Abstract
Abstract. The spatial distribution of the dissipation rate (ε) and diapycnal diffusivity (κ) in the upper ocean of the South China Sea (SCS) is presented from a measurement program conducted from 26 April to 23 May 2010. In the vertical distribution, the dissipation rates below the surface mixed layer were predominantly high in the thermocline where shear and stratification were strong. In the regional distribution, high dissipation rates and diapycnal diffusivities were observed in the region to the west of the Luzon Strait, with an average dissipation rate and diapycnal diffusivity of 8.3 × 10−9 W kg−1 and 2.7 × 10−5 m2 s−1, respectively, almost 1 order of magnitude higher than those in the central and southern SCS. In the region to the west of the Luzon Strait, the water column was characterized by strong shear and weak stratification. Elevated dissipation rates (ε > 10−7 W kg−1) and diapycnal diffusivities (κ > 10−4 m2 s−1), induced by shear instability, occurred in the water column. In the central and southern SCS, the water column was characterized by strong stratification and weak shear and the turbulent mixing was weak. Internal waves and internal tides generated near the Luzon Strait are expected to make a dominant contribution to the strong turbulent mixing and shear in the region to the west of the Luzon Strait. The observed dissipation rates were found to scale positively with the shear and stratification, which were consistent with the MacKinnon–Gregg model used for the continental shelf but different from the Gregg–Henyey scaling used for the open ocean.
Highlights
Turbulent mixing is a crucial mechanism that controls the distribution of nutrients, sediments, freshwater, and pollutants throughout the water column (Sandstrom and Elliott, 1984)
We divide the observations into four regions (Fig. 1): region 1 is located to the west of the Luzon Strait, region 2 is located to the northeast of Hainan Island, region 3 is located in the central South China Sea (SCS), and region 4 is located in the southern SCS
Salinity decreased gradually from the Luzon Strait to Hainan Island and to the central and southern SCS. This trend is reversed in the minimum salinity layer (25.5–27.5 kg m−3), where the salinity slightly increased from the Luzon Strait to Hainan Island and to the central and southern SCS
Summary
Turbulent mixing is a crucial mechanism that controls the distribution of nutrients, sediments, freshwater, and pollutants throughout the water column (Sandstrom and Elliott, 1984). In the past decade, elevated diapycnal diffusivities, i.e., O (10−4 m2 s−1) or higher, have been found in mixing hotspots such as seamounts (Carter et al, 2006; Lueck and Mudge, 1997), ridges (Klymak et al, 2006a; Lee et al, 2006), and canyons (Carter and Gregg, 2002). These elevated mixing events are highly localized. Using a simple averaging scheme, Kunze and Toole (1997) suggested that topographically induced mixing was insufficient to support a basin-averaged diffusivity of O (10−4 m2 s−1) above a 3000 m depth in the North Pacific
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