盛夏南海的水文与环流特征:2000年8月航次再探

Hydrography and circulation of the South China Sea in deep summer:The August 2000 cruises revisited

在国家重点基础研究发展规划的支持下,2000年8月在南海组织了1次多船同步海洋学调查,是迄今为止盛夏期间唯一一次全海盆准同步水文观测。航次间遭遇的多种困难虽被及时化解,然而观测数据的质量已受影响。此前虽已有若干相关研究发表,但未见对观测数据进行质量控制和全面的分析。本文在对CTD和ADCP原始数据仔细校正的基础上,分析和讨论了盛夏期间南海海盆的水文和环流特征,冀留下尽可能完整的记录。分析表明,盛夏期间以海盆SW−NE向轴线为界,南海展现出不同的物理特征。轴线东南受季节性反气旋环流控制,其中次海盆尺度的南沙反气旋环流尤其强大;轴线西北则受各种中尺度现象支配,未见明显的大尺度环流结构。研究证实南沙反气旋环流的水平尺度超过400 km,垂直尺度达近千米,最大流速近1 m/s,是夏季南海最突出的环流特征,但发现其位置和强度存在显著的年际变化。研究并发现,不同于以往的认识,观测期间民都洛海峡交换呈“三明治”结构,表明随强迫条件变化,民都洛海峡交换可出现不同模态。

Funded by the National Key Basic Research Development Program,a multi-vessel oceanography survey was conducted synoptically in the South China Sea(SCS)in August 2000,which appeared to be the unique,available synoptic survey that covered the entire SCS basin in deep summer.A number of difficulties were encountered during the survey that affected the quality of observation,even though they were resolved underway.Although some related research works were previously published,no carefully quality control of the data and a comprehensive analyses had ever appeared before.In this work,the original CTD and shipboard ADCP data were carefully calibrated in the first place,and the hydrography and circulation features were then analyzed and discussed to leave a record of the cruises as complete as possible.The results indicate that,in deep summer,the basin shows different physical characteristics on both sides of its SW–NE oriented axis of the basin.The circulation is generally anticyclonic southeast of the axis,in which the Nansha anticyclonic gyre(NSAG)is especially powerful;the area northwest of the axis,in contrast,appears to be dominated by mesoscale features of different categories,where no obvious structure of large scale circulation is found.The analyses confirm that the scales of the NSAG are over 400 km horizontally and nearly 1000 m vertically,with a maximal swirling speed close to 1 m/s,which appears to be the most remarkable circulation feature of the SCS.Moreover,its position and strength are subject to significant interannual variation.In the Mindoro Strait,the exchange flow showes a sandwich structure during the observation period.This is different from our previous understanding and suggests that the strait exchange may have different modes varying with the forcing condition.