Cross‐shelf water exchange in the East China Sea as estimated by satellite altimetry and in situ hydrographic measurement

Abstract

Combining satellite altimetry and in situ hydrographic measurement, we estimated the cross‐shelf transport (CST) and its spatial and temporal variations across 200 m isobath in the East China Sea (ECS) from 1993 to 2014. The vertically integrated CST can be dynamically divided into three parts: surface Ekman transport, geostrophic transport, and bottom Ekman transport. The results show that the 22 year‐mean, sectionally integrated CST to be 1.7 ± 2.0 Sv (positive in the on‐shelf direction), comprised of bottom and surface Ekman transports of 2.7 ± 1.0 Sv and 0.6 ± 0.6 Sv, respectively, that are partially offset by a geostrophic transport of −1.5 ± 1.7 Sv. The sectionally integrated CST shows significantly high power at roughly annual period from 1999 to 2001, with lower power at intra‐annual period. The vertically integrated CST to the northeast of Taiwan is the main source of sectionally integrated CST. The vertically integrated CST also shows significant variations in the 6–15 month period band to the northeast of Taiwan as well. The temporal variations of the sectionally integrated and vertically integrated CST are both controlled primarily by geostrophic transport and modulated by bottom Ekman transport. In the upper 50 m, the geostrophic current to the northeast of Taiwan exhibits large mean and significant variability. The empirical orthogonal function analysis of vertical structure of geostrophic current shows two significant modes with strong annual signal. The first mode is associated with the migration of Kuroshio axis near Taiwan, while the second mode is associated with the variation of the meander of the Kuroshio to the northeast of Taiwan.

Temporal and Spatial Variations of Cross‐Shelf Nutrient Exchange in the East China Sea, as Estimated by Satellite Altimetry and In Situ Measurements

“…However, the corresponding imbalance is relatively large at the inner boundary (Tss, Tas, Wk, and 200 m), that is, 0.3 Sv (18% of the total input) for water transport and 10.0 kg/s (18% of the total input) for salt transport. The large imbalance at the inner boundary might be caused by the relatively large uncertainty in altimetry data over shallow areas (Ding et al, ). This encouraging result indicates that the water transport data and the associated material transport data are reliable in the study area, particularly at the shelf edge.…”

“…Based on observations and numerical model, the total cross‐shelf water transport in the ECS has been estimated (about 1 to 2 Sv). However, the annual and interannual variation of cross‐shelf water transport from previous studies is not so consistent (Ding et al, ; Liu et al, ), which means that the temporal variation is complicated at the shelf break. The possible mechanisms such as Ekman and geostrophic processes were identified (Guo et al, ; Zhou et al, ).…”

“…Note that the nutrient concentration is not evenly distributed along the 200‐m isobath (Figure ) and the on‐ and off‐shelf water transports are at least 1 order of magnitude larger than the net water transport (Ding et al, ; Zhang et al, ). Therefore, the on‐shelf ( FQ + ) and off‐shelf ( FQ − ) nutrient transports are as important as FQ ; The FQ is separated into FQ + and FQ − , as in equation : …”

“…Calculation and analysis of the cross‐shelf nutrient transport is based on a right‐handed coordinate system ( l , n , z ) along the 200‐m isobaths, as applied by Ding et al (), where l is the along‐shelf coordinate defined to be parallel to the isobath directed from northeast of Taiwan to southwest of Kyushu, n is the cross‐shelf coordinate normal to l in the on‐shelf direction, and z is the vertical coordinate directed upward from the surface.…”

“…The velocity comprises three dynamic components: surface Ekman velocity ( v w ), geostrophic velocity ( v g ), and bottom Ekman velocity ( v b ). Note that all the velocity data used in this paper were from Ding et al (). where t is the time of the season, starting from January 1993.…”