Observational characteristics and dynamic mechanism of low-salinity water lens for the offshore detachment of the Changjiang River diluted water in August 2006
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Abstract: The Changjiang River diluted water (CDW) spreads into the East China Sea (ECS) primarily in a plume pattern, although in some years, low-salinity water lenses (LSWLs) detach from the main body of the CDW. In-situ observations indicate that in August 2006, a LSWL detached from the main body of the CDW near the river mouth. In this paper, the effects of winds, tides, baroclinity and upwelling on LSWLs are explored with a three-dimensional model. The results show that: (1) winds play a crucial role in these detachment events because wind-induced northerly Eulerian residual currents impose an uneven force on the CDW and cut it off, thus forming a LSWL; (2) upwelling carries high-salinity water from the lower layer to the upper layer, truncating the low-salinity water tongue vertically, which is conducive to the detachment and maintenance of LSWLs; and (3) upwelling during the evolution of a LSWL is caused by the combined effects of winds and tides. The influences of wind-induced upwelling are mainly near the shore, whereas the upwelling along the 30 m isobath is predominantly affected by tides, with the effect increasing from neap tide to spring tide.
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Figure 7. Distributions of the 5-day average velocity in the 0–10 m layer during August 2 to 6 (a), August 7 to 11 (b), August 12 to 16 (c) and August 17 to 21 (d). Dashed lines are the 30 m and 50 m isobaths. Filled contours indicate the distribution of the vertical velocity. The vectors represent the Eulerian residual current.
Figure 12. Vertical salinity and upwelling distributions along Line C (marked in Fig. 5c) in CASE06. The upwelling is the average result from August 7 to 11. The upwelling velocity is amplified by a factor of 104. Salinity is the simulated result at 01:00 on August 12.
Table 1. Summary of the numerical experiments conducted to analyze the LSWL detachment mechanism1)
Case Wind Tide Vertical mixing Vertical velocity Baroclinity CASE06 Y Y Y Y Y CASE06a N Y Y Y Y CASE06b Y2) Y Y Y Y CASE06c Y N Y Y Y CASE06d Y Y N Y Y CASE06e Y Y Y N Y CASE06f Y Y Y Y N CASE06g Y Y Y Y Y3) Note: 1) Y means that the dynamic factors are considered in the simulation, and N means that dynamic factors are excluded from the simulation. 2) The wind is changed to a steady southerly wind of 4 m/s. 3) The BPG is calculated by the climatological average of the salinity and sea temperature. Table 2. Summary of the numerical experiments conducted to analyze the upwelling mechanism1)
Case Baroclinity Wind Tide River discharge and open boundary current CASE06h N2) Y Y Y CASE06i N2) N Y Y CASE06j N2) Y N Y CASE06k N2) N Y N Note: 1) Y means that the dynamic factors were considered in the simulation, and N means that the dynamic factors were excluded from the simulation. 2) The salinity of this experiment was fixed and the BPG was set to 0. -
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