1999年与2006年间夏季长江冲淡水变化动力因素的初步分析

A preliminary study on variations of the Changjiang Diluted Water between August 1999 and 2006

根据径流量,1999年和2006年夏季的长江分别处于显著洪季和旱季。此期间的月平均风向也有显著区别。根据同期的海洋现场观测:相对1999年8月,2006年同期的长江口以东、以南毗邻水域表层盐度显著较高,而在长江口东北部海域则相对偏低;长江口附近海域的底层盐度有所偏高,但在浙江中南部沿海底层盐度则相对偏低。利用Regional Ocean Modelling Systems数值模式,对1999年和2006年实际的径流量、风场和黑潮及其分支变化等3个因素对长江冲淡水扩展的影响进行了一系列模拟试验和对比。对比试验表明:相对1999年8月,2006年夏季长江流量大幅度减小是长江口毗邻海域表层盐度升高的主要原因;风场是导致长江冲淡水相对偏北,并使长江口北部出现表层盐度负异常的主要因素;黑潮及其分支在东海北部入侵相对增强、在东海南部入侵相对减弱,使长江口南部表层盐度正异常海域扩大,并促使长江淡水向江口北部扩散增强、而向东部扩散减弱。长江口毗邻海域环流和水团的变化可能对夏季低氧区位置变化产生一定影响。

A large area hypoxia has been already reported respectively by two interdisciplinary surveys off the Changjiang Estuary since summer of 1999 and 2006. The hypoxic zone shows distinct year-to-year variations. Observed oceanographic data are first analysized and reveals a big difference for the Changjiang Diluted Water （CDW） between these two periods, either. These great changes are related to the tremendous reduction of the freshwater discharge and variations of wind fields between these two years. It is also found that the monthly mean intrusion of Kuroshio and its branches has increased in the north of East China Sea （ECS）, but decreased in the south of ECS in August 2006 comparing with 1999 on the base of general circulation models. Then, the Regional Ocean Modelling Systems is applied to the ECS to evaluate the contributions and relative importance of impacts from the river discharge, wind forcing and open boundary data. Our simulations reproduce the phenomena that more fresh water extends northeastward in 2006 and forms a negative SSS anomaly to the northeast of the river mouth comparing with 1999, which is consistent with observations. The five group numerical tests suggest that the wind forcing dominates the CDW variations followed by the Kuroshio and its branches.