潮致余流和潮混合对长江口外东北海域低盐水团的作用

Impact of tide induced residual current and tidal mixing on the low salinity water lens in the northeast out of the Changjiang Estuary

长江冲淡水对黄海、东海水文环境有重要影响,它主要以羽状形态向外海扩展,在某些年份的特殊时间段也存在孤立的低盐水团现象。在低盐水团的动力机制研究中,风、径流量、台湾暖流、天文潮和斜压不稳定的作用已得到讨论。天文潮对冲淡水及低盐水团的影响主要包括潮致余流和潮混合,潮致余流作用仍缺乏讨论。本文对1983年8月低盐水团的动力机制进行数值模拟分析,重点讨论了潮致余流和潮混合的影响。结果表明:潮致Lagrange余流促使一部分冲淡水从口门向北输运,在32°N附近呈舌状转向东,有利于在口外东北海域形成低盐水团;小潮转大潮的垂向混合作用加强,浅水区表层盐度升高的速度快于较深水区,也有利于在口外东北海域形成低盐水团。

The expansion of the Changjiang diluted water （CDW） is a dominant hdrographic phenomenon in the East China Sea ,which is mainly in the shape of plume .However ,there are some low salinity water lenses （LSWL） in partial time of some years .The effects of wind ,runoff ,Taiwan Warm Current ,tide and baroclinic instability on LSWL have been discussed in the previous studies .Tide induced residual current and tidal mixing may both make effect on CDW and LSWL ,but there are lack of discussions on the former .In this paper ,some numerical simula-tions are made to analyze the dynamic mechanism of LSWL in August 1983 ,and more discussions are put on the effects of tide induced residual current and tidal mixing .Tide induced Lagrange current drove part of CDW north-ward from the river mouth ,then turned it eastward in the shape of tongue near 32°N ,which was helpful for forming LSWL in the northeast out of the estuary .Tidal vertical mixing strengthened from neap tide to spring tide ,the sur-face salinity at the shallow water increased faster than that at the deep water ,which was also helpful for forming LSWL in the northeast out of the estuary .