On The Secondary Circulation In The Upper Ocean And Wave-induced Vertical Mixing

The ocean mixed layer is a crucial layer connecting the atmosphere and ocean. Itplays very important role in the exchange of water vapor, heat and the momentumbetween the ocean and atmosphere.The secondary circulation in the mixed layer of the upper ocean can bring theheat, momentum and material from the upper ocean to the subsurface which affect thevertical transport, exchange and mixing in this layer. Therefore, the studies on thesecondary circulation, including its formation mechanism and the interaction withother dynamic process, have very important scientific significance. The correctunderstanding of the secondary circulation can also enrich our understanding of theocean upper layer. In this thesis, firstly we use the linear stability theory analyzestability of classical Ekman flow. The secondary circulation generated by theinstability of the Ekman flow has extensive space and time scales, and the scales areclosely related with the Reynolds number, decaying rate of Ekman flow and the ratiobetween the horizontal and the vertical turbulent viscosity coefficients etc. In addition,the deflection angles between secondary circulation and the main stream is affectedby the coriolis force, in the northern hemisphere the secondary circulation turns to theleft of the mainstream, and to the right in the southern hemisphere; However, thedeflection angle does not obey this rule when the ratio between the horizontal and thevertical turbulent viscosity coefficients is large enough. The current measurementfrom a Doppler current profile instrument (ADCP) off Qingdao is used to analyze itsstability. The spatial scale of secondary circulation fits well with the observation byQiao et al.(2009) from the banded structure of surface drifting macroalgae.In addition, the surface wave is important for the thermodynamics of the uppermixed layer. In this paper, the effects of the surface wave-induced vertical mixing areevaluated by carrying out two numerical tests based on one-dimensional Mellor-Yamada turbulence closure model (hereinafter referred to as MY) against datafrom Papa Station. Numerical results show that the modified model can successfullyovercome the shortcoming of the MY, which predicts too shallow upper mixed layerdepth during summertime and consequently overheated sea surface temperatures. Thedeviation between the simulated and the observed SST from June to Septemberdecreases from0.95℃to-0.06℃. The maximum simulation deviation can even reach5.0℃in the original MY. By adding the wave-induced mixing of Bv, the temperaturedeviation can be dramatically reduced to about0.5℃, while the relatively largetemperature deviation persists for quite short period of time in late September. Fromthe statistical analysis, the numerical model with Bv can reduce the mean absoluteerror (MAE), and root mean square error (RMSE) of temperature in the sub-surfacelayer between20-60m0.67℃and1.05℃, respectively, and increase the meancorrelation coefficient between model and observed temperature from0.22to0.94.The including of the wave-induced mixing of Bv can improve the performance of theocean model dramatically. Artificially increasing background mixing can’t get thesame achievements as those by adding the wave-induced mixing of Bv.