833 / 2024-09-19 15:48:52

Effects of the Coupled Diurnal Tide-Heat Forcing on the Evolution of Surface and Bottom Boundary Layers in the Coastal Ocean

Boundary Layer; Turbulence; tidal current; heat forcing; Large Eddy Simulation

The turbulent processes within the coastal boundary layer play a crucial role in the internal mixing and the vertical transport within the ocean. The oceanic boundary layer consists of two parts: the surface boundary layer, driven by external forces such as wind, waves, and buoyancy, and the bottom boundary layer, driven by the current shear. Due to the shallow water depth in nearshore areas, strong dynamic coupling occurs between the surface and bottom boundary layers, resulting in strong disturbances in each boundary layer and different evolution processes. Additionally, the external forces driving the nearshore boundary layers exhibit pronounced diurnal periodicity, causing considerable daily variation in the boundary layer structure. We investigate the influence of semidiurnal tidal and diurnal thermal forcing on the evolution and merging processes of the nearshore surface and bottom boundary layers, along with their turbulence characteristics using large eddy simulations. The results show that the phase of tidal current and thermal forcing greatly influences the interaction between surface and bottom boundary layer. When the wind and the current are aligned, turbulent mixing is enhanced, leading to the formation of Langmuir supercells extending throughout the entire water column. Conversely, mixing intensity decreases as a stratified layer forms in the middle layer, separating the surface and bottom boundary layers. Furthermore, this stratification generated by the heating phase at the sea surface suppresses the vertical mixing. Turbulence-induced disturbances on the stratified layer give rise to internal waves within the water column.