The cruise observation of turbulent mixing in the upwelling region east of Hainan Island in the summer of 2012

Abstract

The turbulent mixing in the upwelling region east of Hainan Island in the South China Sea is analyzed based on in situ microstructure observations made in July 2012. During the observation, strong upwelling appears in the coastal waters, which are 3°C cooler than the offshore waters and have a salinity 1.0 greater than that of the offshore waters. The magnitude of the dissipation rate of turbulent kinetic energy e in the upwelling region is O (10–9 W/kg), which is comparable to the general oceanic dissipation. The inferred eddy diffusivity Kρ is O (10–6 m2/s), which is one order of magnitude lower than that in the open ocean. The values are elevated to Kρ≈O (10–4 m2/s) near the boundaries. Weak mixing in the upwelling region is consistent with weak instability as a result of moderate shears versus strong stratifications by the joint influence of surface heating and upwelling of cold water. The validity of two fine-scale structure mixing parameterization models are tested by comparison with the observed dissipation rates. The results indicate that the model developed by MacKinnon and Gregg in 2003 provides relatively better estimates with magnitudes close to the observations. Mixing parameterization models need to be further improved in the coastal upwelling region.