影响长江口毗邻海域低氧区多种时间尺度变化的水文因素

Hydrographic analysis on the multi-time scale variability of hypoxia adjacent to the Changjiang River Estuary

对2006年6月、8月和10月与1999年8月在长江口向东至125°E、27°30′—33°30′N之间海域(统称长江口毗邻海域)开展的4次多学科综合海洋调查资料进行分析,了解长江口毗邻海域低氧现象的季节变化、年间变化及其与水团变化的关系。研究再次发现该海域存在长江口和浙江近海2处低氧水体、且2处低氧水体具有不同的季节演替和年际变化特征:长江口附近海域低氧水体的溶解氧浓度低、覆盖的面积大,低氧持续时间相对较短、溶解氧浓度的季节变化较大;浙江沿海低氧水体面积较小、溶解氧浓度的季节变化较小、但持续时间较长(6—10月份);2处水体低氧现象的年际变化均很显著。长江口毗邻海域的多种水动力因素及其相互作用导致了水团消长的季节和年际变化,并与该海域低氧现象的季节和年际变化具有较好的关联。季节性跃层的成长是近底层低氧形成的必要条件,而水团迁移和消长过程及其季节和年际变动是导致低氧区不同时间尺度变化的重要物理因素。针对2006年与1999年夏季长江口低氧区的显著变化给出观测证据,提出该时期内长江口的水团结构发生了变化,是导致低氧核心区的位置偏北的主要动力原因。2006年和1999年夏季长江口附近低氧水体的年际变化与同时期叶绿素高浓度区的位置变动是一致的,也为此期间水团消长情况提供了证据。

The extremely low concentration of dissolved oxygen（DO） phenomenon,also known as hypoxia if DO ≤ 2 mg/L,is extraordinary significant in the near bottom water of the Changjiang River Estuary and its adjacent waters in summer.The area of hypoxic zone in this region has a clear trend of increasing during last decades.The spatial distribution of hypoxic zone shows a significant inter-annual variability.Most of published work has mainly concentrated on the formation of hypoxia while only a few attentions have been paid on the mechanism of hypoxia variability and its association with dynamical factors.To understand the evolution and variability of hypoxia and their relationship with the hydrographic conditions in the above area,data collected during four interdisciplinary cruises in June,August and October 2006 and in August 1999 were comparatively analyzed in this paper.The hypoxia off the Changjiang River Estuary was not significant in early summer（June） but already disappeared in early autumn（October）.By contrast it continued along the coastal waters off Zhejiang Province throughout three cruises in 2006.In mid-summer（August）,there were two hypoxic zones co-existed in the investigating areas as in 1999.It was obvious that these two low DO zones evolved in different ways with different seasonality.The area of the north hypoxic zone was significantly larger even though it took place later.In the meantime,the DO concentration there was much lower than that of the south part,but endured for a relatively short time.The inter-annual variability was also much stronger especially for the north hypoxic zone.A comparison was conducted to show those variations between the mid-summer of 1999 and 2006 were unprecedentedly large marked by the enormous intensification of northward spreading of the hypoxic zone in 2006.The prominent multi-time scale variability of the hypoxia is associated with variable physical forcing.（1）The seasonal evolution of hypoxia is relevant to the corresponding changes of thermo-and hydro-dynamic factors.The development of vertical stratification is necessary for the formation of the bottom hypoxic condition.（2）The competition among various waters masses,like the Changjiang Diluted Water（CDW） and the Taiwan Warm Current Water（TWCW）,result in migrations of hypoxic zone in different years.The distribution of satellite observed sea surface chlorophyll a provides supporting evidence for the changes of waters masses.According to these observations,the 3D primitive equation model-Regional Ocean Modeling Systems（ROMS）-was applied to the East China Sea to examine the variability of the CDW with the agency of sea surface salinity.A set of numerical simulations were conducted to understand the individual and joint effects of three forcing： wind,river runoff and the Kuroshio.The significant disparity between 1999 and 2006 can be explained by the CDW retreat and/or the TWCW expansion induced by these factors,i.e.the change of the wind fields,the tremendous reduction of the freshwater runoff and variant intrusion of the TWCW.Among of which,the changing wind forcing plays the dominant role to the changed CDW.This paper suggests that more attentions need to be paid on the roles of hydrography played on the hypoxia formation and evolutions.