The impact of surface waves on the mixing of the upper ocean
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The impact of surface waves on the mixing of the upper ocean WANG Zhifeng1, WU Kejian2*, XIA Changshui3, ZHANG Xiaoshuang4 1 College
of Engineering, Ocean University of China, Qingdao 266100, China of Physical and Environmental Oceanography, Ocean University of China, Qingdao 266100, China 3 First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China 4 Key Laboratory of Marine Environmental Information Technology, National Marine Data and Information Service, State Oceanic Administration, Tianjin 300171, China 2 College
Received 28 March 2013; accepted 9 June 2013 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2014
Abstract A new three-dimensional numerical model is derived through a wave average on the primitive N-S equations, in which both the“Coriolis-Stokes forcing” and the“Stokes-Vortex force” are considered. Three ideal experiments are run using the new model applied to the Princeton ocean model (POM). Numerical results show that surface waves play an important role on the mixing of the upper ocean. The mixed layer is enhanced when wave effect is considered in conjunction with small Langmuir numbers. Both surface wave breaking and Stokes production can strengthen the turbulent mixing near the surface. However, the influence of wave breaking is limited to a thin layer, but Stokes drift can affect the whole mixed layer. Furthermore, the vertical mixing coefficients clearly rise in the mixed layer, and the upper ocean mixed layer is deepened especially in the Antarctic Circumpolar Current when the model is applied to global simulations. It indicates that the surface gravity waves are indispensable in enhancing the mixing in the upper ocean, and should be accounted for in ocean general circulation models. Key words: surface waves, Stokes drift, Langmuir turbulence, mixing Citation: Wang Zhifeng, Wu Kejian, Xia Changshui, Zhang Xiaoshuang. 2014. The impact of surface waves on the mixing of the upper ocean. Acta Oceanologica Sinica, 33(9): 32–39, doi: 10.1007/s13131-014-0514-6
1 Introduction The air-sea interface, more broadly the ocean mixed layer (or ocean boundary layer, OBL), is the link between the atmosphere and the deep ocean. It directly affects the air-sea fluxes of momentum, heat, and gases, playing an important role in geophysical flows, biological productivity, and marine pollution. Surface gravity waves are capable of generating a mean Lagrangian current called Stokes drift (Stokes, 1984). The Stokes drift can affect the large-scale sea state in a sometimes dramatic way (Longuet-Higgins and Stewart, 1960, 1961). Hasselmann (1970) showed that the Stokes drift is capable of inducing an opposing mean Eulerian current through the “Coriolis-Stokes forcing” f × us ( f is the Coriolis parameter, us is the Stokes drift). Webber (1983, 2003) analyzed the transfer of momentum to the Ekman layer due to winds and waves. The solutions suggest that the wave-induced current and the Ekman current on the surface could be of the same magnitude. For a monochroma
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