Temporal and Spatial Variations of Cross‐Shelf Nutrient Exchange in the East China Sea, as Estimated by Satellite Altimetry and In Situ Measurements. Issue 2 (27th February 2019) - Ding, Ruibin
Abstract: Based on velocity and nutrient data from satellite and in situ measurements, we studied the temporal and spatial variations for 22‐year (1993–2014) cross‐shelf nutrient exchange at the 200‐m isobath section in the East China Sea. Since nitrate and phosphate concentrations were highly correlated (correlation coefficient of 0.95), only nitrate transport was investigated in detail. The cross‐shelf nutrient exchange is explored from whole to detail. The results show that the 22‐year averaged section‐integrated nitrate transport ( FQ ) is 22.0 ± 12.4 kmol/s (+ in the on‐shelf direction). Its interannual variation is larger than its seasonal variation, but no persistent interannual signal exists. The on‐ and off‐shelf components of FQ are about twice as large as FQ itself, and the temporal variation of FQ is mainly determined by its on‐shelf component. The depth‐integrated nitrate transport ( Fq ) along the 200‐m isobath shows that the largest on‐ and off‐shelf nitrate transports occur northeast of Taiwan, whereas the on‐shelf nitrate transport at the middle East China Sea is also important. Moreover, the on‐shelf component of Fq northeast of Taiwan mainly determines the temporal variation in FQ . The mean and standard deviation of the sectional nitrate flux ( F ) suggest that the largest variation in F occurs northeast of Taiwan, 150 m below the surface. Combining results from empirical orthogonal function analysis, the horizontal and vertical structures of F are determined by velocity and nitrate concentration, respectively, whereas the temporal variation of F is mainly determined by velocity and the geostrophic component. Plain Language Summary: The nutrient exchange at the 200‐m isobath in the East China Sea has been studied extensively by using in situ observational data and numerical simulated data. The former one can provide detailed local information but suffered from spatial and temporal limitations. The latter one can provide the information at the entire shelf break but suffered from uncertainties due to numerical schemes and driving forces. Based on velocity and nutrient data from satellite and in situ measurements, we established the motion equation at the shelf break and estimated the 22‐year (1993–2014) cross‐shelf nutrient exchange at the 200‐m isobath section in the East China Sea. We aimed to (i) explore the spatial and temporal variations in nutrient transport by collecting observational data with a large spatial coverage and a long time span and (ii) explore the governing mechanisms of this transport. Nitrate transport showed a significant 3‐D spatial structure with annual and interannual variations. The spatial structure of the nitrate transport was determined by velocity in the horizontal direction and nitrate concentration in the vertical direction, while the temporal variation in nitrate transport was mainly determined by velocity and particularly by its geostrophic component. This study will provide a reasonable guide for future study, particularly for numerical simulation. Key Points: Nitrate transport shows a significant three‐dimensional spatial structure with annual and interannual variations Spatial structure of nitrate transport is determined by velocity in the horizontal and nitrate concentration in the vertical Temporal variation in nitrate transport is mainly determined by velocity and particularly by its geostrophic component