Seasonal Variability Of 3D Volume And Nutrient Transports In The Northwestern South China Sea

The mass transport directly affects the ecological dynamics in shelf seas,and plays important role in the local fishery,pollutant diffusion,and climate variation.The cross-shelf transport also restricts the exchange processes between the shelf sea and the open the ocean.The typhoon process in shelf sea affects the concentration of chlorophyll,which in turn affects primary productivity.Using cruise observations in July 2012,December 2012,and July and August 2013 as well as monthly mean data of ECC02,this study diagnoses the vertical velocity with Omega equation,and obtains the 3D,i.e.,along-shore,cross-shelf and vertical,volume transports as well as heat,salt and nutrient fluxes in continental shelf west of Guangdong(WG)and east of Hainan Island(EH)in the northwestern South China Sea(SCS).Then seasonal variations of the transports and fluxes and the disturbation by typhoon Chebi(2013)are investigated.Meanwhile,dynamic factors controlling the vertical flux are analyzed.The results show that,in winter(December),average southwestward along-shore volume transports of-6 Sv are dominant in WG and EH(minus represents southwestward in alongshore direction),while the seaward cross-shelf transports are+0.07 Sv and+0.46 Sv in WG and EH,respectively(plus represents shoreward in alongshore direction).Vertically,the volume transport is upward in WG but downward in EH.The peak values are 0.01 Sv.In summer(July),the along-shore transport is still southwestward-0.56 Sv in WG,but reverses to northeastward 0.64 Sv in EH,one order lower than that in winter.The average cross-shelf transport in EH is shoreward-0.27 Sv,while the vertical transport is similar with that in winter.The direction of salt fluxes follows the volume transport.In WG,the alongshore salt fluxes are both southwestern in two seasons,with value of-1.97×1011 kg s-1 in winter,one order lager than the summer value.The along-shore transports in EH is in similar orders of O(1×1010)kg s-1 while cross-shelf is in O(1×109)kg s-1.They are both higher in summer than that in winter,and the directions are opposite.Disturbed by typhoon Chebi(2003),the along-shore transport in EH reversed to southwestward with flux of-0.34 Sv in 1?2 days before typhoon,while the cross-shelf volume was shoreward-0.20 Sv.In 4?8 days after typhoon,the along-shore transport in nearshore waters recovered to be northeastward as that in summer without typhoon.The along-shore transport was 0.16 Sv,which is about a quarter of transport in the period without typhoon.Meanwhile,the cross-shelf volume is shoreward-0.18 Sv as that before typhoon.The vertical transports during typhoon passage are one order larger than that without typhoon,and downward across the layers of 15?50 m.The variation of horizontal nutrients fluxes follows the volume transports.For NO3-and SiO32-,the along/cross-shelf fluxes of have the largest values during the period without typhoon.The fluxes per unit area are 199.69 ?mol s-1/-237.84 ?mol s-1 for NO3-,and 311.00?mol s-1/-370.41 ?mol s-1 for SiO32-.For NO2-,the largest fluxes of-22.31 ?mol s-1(alongshore)and-29.85 ?mol s-1(cross-shelf)appeared during the period before typhoon.For PO43-,the horizontal fluxes were similar during three periods,and the order is about O(1×102)?mol s-1.For vertical nutrient fluxes,the net transport Fe is calculated as the sum of the convection term Fadv,the mixed diffusion term Fdiff and the aggregating divergence term Fh.In the period after typhoon,the net vertical fluxes for NO3-?NO2-?PO43-?SiO32 and Ch1 a are-0.07 ?mol m-2 s-1,-0.03 ?mol m-2 s-1,-8-10-3 ?mol m-2 s-1,-0.04?mol m-2 s-1,and-9×10-3 ?g m-2 s-1.The vertical net fluxes have the largest values at depth of 5 m,which are one order higher than that at other depths.At depths from 20 to 30 the net flux of NO3-after typhoon is three times higher than that without typhoon,and the fluxes of NO2-,PO43-and SiO32-after typhoon are one order of magnitude higher than that without typhoon.The dynamic analysis shows that the typhoon disturbance increases the ageostrophic advection term Sadv more than one order of magnitude,thus enhancing the vertical convection.The enhanced mixing rate and nutrient concentration gradient during typhoon significantly affect the diffusion term and thus the net vertical nutrient fluxes.