Temporal And Spatial Variations Of Cross-shelf Water Exchange And Nutrient Exchange In The East China Sea As Estimated By Satellite And In Situ Measurements

The cross-shelf water and nutrient exchange are of vital importance to the circulation structure,water mass distribution and biochemical process in the East China Sea(ECS),and its spatial and temporal variations have raised attention and discussion.Usually the cross-shelf water and nutrient transports(cross-shelf transports)are estimated by the box model and satellite altimetry data,The former one provides a convenient way to derive the total cross-shelf transports,but lacks the knowledge related to their spatial structure along the 200 m isobath and their long-term temporal variation.The latter one can solve those problems,but only contains the information at the surface,hence uncertainty still lies in the calculation of the cross-shelf transport.According to the problems mentioned above,this study firstly uses satellite altimetry data as the reference velocity,and in situ salinity and temperature data as the horizontal density gradient,the vertical distribution of cross-shelf geostrophic velocity is achieved by applying the thermal wind equation.Then,based on surface wind data and bottom geostrophic velocity,this study also calculates the cross-shelf velocity at the upper and lower boundary layer.Finally the cross-shelf velocity at 200 m isobath from 1993 to 2014 is derived by summing three components.Besides,based on in situ nitrate and phosphate data,the cross-shelf nutrient transport was also reconstructed to match with the cross-shelf water transport.Using the calculated results,the temporal and spatial variation of the cross-shelf transports and their related mechanisms were explored from whole to detail:temporal variation of the total cross-shelf transport;Spatial and temporal variations of local cross-shelf transport in horizontal;Spatial and temporal variations of local cross-shelf transport in vertical.The main conclusion is as follows:(1)Revealed the spatial structure of cross-shelf water and nitrate transports in the horizontal and vertical directions.The nitrate transport was used to represent the nutrient transport because phosphate transport can be estimated by the ratio(13.75)of nitrate and phosphate concentrations.The 22-year averaged total cross-shelf transports were 1.7±2.0 Sv and 22.0± 12.9 kmol s-1,respectively,positive in the on-shelf direction.Their on-and off-shelf components were much larger than their net transports.Based on the horizontal distribution of cross-shelf transports,the 200 m isobath was separated into three regions,namely,northeast of Taiwan,middle ECS and southwest of Kyushu.The northeast of Taiwan had the largest on-and off-shelf water and nitrate transports.Middle ECS also exhibited obvious on-shelf nitrate transport and off-shelf water transport,whereas the nitrate and water transports at southwest of Kyushu cannot be neglected.The vertical distribution of cross-shelf transports further showed that the largest velocity and nitrate flux were located at northeast of Taiwan,50 m below the surface and 50 m above the bottom,respectively.The area 50 m above the bottom at middle ECS and at southwest of Kyushu showed obvious on-shelf nitrate transport,but the water transport changes little in vertical.(2)Revealed the seasonal and interannual variation of cross-shelf water and nitrate transports.The net transports showed significant 1-year period,however,the magnitude of seasonal variation was significantly smaller than that of inter-annual variation.Their on-and off-shelf components not only exhibited 1-year significant period,but also showed a significant period with 2 and 4 years.Further,they exhibited a significant inter-annual variation with an increasing trend.The temporal variation of the total cross-shelf transports were mainly determined by the on-shelf component of cross-shelf transports at northeast of Taiwan.The vertical distribution of cross-shelf transports further showed that the largest variation of velocity and nitrate flux were located at northeast of Taiwan,50 m below the surface and 50 m above the bottom.The empirical orthogonal function showedtwo significant spatial pattern at northeast of Taiwan,and the seasonal variation of the first mode was related to the seasonal shifting of the Kuroshio mam axis,that is,the westward movement of the Kuroshio main axis causes increase of on-shelf transport near Taiwan,and vice versa.(3)Explored the detailed relationship between nitrate transport,velocity,and nitrate concentration,and further determined the related mechanisms of cross-shelf water and nitrate transports.Based on the horizontal and vertical distribution of velocity and nitrate concentration,the spatial structure of nitrate flux was decided by the velocity in horizontal and nitrate concentration in vertical;The temporal variation of nitrate flux was mainly determined by velocity,while its long-term trend was determined by nitrate concentration.By separating the cross?shelf transports into three dynamical components,namely surface Ekman component,geostrophic component,and bottom Ekman component,this study also discussed the related mechanisms.Both the 22-year mean total cross-shelf water and nitrate transports were decided by the on-shelf bottom Ekman component and the off-shelf geostrophic component.Specifically,the temporal variation of total and local water transports was mainly determined by its geostrophic component,and northeast of Taiwan exhibited the largest variation of geostrophic component.Differently,the temporal variation of total and local nitrate transport was determined by its bottom Ekman component and geostrophic component,respectively.The geostrophic component exhibited the largest variation at northeast of Taiwan,while the bottom Ekman component also modulated the variation at middle ECS,50 m above the bottom.