Water exchange across isobaths over the continental shelf of the East China Sea

Daily reanalysis data (from January 1993 to December 2012) from a data assimilative ocean model system, Japan Coastal Ocean Predictability Experiment 2(JCOPE2), are used to study water exchange across isobaths over the continental shelf over the East China Sea. We calculated onshore, offshore, and net volume transports across three representative isobaths (50 m, 100 m and 200 m) and examined their detailed vertical structure and temporal variations. The yearly and horizontally averaged velocities across 100 m and 50 m isobaths are in onshore direction in bottom layer but in offshore direction in surface layer. Differing from such two-layer structure, the yearly and horizontally averaged velocities across 200 m isobath are in onshore direction in the surface and bottom layers but in unstable direction in the middle layer. The onshore and offshore components of current in surface layer mainly results from the northeastward currents flowing across the ladder-shaped isobaths and therefore show a subsequent alternation of current in two directions. However, the onshore component of current dominates in the bottom layer over the continental shelf where the bottom Ekman dynamic plays an important role in maintaining the onshore currents. The seasonal variations in above onshore and offshore currents change with water layers. The seasonal variations of current across 200 m isobath in surface layer are consistent with change in wind direction, and those in middle layer are subject to change in buoyant forcing. In the surface layer of 100 m and 50 m isobath, the seasonal variations of cross-isobath currents are mainly related to the Taiwan Strait Current. The interannual variations of currents across the isobaths are weaker than their seasonal variations. The bottom layers of the three isobaths are correlative and suggest that bottom layer is an important pathway for the open sea water to intrude over the continental shelf.