Dynamical Response Of Changjiang River Plume Extension To Typhoon Events

As the largest river in China,the Changjiang River discharges 8.9×10¹¹m³freshwater and 1.22×10⁸t sediments annually into the adjacent sea.The enormous water and sediment fluxes have an enormous effect on the Changjiang Estuary,including hydraulic dynamics,sedimentations,geomorphological processes and coastal ecological environment.The Changjiang River plume,formed by the freshwater from the Changjiang River mixing with seawater,carries lots of nutrients to stimulate the primary production in the Changjiang River Estuary and its adjacent seas.Sometimes,it can even lead to red tide and other environmental disasters.Normally,the Changjiang River plume extends offshore toward the Jeju Island in summer under the upwelling-favorable southerly summer monsoon,unlike the wintertime plume that extends southward under the downwelling-favorable northerly winter monsoon.Among these forcings,tropical cyclones（i.e.,typhoons or hurricanes）,the most energetic ones acting on some coastal oceans,i.e.,East China Seas（ECSs）,significantly modulate the horizontal distribution and vertical stratification of the Changjiang River plume.However,the existing researches on the response of the Changjiang River plume to a typhoon are still rare due to lacking of observation data in the severe weather.Therefore,in this study,we investigated the response of the summertime Changjiang River plume to typical typhoons with numerical simulation,as well as the destruction and recovery mechanism and potential environmental disasters after being hit by a typhoon.Therefore,a well-validated model including surface wave induced mixing was used in this study.We investigated the variation of the horizontal distribution and vertical stratification of the Changjiang River plume,taking the typhoon Chan-hom（1509）,the typhoon Morakot（0908）,the typhoon Fengshen（0209）as typical typhoons.In addition,we discussed the destruction and recovery mechanisms of the Changjiang River plume as well as the associated ecological effects.The results showed that most of（about 75%）typhoons affecting the Changjiang River Estuary modulated the core area of the Changjiang River plume with northeasterly wind when they approached the Changjiang River Estuary.Taking the typhoon Chan-hom（1509）as an example,the typhoon wind did intensify the vertical mixing.The vertical stratification was completely destroyed and the surface seawater salinity（SSS）sharply increased.In addition,the typical offshore-extending summer state of the Changjiang River plume completely disappeared under the influence of typhoon wind.Instead,it extended southward along the Zhejiang and Fujian（Zhe-Min）coast as a typical winter state of the Changjiang River plume.A large amount of freshwater was transported to the Zhe-Min coast,contributing to stratification enhancement and decrease of the SSS.After the typhoon passage,the stratification of the Changjiang River plume and the SSS gradually recovered to the normal state with the continuous supplement of the Changjiang River freshwater.However,after the typhoon Chan-hom（1509）passage,the freshwater was still continuously transported to the Zhe-Min coast.Through calculation,we found that the freshwater transported to the Zhe-Min Coastal Water reached about 4.7×10¹⁰ m³ as influenced by the typhoon Chan-hom（1509）,which was about 5%of the total Changjiang River discharge in 2015.After that,the freshwater began to retreat to northward and offshore when the southerly wind became strong enough.The SSS also gradually recovered to the normal state from a low salinity which maintained for about one week.The results showed that about 17%typhoons affecting the Changjiang River Estuary modulated the core area of the Changjiang River plume with southeasterly wind when they approached the Changjiang River Estuary.For example,the typhoon Morakot（0908）,had the similar effect on the core area of the Changjiang River plume as the typhoon Chan-hom（1509）that the typhoon intensified the vertical mixing,destroying the vertical stratification of the Changjiang River plume and the SSS sharply increased.However,little freshwater was transported to the Zhe-Min coast.Instead,the freshwater along the Subei Coast reached farther than normal state.After that,the Changjiang River plume gradually recovered to the normal state in the core area.Another interesting case is that only 8%typhoons affecting the Changjiang River Estuary modulated the core area of the Changjiang River plume with northwesterly wind when they approached the Changjiang River Estuary.In view of the typhoon Fengshen（0209）had weakened to tropical storm as it approached the Changjiang River Estuary,it had little impact on the Changjiang River plume.The strong typhoon wind was the most significant contributor for the breakdown of Changjiang River plume.It included that the typical offshore-extending summer state of the Changjiang River plume was completely disappeared and the vertical stratification was destroyed.Besides,under the influence of the typhoon affecting the Changjiang River Estuary with northeasterly wind like the typhoon Chan-hom（1509）,lots of freshwater were abnormally transported to the Zhe-Min coast.On the other hand,in the recovery process,the vertical stratification of the Changjiang River plume in the core area gradually recovered after the dissipation of typhoon wind.For freshwater transported to the Zhe-Min coast,the recovery period was divided into the retention phase and the retreating phase under the competition between wind-driven current and buoyancy-driven current.The results showed that water with high chlorophyll-αconcentration coincided with the movement of the Changjiang River plume when the Changjiang River plume recovered from the typhoon.The remote sensing data of chlorophyll-αfrom GOCI and MODIS also showed that significant algal blooms occurred via nutrient-rich plume water redistribution induced by typhoon.This study showed that one single typhoon can dramatically change the fate of terrestrial materials from the Changjiang River.