Spatial-temporal variation and salt transport in the intermediate water of the northern South China Sea
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摘要:
为探讨南海北部中层水盐度的时空分布特征及与外海水的交换，选用1871—2010年共140年的月平均海洋同化数据（SODA2.2.4），利用EOF、小波等分析方法，分析南海北部中层水盐度的时空分布、变化周期、盐通量等特征。结果表明：① 南海北部中层水的盐度月际变化特征为：1—3月，南海北部中层水受到越南东部沿岸向东北方向延伸出的高盐舌的影响（＞34.45 psu），盐度偏高，中部盐度较低；4月南海北部中层水的盐度分布均一，盐度范围在34.40～34.45 psu；5—8月，夏季南海北部的中层气旋式环流将北太平洋中层水（North Pacific Intermediate Water，NPIW）携带至南海中部，在南海中部偏北形成一低盐水团（＜34.42 psu）, 并在此时间段内低盐水团逐渐向东北方向移动，直至9—10月，上述低盐水团与NPIW混合；11—12月，NPIW东撤，收缩至吕宋海峡西北侧。② 过去140年，南海北部中层水的盐度值波动上升，其中，1985年中层水盐度值最高，可能与当年夏季风爆发早且强有关。1963—2002年，存在较强的16～21 a的周期性，小波系数的实部正相位的峰值和负相位的谷值所对应的年份，与ENSO冷事件具有较好的对应。③ 对EOF第一模态分析显示，整个研究海域为单极子变化特征，呈一致性变化。第二模态的空间分布呈偶极子形态，一正一负相位中心沿NW-SE走向对称分布。④ 南海北部中层水全年的净盐通量均由南海向东流入太平洋，夏季最少，冬季最多。1871—2010年的140年间，中层水的盐通量一直处于下降的趋势，下降的趋势十分缓慢。
Abstract:To explore the spatial-temporal distribution of salinity in the intermediate water and water exchange with the external waters of the northern South China Sea(SCS), the monthly mean oceanic assimilation data of past 140-year from 1871 to 2010 (SODA2.2.4) were selected, on which the spatial-temporal variation, periodicity, and salt transport of intermediate water salinity in the SCS and adjacent northwest Pacific Ocean were analyzed using wavelet analysis and EOF decomposition. The result show that the monthly salinity variation of intermediate water in the northern SCS is higher in January-March than that of the central part due to northeastward extending of high salinity water from the eastern coast of Vietnam (＞34.45 psu). The salinity distribution of the intermediate water in the northern SCS is uniform in April, and the salinity ranged from 34.40 to 34.45 psu. In May-August, there is a cyclonic circulation in the intermediate water of the northern SCS in summer, which can carry the North Pacific Intermediate Water (NPIW) to the central part of SCS, and then forms a low-salinity water lens (＜34.42 psu) in the north-central SCS. Meanwhile, the low-salinity water lens move northeastward gradually until mix with NPIW in September-October. In November-December, NPIW withdraws eastward and shrinks to the northwest of the Luzon Strait. In the past 140 years, the salinity of intermediate water in the northern SCS has been rising periodically, reaching the peak in 1985, which may be related to the early and strong summer monsoon. From 1963 to 2002, a strong 16-21–year periodicity is shown, and the years of the maximum (minimum) values of the real part of wavelet coefficients well agree with corresponding ENSO cold events. The first EOF analysis shows that the intermediate water salinity in the northern SCS is in a monopole and consistent variation pattern. The positive and negative phase centers identified by the second mode is symmetrical in distribution in NW-SE strike. The annual net salt fluxes of the intermediate water in the northern SCS flow eastward from the South China Sea to the Pacific Ocean, the least in summer and the most in winter. During the 140 years from 1871 to 2010, the salt fluxes in the intermediate water has been decreasing constantly and slowly.
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Key words:
- intermediate water /
- Northwest Pacific /
- seawater salinity /
- salt transport /
- the South China Sea
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表 1 小波系数的实部正（负）位相年份与ENSO事件的对应
Table 1. The years of maximum (minimum) value of the real part of wavelet coefficients vs ENSO events
年份 小波实部正/负相位 ENSO事件 起止年月 峰值时间 强度 1965年 负相位 冷事件 1965.05—1966.05 1965.11 中等 1972年 正相位 冷事件 1972.05—1973.03 1972.11 强 1979年 负相位 冷事件 1979.09—1980.01 1980.01 弱 1986年 正相位 冷事件 1986.08—1988.02 1987.08 中等 1992年 负相位 冷事件 1991.05—1992.06 1992.01 中等 1995年 正相位 冷事件、
暖事件1994.09—1995.03、1995.09—1996.03 1994.12、1995.11 中等、弱 
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表 2 南海中层海水各模态贡献率
Table 2. Contribution of each mode to the intermediate seawater salinity of the South China Sea
第一模态 第二模态 第三模态 第四模态 贡献率 0.5973 0.1735 0.0533 0.0397
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