Physical processes causing the formation of hypoxia off the Changjiang estuary after Typhoon Chan-hom,2015

摘要

Severe hypoxia was observed in the submarine canyon to the east of the Changjiang estuary in July 14,2015,two days after typhoon Chan-hom.The oxygen concentration reached as low as 2.0 mg/L and occupied a water column of about 25 m.A ROMS model was confi gured to explore the underlying physical processes causing the formation of hypoxia.Chan-hom passed through the Changjiang estuary during the neap tide.The stratifi cation was completely destroyed in the shallow nearshore region when typhoon passing.However,it was maintained in the deep canyon,though the surface mixed layer was largely deepened.The residual water in the deep canyon is considered to be the possible source of the later hypoxia.After Chan-hom departure,not only the low salinity plume water spread further off shore,but also the sea surface temperature(SST)rewarmed quickly.Both changes helped strengthen the stratifi cation and facilitate the formation of hypoxia.It was found that the surface heat fl ux,especially the solar short wave radiation dominated the surface re-warming,the off shore advection of the warmer Changjiang Diluted Water(CDW)also played a role.In addition to the residual water in the deep canyon,the Taiwan Warm Current(TWC)was found to fl ow into the deep canyon pre-and soon post-Chan-hom,which was considered to be the original source of the hypoxia water.