Sustention Mechanisms And Transport Processes Of Buoyant Coastal Currents On Tidal Continental Shelves

Rivers play key roles in global biogeochemical cycles,maintaining global water balances and exporting huge amounts of terrigenous materials into the ocean from vast river basins.After flowing into the sea,river runoffs will form river plumes due to their significant density and momentum differences with adjacent sea water.River plume is the continuation of a river in the ocean.It carries a large amount of fresh water,terrigenous materials and energy from the upstream river,transporting and spreading in nearshore waters.Therefore,it imposes profound impacts on circulation systems,biogeochemical cycles and sedimentary systems in nearshore waters and even in the open ocean.Theoretical studies have categoried river plumes into two forms: surfaceadvected river plumes and bottom-trapped river plumes.Bottom-trapped river plumes are widely found in nature.This type of river plumes and buoyant coastal currents driven by them are imposing great influences on nearshore environments.However,it was frequently found that realistic river plumes are inconsistent with what were predicted by theoreties.The reason of such a gap is that theoretical researches often ignored the tidal forcing.There are many river plume systems that are influenced by energetic tidal forcing,whose dynamics and transport processes are evidently modulated by tides.The Changjiang River plume is a typical case.Taking the Changjiang River plume and the associated Zhejiang-Fujian(Zhe-Min)Coastal Current(ZMCC)as examples,this study investiagated the sustention mechanisms and transport processes of river plumes and associated buoyant coastal currents under strong tidal forcing.Based on observation data and numerical model results,it is found that there are evident seasonal variations in the river plume of the Zhe-Min Coastal Water(ZMCW).The Changjiang River discharge and sea surface winds are the main driving factors that control the seasonal changes of the river plume of the ZMCW.The Changjiang River discharge controls the overall scale of the Changjiang River plume,while the wind forcing directs the expansion of the plume.Despite great seasonal variations of the surface plume,the bottom front of it always stays between 20 m and 40 m isobaths,which indicates that the river plume of the ZMCW is bottom-trapped all the year round.The ZMCC also exists in all seasons whose velocity maximum occurs above the bottom front of the plume.The ZMCC is a surface enhanced baroclinic current,and the baroclinic geostrophic balance controls its cross-shelf momentum balance.The contributions of wind driven currents and the buoyant coastal current driven by the plume front in the ZMCW to the ZMCC are quantified through numerical experiments.It is found that the direct contribution of wind driven currents only accounts for 23% of the ZMCC,indicating the beroclinic effects of the river plume is the main direct driving forcing of the ZMCC.However,by adjusting the expansion of the Changjiang River plume,the surface winds can modulate the intensity of the plume front in the ZMCW,and then indirectly regulates the scale of the ZMCC making it strongest in autumn but weakest in summer.Previous observation studies found summer counter-wind coastal current in the ZMCW.Through a series of numerical experiments,tidal mixing is found to be the sustension mechanism of the summer ZMCC.Under summer monsoon winds,the offshore expansion of the surface river plume in the ZMCW enhances the vertical stratification.The tidal mixing keeps the plume front bottom-trapped under the strong stratification induced by the plume water.The tidal induced plume front drives a buoyant coastal current,while the buoyant coastal current transports the plume water from the core of the Changjiang River plume to the ZMCW,thereby maintaining the stratification.The spring-neap variation of tidal mixing controls the cross-shelf Ekman transport at the bottom of the plume front by adjusting the height of the bottom boundary layer,thereby promoting the cross-shelf movement of the plume front.During spring tides in summer,the plume's trapping depth becomes deeper,increasing the baroclinic forcing of the plume front,which drives a stronger buoyant coastal current.There are significant seasonal differences in the sediment transport along the ZMCW.Under strong dynamic conditions in winter,along-shelf sediment transport reaches its peak.The buoyant coastal current contributes more than 50% to the total sediment transport.The tidal forcing also greatly modulates the along-shelf sediment transport in the ZMCW.Due to strong mixing in spring tides,a large amount of sediment suspended into the upper water column where locates the core of the ZMCC.At the meantime,the lower sediment-rich water column obtains more southward momentum flux from the upper water column because strong tidal stiring thickens the bottom boundary layer.The combined effect results in a significant rise in sediment transport along the ZMCW.In spring and summer seasons,strong tidal mixing during spring tides sustaines a southward buoyant coastal current and high suspended sediment concentration,thereby maintaining a southward sediment transport under southerly winds.