Numerical Study On The Response Of Ocean Surface Mixed Layer To The Typhoon-Generated Waves

The ocean surface mixed layer plays an important role in the processes of the momentum, heat and mass exchange between the ocean and the atmosphere. It is significant to investigate the effects of the surface wave and the related physical processes on the ocean surface mixed layer.Firstly, the results and the existing problems in the literature are summarized in this paper, and also several methods to define the mixed layer depth (MLD) are reviewed. Secondly the development of the turbulence model and the ocean mixed layer model are introduced briefly. Finally, a series of numerical experiments are made to investigate the response of ocean surface mixed layer to the constant wind and to the typhoon-generated waves, considering the effects of wave breaking and the Stokes drift induced by the waves.The effects of the wave breaking and the Stokes drift on the mixed layer are analyzed in detail. The method of incorporating the effect of the wave breaking into the dynamics of ocean surface mixed layer is determined by modifying the upper boundary conditions of the turbulence kinetic energy equation and the roughness length of air-sea interface in the sea-side. The effect of the Stokes drift to the mixed layer is introduced by adding the Coriolis-Stokes forcing to the horizontal momentum equations.In the numerical investigations, the response of the ocean surface mixed layer to the 30m/s constant west wind was simulated firstly by using the one-dimensional ocean model , including the M-Y2.5 level turbulence closure scheme. The results show that when considering the effect of the wave breaking, the MLD and the sea surface temperature (SST) change insignificantly, while the surface current velocity and the vertical gradient of the current velocity near the surface layers reduce remarkably. Moreover, the turbulent shear production reduces largely, but the turbulent dissipation increases by two orders of magnitude. The affecting depth of wave breaking is only limited in one significant wave height. The effects of the Coriolis-Stokes forcing on the MLD and SST are a little larger than those of wave breaking, but the variations are not so remarkable either. However, the effect of the Coriolis-Stokes forcing changes the current profile substantially within the whole depth of the mixed layer. When considering the effect of wave breaking and Coriolis-Stokes forcing together, the properties of the mixed layer, such as the current profile and the turbulence structure, are simulated well with the existing observations. So it can be concluded that considering the effects of wave breaking and Stokes drift is necessary in the numerical simulation of the ocean surface mixed layer.Based on the results of constant wind blowing case, the response of the ocean surface mixed layer to the typhoon-generated waves is simulated with combining the wave model WAM and the three-dimensional ocean model POM. For this numerical investigation, the wind field generated by typhoons moving with different velocities is computed with the ideal Holland model, and then the typhoon-generated waves are simulated with the WAM model. Finally, the characteristics of typhoon-generated waves are introduced to the POM model to simulate the response of the ocean surface mixed layer, in which the methods of considering the effects of the wave breaking and the Stokes drift are as the same as the one-dimensional model.