The effect of tides on eddy viscosity via K-profile parameterization in the South China Sea near Luzon Strait

Abstract

The non-local K Profile Parameterization (KPP) is a one-dimensional parameterization of the vertical turbulence mixing in the water column. It is the main mixing scheme used in the HYbrid Coordinate Ocean Model (HYCOM). There are two distinct mixing regimes in KPP. In the ocean surface boundary layer (OSBL), the mixing is driven by surface forcing. In the ocean interior, vertical mixing is driven by resolved shear instability, internal wave background, and double diffusive mixing. In this research, two global HYCOM simulations conducted with and without tidal forcing are used to study how tides affect eddy viscosity in the South China Sea (SCS) near Luzon Strait in 2019. Our analysis reveals that tides play a crucial role in modifying eddy viscosity. Internal tides generated at Luzon Strait propagate into the SCS under tidal forcing conditions. They increase the vertical shear of the velocity and consequently enhance eddy viscosity in the ocean interior. Notably, the HYCOM simulation with tides demonstrates substantial eddy viscosity, reaching the order of 10⁻³ m²s⁻¹ at a depth of ∼2000 m at the Luzon Strait to the north of 20.3°N. The clear signature of spring-neap tidal cycle in the strong eddy viscosities in the ocean interior attribute their generation to internal tides. Due to the existence of the high-salinity North Pacific Tropical Water in the upper ocean of SCS (∼100 m), double diffusive mixing generated by salt fingering is shown to be more important than the background internal wave contribution at these depths. Tides also enhance the net downward surface heat flux and reduce the surface stress at Luzon Strait in both summer and winter of 2019. But tides mainly reduce (deepen) the OSBL depth at Luzon Strait in June (December) 2019 and lead mainly to a reduction (increase) of eddy viscosity in the OSBL in June (December) 2019.