Abstract
The nominal spatial distribution of diapycnal mixing in the South China Sea (SCS) is obtained with Thorpe-scale analysis from 2004 to 2020. The inferred dissipation rate ε and diapycnal diffusivity Kz between 100 and 1500 m indicated that the strongest mixing occurred in the Luzon Strait and Dongsha Plateau regions, with ε ~ 3.0 × 10−8 W kg−1 (εmax = 5.3 × 10−6 W kg−1) and Kz ~ 3.5 × 10−4 m2 s−1 (Kzmax = 4.2 × 10−2 m2 s−1). The weakest mixing occurred in the thermocline of the central basin, with ε ~ 6.2 × 10−10 W kg−1 and Kz ~ 3.7 × 10−6 m2 s−1. The ε and Kz in the continental slope indicated that the mixing in the northern part [O(10−8) W kg−1 and O(10−4) m2 s−1, respectively] was comparatively stronger than that in the Xisha and Nansha regions [O(10−9) W kg−1 and O(10−5) m2 s−1, respectively]. The Kz in the continental slope region (200–2000 m) decayed at a closed rate from the ocean bottom to the main thermocline when the measured depth D was normalized by the ocean depth H as D/H, whether in the shallow or deep oceans. The diapycnal diffusivity was parameterized as Kz = 3.3 × 10−4[1 + (1 − D/H)/0.22]−2 − 6.0 × 10−6 m2 s−1. The vertically integrated energy dissipation was nominally 15.8 mW m−2 for all data and 25.6 mW m−2 for data at stations H < 2000 m. This was about one order of magnitude higher than that in the open oceans (3.0–3.3 mW m−2), which confirmed the active mixing state in the SCS.
Significance Statement
Diapycnal mixing (mixing across surfaces of constant density) is a dominant process in the ocean. It can affect ocean circulation and climate change. The South China Sea (SCS) is known as a strongly mixed marginal sea, but little research has been done on the SCS as a whole, and relatively little attention has been given to its central and southern regions. This study aimed to explore the nominal spatial variability of mixing in the entire SCS using Thorpe-scale analysis. Our study showed that the diapycnal mixing was stronger in the continental slope than in the central basin and that it followed a decay model from the bottom up in the region of the continental slope. According to our results, the averaged energy dissipation in the SCS was about one order of magnitude higher than that in the open oceans.
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