Turbulent mixing above the Atlantic Water around the Chukchi Borderland in 2014

This study presents an analysis of the CTD data and the turbulent microstructure data collected in 2014, the turbulent mixing environment above the Atlantic Water（AW） around the Chukchi Borderland region is studied.Surface wind becomes more efficient in driving the upper ocean movement along with the rapid decline of sea ice,thus results in a more restless interior of the Arctic Ocean. The turbulent dissipation rate is in the range of4.60×10–10（–3.31×10–9 W/kg with a mean value of 1.33×10–9 W/kg, while the diapycnal diffusivity is in the range of1.45×10–6–1.46×10–5m2/s with a mean value of 4.84×10–6 m2/s in 200–300 m（above the AW）. After investigating on the traditional factors（i.e., wind, topography and tides） that may contribute to the turbulent dissipation rate, the results show that the tidal kinetic energy plays a dominating role in the vertical mixing above the AW. Besides, the swing of the Beaufort Gyre（BG） has an impact on the vertical shear of the geostrophic current and may contribute to the regional difference of turbulent mixing. The parameterized method for the double-diffusive convection flux above the AW is validated by the direct turbulent microstructure results.

Estimates of mixing on the South China Sea shelf

Mooring observations aimed at understanding the vertical mixing were carried out on the outer shelf of the South China Sea from April to May in 2002. Temporal and vertical distributions of horizontal velocity shear and Brunt- V!is!l! frequencies are calculated with these observations. Dissipation rate and diapycnal diffusivity are then inferred from the fine-scale parameterization. The temporally and vertically averaged dissipation is 15 nW/kg and the associated diapycnal diffusivity is 2×10-5 m2/s. Daily-averaged diapycnal diffusivity is well related to the tides, larger during the spring tide, and smaller during the neap tide. Depth-averaged diapycnal diffusivity, which is as larger as 5×10-5 m2/s during the spring tide, is 8.3 times that of the neap tide, which is only 6×10-5 m2/s. This is in proportion to the vertical energy flux from barotropic tide to baroclinic tide. During the spring tide, the energy flux from the semi-diurnal and diurnal barotropic tide to the internal tide is 160 mW/m, while it is only 35 mW/m during the neap tide. Vertically, monthly-averaged dissipa- tion rate and associated diapycnal diffusivity are large near the upper mixing layer and the bottom boundary. Dissipation rate is about 30￣100 nW/kg, diapycnal diffusivity is about 4×10-5￣10×10-5 m2/s. However, both of them are quite small in the mid column, where dissipation rate is 3￣10 nW/kg and diapycnal diffusivity is 4×10-6￣40×10-6 m2/s.

The influence of turbulent mixing on the subsurface chlorophyll maximum layer in the northern South China Sea

We present observations from deployments of turbulent microstructure instrument and CTD package in the northern South China Sea from April to May 2010.From them we determined the turbulent mixing(dissipation rateεand diapycnal diffusivityκ),nutrients(phosphate,nitrate,and nitrite),nutrient fluxes,and chlorophyll a in two transects(A and B).Transect A was located in the region where turbulent mixing in the upper 100 m was weak(κ~10-6-10-4 m^(2)/s).Transect B was located in the region where the turbulent mixing in the upper 100 m was strong(κ~10-5-10-3 m^(2)/s)due to the influence of internal waves originating from the Luzon Strait and water intrusion from the Western Pacific.In both transects,there was a thin subsurface chlorophyll maximum layer(SCML)(>0.25 mg/m^(3))nested in the upper 100 m.The observations indicate that the effects of turbulent mixing on the distributions of nutrients and chlorophyll a were different in the two transects.In the transect A with weak turbulent mixing,nutrient fluxes induced by turbulent mixing transported nutrients to the SCML but not to the upper water.Nutrients were sufficient to support a local SCML phytoplankton population and the SCML remained compact.In the transect B with strong turbulent mixing,nutrient fluxes induced by turbulent mixing transported nutrients not only to the SCML but also to the upper water,which scatters the nutrients in the water column and diffuses the SCML.