南海北部海区上层水体平均声速场的变化

Variation of sound velocity of upper water in the northern South China Sea

对所收集到的南海北部海区的温度、盐度和深度历史资料进行了声速的计算、统计与分析 ,结果表明该海区上层水体的平均声速场有较明显的年际变化 :表层声速的平面分布显示出沿岸水与外海水强弱交替变化的特征 ,声速等值线的走向几乎与岸线平行 ,等声速线的值自近岸向外海增加 ,大陆架外缘海区声速的水平梯度较大 ;下层声速的分布以环流和水体共同作用的形式出现 ,50m层声速平面分布的趋势除冬季与表层稍为相似外 ,其余季节与表层有明显的差别 ,春、夏季节 50m层声速自西向东增加 ,而秋季与其表层分布相反 ,自近岸向外海减小 ;声速的垂直分布受海水升温与降温的影响显著 ,春、夏季表层海水升温 ,海表声速最大 ,声速自海表随深度的增加而减小 ;秋、冬季表层附近水层降温 ,声速稍偏低 ,普遍出现正梯度现象 ,最大声速移至表层以下的水层 ,这个深度随季节的改变而改变 ,随海区的不同而不同 .该海区的平均声速有年波动现象 ,表层波幅最大 ,随深度的增加波幅变小 ,至 1 0 0m水层其波动的位相几乎与表层相反 .

Based on the temperature, salinity and depth data measured from 1975 to 1998 in the northern South China Sea, variation of sound velocity of upper water (0～100 m) is discussed. The results show that annual variation of sound velocity of sea water is of importance. Distribution of sound velocity of sea water in the surface is notably influenced by water masses. There is a largest horizontal gradient near the continental shelf margin in the northern South China Sea in winter, it gets weaker and weaker from spring to summer, to autumn. Distribution of sound velocity of sea water in 50 m depth is complex because of joint action of circulation and water masses. In spring and summer sound velocity decreases as depth increasing, but in autumn and winter it increases as depth increases in the upper layer and then decreases as depth increasing. Variation of monthly averaged sound velocity looks like a annual wave variation and is similar to variation of temperature. Variation amplitude decreases as depth increasies and its phase in the surface is almost opposed to that in 100 m depth.