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

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.