THE VARIATION CHARACTERISTICS OF 10~30 DAYS OSCILLATION INTENSITY OF SPRING OVER THE SOUTH CHINA SEA-WESTERN PACIFIC OCEAN AND THEIR EFFECTS ON THE SOUTH CHINA SEA SUMMER MONSOON ONSET
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摘要: 采用NCEP再分析资料，揭示了南海-西太平洋春季对流存在显著的10~30天振荡周期。在年际尺度上，南海-西太平洋春季对流10~30天振荡强度（简称SCSWP_SISO）与南海夏季风爆发日期存在显著的负相关关系。当春季菲律宾和西太平洋海温偏高、赤道太平洋中部及以东地区海温偏低时，索马里、110 °E越赤道气流会加强，南海-西太平洋偏西风加强，产生异常气旋性环流，垂直上升运动增强，水汽异常偏多，东西风切变增强，有利于SCSWP_SISO增强。而SCSWP_SISO增强时，有由南往北、自西向东的异常气旋传播，从而减弱低层副热带高压使之较早撤出南海，南海夏季风得以较早爆发。反之亦然。在不同的年代际背景下，SCSWP_SISO经历了偏弱、较弱和偏强的变化，但影响其变化的因子并不完全一致。在第一阶段（1958—1976年），主导因子是南海-西太平洋冷的海温与异常下沉运动、异常减弱的水汽-对流条件。在第二阶段（1977—1993年），主导因子为中东太平洋异常偏冷的海温以及局地异常减弱的风场垂直切变。在第三阶段（1994—2011年），主导因子为热带海温的整体偏暖、风场垂直切变的增强以及水汽-对流的加强。但随着SCSWP_SISO的年代际增强，其与南海夏季风爆发日期的相关关系却呈现下降趋势。Abstract: Based on the NCEP reanalysis data, the 10~30 days significant sub-seasonal variability period of spring (March-May) convection over the South China Sea-Western Pacific Ocean is revealed. On interannual scale, there exist a negative significant correlation between the intensity variation of the 10~30 days oscillation over the South China Sea-Western Pacific Ocean regions (SCSWP_SISO) and the South China Sea summer monsoon onset. When the sea surface temperature anomalies (SSTA) of the Philippines and the western Pacific Ocean is high, the central and east equatorial Pacific SSTA is low, the crossing equatorial currents of Somali and around 110~120 °E areas and westerly winds from the tropical Indian Ocean will strengthen. Meanwhile, an obvious anomalous cyclonic circulation over the South China Sea-Western Pacific Ocean occurs, along with enhancing vertical ascending motion, increasing water vapor anomalies and strengthening wind vertical shear, which results in enhancing SCSWP_SISO. Due to the SCSWP_SISO increase, propagation of an anomalous cyclone from south to north and west to east leads to early withdrawal of lower subtropical high from the South China Sea and the South China Sea summer monsoon breaks out earlier. And vice versa. Under the background of different decadal, SCSWP_SISO has experienced the change of the weak, the weaker and strong, while the influencing factors are not entirely consistent. In the first stage (1958—1976), the negative SSTA over the South China Sea-Western Pacific and abnormal subsidence, decreasing water vapor-convection play a leading role. In the second stage (1977—1993), the main factors are the abnormal cold SST in the Middle East Pacific and local anomalies reducing vertical shear of wind field. In the third stage (1994—1994), the overall warming SST in the tropical, abnormal increasing vertical shear of wind field and convection are the key factors. But, as the SCSWP_SISO increases, its relation with the date of the South China Sea summer monsoon onset is a declining trend.
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图 1 1958—2011年南海-西太平洋（105~130 °E，5~20 °N）850 hPa涡度逐日变化的功率谱多年平均曲线实线为涡度功率谱曲线，虚线为0.05显著性水平线。
图 2 1958—2011年南海夏季风爆发日期（黑直方）以及南海-西太平洋（105~130 °E，5~20 °N）季节内振荡强度指数（虚线）变化曲线R为两者间的相关系数，水平实线和长虚线分别表示两个曲线在3个年代的平均线。
图 3 春季变量合成异常场分布（振荡强年减去弱年）
a.海温（深、浅阴影表示正、负海温，*表示通过0.05显著性水平检验，单位：℃）；b. 850 hPa风场（矢量，粗箭头表示通过0.05显著性水平检验，单位：m/s）和700 hPa水汽（阴影，#号表示通过0.05显著性水平检验，单位：g/kg）；c. 1 000~100 hPa共12层的经向风场V和气压垂直速度W构成的Hadley环流（经度平均取105~130 °E）（深浅阴影为通过0.05显著性水平检验，V单位：m/s，W单位：Pa/s，放大100倍）；d. 1 000~100 hPa共12层的纬向风差值分布（经度平均取105~130 °E）（深、浅阴影表示通过0.05显著性水平检验，单位：m/s）。
图 4 春季影响南海夏季风强度的关键区（105~130 °E，5~20 °N）10~30天振荡对应的温度（a, 单位：K）、水汽（b, 单位：g/kg）、湿静力（c, 单位：J/kg）、位温（d, 单位：K）、垂直速度（e, 单位：Pa/s）、散度（f, 单位：10-6 s-1）、相对湿度（f, 单位：%）及水汽通量散度（h, 单位：g/(kg·s)）面积平均曲线强年和弱年随高度变化的异常分布（减去气候态）
图 6 春季影响南海夏季风强度的关键区（105~130 °E，5~20 °N）10~30天振荡对应的温度（a, 单位：K）、水汽（b, 单位：g/kg）、湿静力（c, 单位：J/kg）、位温（d, 单位：K）、垂直速度（e, 单位：Pa/s）、散度（f, 单位：10-6 s-1）、相对湿度（f, 单位：%）及水汽通量散度（h, 单位：g/(kg·s)）面积平均曲线在3个不同年代际背景下随高度变化的差值分布（减去气候态）
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