Abstract:

An eddy-permitting state estimate for the South China Sea (SCSSE) spanning 2012-2016 is developed and used to investigate the Luzon Strait dynamics through adjoint sensitivity analysis. The estimate is constrained by the World Ocean Atlas 2009 climatological datasets, the NOAA Optimum Interpolation Sea Surface Temperature Version 2 dataset, and the Aquarius Sea Surface Salinity Level 3 Version 5 dataset. Iterative optimization is employed, adjusting initial states, atmospheric forcings, and vertical eddy diffusivity as controls. Comparison with available observations indicates good agreement, supporting the reliability of the SCSSE. The primary benefit of the SCSSE is its improved representation of the South China Sea dynamics by merging numerical modeling with observational data while preserving dynamical consistency. Using the adjoint method, the sensitivity of the persistent westward abyssal overflow at the Luzon Strait is analyzed to investigate the underlying dynamics. Results reveal strong sensitivity to vertical diffusivity within the South China Sea, with a positive correlation above and a negative correlation below a bifurcation depth, where isopycnal tilting begins in the east of the strait. By combining the observed eastward middle-layer flow and theoretical attribution of the overflow to density difference between the bifurcation and sill depths east of the strait, it is argued that the vertical gradient of vertical mixing around the bifurcation depth within the South China Sea governs the overflow, by modulating the density structure in the western Pacific. 

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