Effects Of Waves On The Ekman Current And Mixing In The Oceanic Mixed Layer

The oceanic mixed layer is where momentum transport, heat transport andmoisture transport are very active. It is very important for climate change,environmental pollution and biological processes. Therefore, it is very significant tostudy the impact of waves on the kinetic processes in the oceanic mixed layer.Firstly, the DP spectrum extended to the high-frequency range (denoted by EDPspectrum) and Combi spectrum are used to calculate the impact of high-frequencysurface waves on the Stokes drift. It is shown that high-frequency waves obviouslychanges the Stokes drift velocity on the sea surface, but has little effect on the Stokestransport. Next, the EDP spectrum, wind input and wave dissipation expressions inHasselmann et al.(1988) and Komen et al.(1994)(denoted by HK) and Combispectrum, wind input and wave dissipation expressions in Tsagareli et al.(2010) andBabanin et al.(2010)(denoted by TB) are used to study the effect of high-frequencywaves on the steady Ekman current solutions. The result shows that the formersignificantly changes the Ekman velocity on the sea surface, while the latter has littleinfluence. And two sets of Ekman current solutions are compared with the observeddata. It can be seen that the first set result is not close to the observation, but thesecond set is close to the observation. However, the impact of high-frequency wavesin the second set result is small and almost negligible.Due to the above inconsistent conclusion, the differences between the EDPspectrum and Combi spectrum and between the wind input and wave dissipationexpressions in HK and TB are further studied. The results show that the difference inthe spectral shape is very small, but the difference in the wind input and wavedissipation expression is obvious and the caused difference in the Ekman currentsolutions can not be ignored. Thus, the linear eddy viscosity solutions calculated bywind input and wave dissipation expressions in HK and TB are compared with theobservations. It is shown that the solution calculated by one in TB is closer to theobservation. In addition, the linear eddy viscosity solution and “K ProfileParameterization”(denoted by KPP) solution calculated by wind input and wavedissipation expression in TB are also compared with the observation. The resultshows that the KPP eddy viscosity is superior to the linear eddy viscosity. The vertical eddy viscosity coefficient reflects the strength of mixing in theocean and has a direct impact on the Ekman current. The steady Ekman currentsolutions calculated by a constant eddy viscosity, linear eddy viscosity, KPP eddyviscosity and the unsteady Ekman current solution calculated by wave-modifiedLI_MY2.5model are compared with the observation. It can be seen that thenumerical solution given by wave-modified LI_MY2.5model is closest to theLOTUS3observation, and the KPP solution is closest to the EBC observation.Finally, based on the Navier-Stokes equation and after taking the variablesdecomposition, the ensemble average and Lagrangian mean, the wave-modifiedmean field equations can be deduced. Using the turbulent momentum equation, theturbulent kinetic equation is derived after a series of calculations. Using thetwo-point correlation, the length scale equation can be deduced. Then, thewave-modified parameterization schemes of momentum flux and heat flux are givenaccording to Mellor and Yamada (1974,1982). Therefore, the wave-modified MY2.5model has been completed theoretically. By the ideal experiments under the differentwind speeds and heat fluxes, the impact of waves on the oceanic mixed layer isstudied. The results show that when the waves are considered, the mixing isstrengthened, the mixed layer depth is deepened and the sea surface temperature isdecreased.