Impact Of Stokes-drift On The Upper Ocean

The Stokes-drift not only modifies the vertical profiles of current in the upperocean, but also affects the mechanical energy input, turbulence production and uppermixing. It plays an important impact on the geophysical fluid dynamics andthermodynamics of the mixed layer, which is the connection of atmosphere andinterior ocean, so that the marine ecology and global climate would be influencedindirectly.In this dissertation, a new three-dimensional numerical model is derived througheulerian wave average on the primitive basic equations. This model is the theoreticalbasis of this dissertation, because it includes not only the dynamical andthermodynamic influence, but also the turbulence and mixing effort induced by theStokes-drift.The wave forcing large-Scale ‘Coriolis-Stokes forcing’ and small-scale ‘Stokes-Vortex forcing’ induced by Stokes-drift affects the upper ocean together. The particlepaths and vertical current profiles are changed as the effort of ‘Coriolis-Stokesforcing’. The ‘Stokes-Vortex forcing’ is the most important source of Langmuircirculations. The vertical shear instability strengthens due to Langmuir circulations.Forther more, the turbulence is strengthened because Langmuir circulations couldcontribute to TKE with the form of Langmuir Turbulence. Vertical convectioninduced by Langmuir circulations enhances water exchange through the effort called‘entrainment’, so that the result is mixing is deepened in the upper ocean. Bycomparing with wave induced mixing Bv which introduce by Qiao (2004), it could befound that Bv is explained as the direct effort of wave stirring, on the other hand,mixing induced by Stokes-drift is indirect through enhancement of vertical sheardriven turbulence and Langmuir circulations.POM model is applied to satisfy the three-dimensional numerical model. Severalparameters are selected for research after the equation is non-dimensionalized, whichare Rossby number, Langmuir number, Hoenikker number, wave number, and besides, wave breaking is considered. By ideal experiments, the results show that, the TKEand mixing coefficients change little in both large and middle scales if the externalconditions are uniform. When Langmuir numbers decreases, the influence ofStokes-drift on the mean current improves. The instability of mixed layer is enhancedand mixing coefficients increase. When Hoenikker numbers become negative, thetemperatures decrease, but the mixing coefficients and influenced depths increase. Onthe contrary, if Hoenikker numbers become positive, the temperatures increase, butthe mixing coefficients and influenced depths are weakened. With the wave numberlargening, the decreasing amplitude increases, but the Stokes-depth become shoaling.The TKE and mixing coefficients increase rapidly when wave breaks, however, theinfluence is limited in a thin layer, but the Stokes-drift could affect the whole mixedlayer.The wave model WAVEWATCH III is applied to simulate the global waves, andthe results are reasonable comparing with European Centre for Medium-Range Wea-ther Forecasts reanalysis data. Global Stokes-drift is calculated base on the simulationresults. It could be found that the magnitude of Stokes-drift could compare with theEulerian mean current. The value of Stokes drift in the Antarctic Circumpolar Currentis the most outstanding annually, and most of Stokes-depth is not large than20m.With the analysis of Langmuir numbers, the results show that classic shear driventurbulence and Langmuir Turbulence occurs together in most of global ocean, andLangmuir turbulence dominate the turbulence in some sea areas moreover. So, it issuggested that the effects of Stokes-drift on mixing in the oceanic mixed layer areimportant and should be accounted for in ocean general circulation models in parame-terrizationsThe global circulations are simulated using the ocean model POM. With thecirculation results as background fields, wave simulation data as external force, themodified model POM including effort of Stokes drift is used for global circulationssimulation once more, in order to learn about the changes of ocean currents, verticalmixing coefficients and vertical temperature in the upper ocean. By compared withorigin simulation results, it could be found that the Eulerian mean current and Lagranian drift is both changed after Stokes-drift is considered. The variable quantitydepends on Stokes drift. The combined result is Stokes drift if variations of Eulerianmean current and Lagranian drift are added. The vertical mixing coefficients increasesafter Stokes-drift is considered. In the high latitude where vertical mixing coefficientsincreases rapidly, the vertical mixing of temperature deepens obviously. In the lowlatitude, however, the stratification phenomenon dominates due to the warming of seasurface with huge heat flux, so that the vertical mixing of temperature is not soremarkable even if the vertical mixing coefficients increase rapidly.