second phase with that in the first phase, it is found that the average intensity is weaker, the amplitude of annual variation is greater, the periods are shorter and the ascending flow over the south and center of SCS is stronger in the second phase than that in the first phase. The temperatures in whole troposphere in summer drop over the China continent and rise over the surrounding ocean, leading in the increase of geopotential height in lower and middle levels in troposphere over China continent, and it is more than that over the ocean by the thermodynamic forcing process. The weakening of the pressure gradient between the continent and ocean can result in the enhancement of an anticyclonic anomalous circulation over the continent, leading to the weakening of the southwesterly over the north and center SCS. The temperatures in the troposphere in summer drop over the China continent and rise over the surrounding ocean, leading in geopotential height in lower and middle levels of troposphere over China continent increasing more than that over ocean through the thermodynamic forcing process. The weakening of pressure gradient between the continent and ocean can trigger an anticyclonic anomalous circulation over the continent, leading to the weakening of the southwesterly over the north and center SCS. As shown in the distribution of divergence wind field and vertical motion, it implies that the remarkable increase of sea surface temperature over the eastern equatorial Pacific Ocean may play an important role in enhancing the ascending motion over southern and center of SCS.
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图 1 1960—2000年南海夏季风强度指数序列变化图 (a) 和南海夏季风指数的Mann-Kendall法突变检验图 (b)
(其中直线为平均值, 点线为均方差, 曲线为11年滑动平均)
Fig. 1 Variations of South China Sea summer monsoon (SCSSM) indices for the period of 1960—2000 (a) and detecting abrupt climatic change of SCSSM by using Mann-Kendall technique (b)
(horizontal lines denote mean values, the dot lines standard denote deviations and the curve line denotes 11-year running mean)
图 2 南海夏季风强度指数序列的小波变换图 (a) 和小波变换方差的比较 (b)
(虚线为95%信度曲线)
Fig. 2 The wavelet transform of the South China Sea summer monsoon indices (a) and comparison of the wavelet powers for 1960—1976 and 1980—1998 periods (b)
(dash line is for 95% confidence level)
图 3 850 hPa (a) 和500 hPa (b) 等压面平均风场差值 (矢量, 单位:m/s) 及其显著性检验
(阴影区和等值线分别为纬向风和经向风平均值差异通过95%信度区域)
Fig. 3 850 hPa (a) and 500 hPa (b) of average wind differenes and their significant test
(vectors are for wind differences, unit: m/s; shaded areas and isolines denote the differences of mean zonal winds and meridional winds exceed 95% level, respectively)
图 4 850 hPa (a) 和200 hPa (b) 势函数差值 (等值线, 单位:10-6 m/s; 阴影区为通过95%信度检验区域) 和辐散风差值 (矢量, 单位:m/s) 分布
Fig. 4 850 hPa (a) and 200 hPa (b) velocity potential differences (isolines, unit: 10-6 m/s; shaded areas denote exceeding 95% level) and divergent wind differences (vectors, unit: m/s)
图 5 850 hPa (a) 和500 hPa (b) 流函数差值 (等值线, 单位:m2/s; 阴影区为通过95%信度检验区域) 和旋转风差值 (矢量, 单位:m/s) 分布
Fig. 5 850 hPa (a) and 200 hPa (b) stream function differences (isolines, unit: m2/s; shaded areas denote exceeding 95% level) and rotation wind differences (vectors, units: m/s)
图 6 850 hPa上平均位势高度 (单位:gpm)(a) 和平均温度 (单位:℃)(b) 差值场
(阴影区表示通过95%信度检验区域)
Fig. 6 Potential height differences (unit:gpm)(a) and air temperature differences (unit: ℃)(b) at 850 hPa
(shaded areas denote exceeding 95% level)
图 7 气温差值垂直剖面图 (单位:℃, 阴影区为气温平均值的差异通过95%信度检验)
(a) 纬向平均 (105°~120°E), (b) 经向平均 (20°~40°N)
Fig. 7 Vertical section of air temperature differences (unit: ℃; areas exceeding 95% level are shaded)
(a) zonal mean (105°—120°E), (b) meridional mean (20°—40°N)
图 8 纬向平均 (105°~120°E)(a) 和经向平均 (15°S~15°N)(b) 垂直速度剖面差值图 (单位:Pa/s) 以及经向平均 (15°S~15°N) 夏季海温差值随经度的变化曲线 (单位:℃)(c)
(阴影区为垂直速度平均值的差异通过95%信度检验区域)
Fig. 8 (a) Section of zonal mean vertical velocity in 105°—120°E (unit: Pa/s), (b) section of meridional mean vertical velocity in 15°S—15°N (unit: Pa/s), (c) differences of meridional mean (15°S—15°N) summer sea surface temperature (unit:℃)
(shaded areas denote vertical velocity exceeding 95% level)
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