Water Exchange across Isobaths over the Continental Shelf of the East China Sea

Abstract

Onshore and offshore currents and the associated volume transport across three isobaths (50, 100, and 200 m) over the continental shelf of the East China Sea were examined using daily reanalysis data in 1993–2012. After being averaged along the isobaths, the velocities across 100 and 50 m are onshore in the bottom layer but offshore in the surface layer. In contrast, those across the 200-m isobath are onshore in the surface and bottom layers but without a clear direction in the midlayer, suggesting a three-layer structure. The surface offshore current across the 100-m isobath mainly arises from the Taiwan Strait Current, while the surface onshore current across the 200-m isobath mainly arises from the Kuroshio, both of which converge in the area between the 100- and 200-m isobaths and flow toward the Tsushima Strait. The control of bottom Ekman dynamics on the onshore bottom currents is important at the 100-m isobath, partly important at the 200-m isobath, and slightly important at the 50-m isobath. The seasonal variations of onshore and offshore currents in the surface layers across the three isobaths are likely caused by local winds, the Taiwan Strait Current, and the Changjiang discharge, while those in midlayer across the 200-m isobath demonstrate a strong geostrophic control and can be interpreted from a traditional viewpoint on the Kuroshio intrusion over the entire water column across the shelf slope. The close connection of bottom onshore currents across the three isobaths suggests that the bottom layer is an important pathway for water exchange of shelf water and the open sea.

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

“…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 : …”

“…The larger uncertainty in the numerically simulated volume transport along the 200-m isobath, particularly in the middle ECS, is also demonstrated by the numerical simulation. The depth-averaged velocity along the 200-m isobath presented by Zhang et al (2017, their Figure 4) and that presented by Zhao and Guo (2011;their Figure 11) exhibit different spatial distributions northeast of Taiwan and (particularly) at the middle ECS. The off-shelf velocity at the middle ECS obtained by Zhang et al (2017) is much smaller than that of Zhao and Guo (2011).…”

“…The depth-averaged velocity along the 200-m isobath presented by Zhang et al (2017, their Figure 4) and that presented by Zhao and Guo (2011;their Figure 11) exhibit different spatial distributions northeast of Taiwan and (particularly) at the middle ECS. The off-shelf velocity at the middle ECS obtained by Zhang et al (2017) is much smaller than that of Zhao and Guo (2011). The net volume transport reported by Zhang et al (2017) is in the on-shelf direction, rather than the off-shelf direction as reported by Zhao and Guo (2011), indicating that the large negative nitrate transport (−19.7 kmol/s) of Zhao and Guo (2011) in the middle ECS may become positive if using the velocity data of Zhang et al (2017).…”

“…From the dynamic point of view, the mean FQ is mainly determined by the on-shelf bottom Ekman component ( Figure 4, first row, FQ b = 24.0 ± 11.1 kmol/s), whose importance is also mentioned by Zhang et al (2017). The geostrophic component (FQ g = −3.0 ± 7.4 kmol/s) mainly contributes to FQ in the off-shelf direction, whereas the surface Ekman component has the smallest contribution in the on-shelf direction (FQ w = 1.0 ± 1.3 kmol/s).…”

“…Note that the nutrient concentration is not evenly distributed along the 200-m isobath ( Figure 2) and the onand off-shelf water transports are at least 1 order of magnitude larger than the net water transport (Ding et al, 2016;Zhang et al, 2017). 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 (9):…”