Wind

If almost $1 TW$ of power is contributed to mixing in the abyss by the tides (Egbert and Ray, 2001), the rest of the estimated $2 TW$ required to maintain the abyssal stratification may be provided indirectly by the wind. The wind can contribute to the abyssal mixing through the generation of large-scale and mesoscale currents or the generation of internal waves. The work done by the wind on the ocean circulation is estimated to be $1 TW$ (Wunsch, 1998). The ensuing large-scale currents create drag in areas of rough topography and in deep passages (Ferron et al., 1998) or over sills (Lukas et al., 2001). This drag creates turbulent mixing and contributes an estimated $0.2 TW$ of power available for the deep ocean mixing (Wunsch and Ferrari, 2004). The wind also contributes to deep ocean mixing via the generation of internal waves, and their subsequent dissipation (Watanabe and Hibiya, 2002; Alford, 2003; Alford, 2001; Wunsch, 1998). The wind excites inertial motions in the surface mixed layer, and these motions pump internal waves in the stratified fluid below the mixed layer (Nagasawa et al., 2000; D'Asaro, 1995; D'Asaro, 1985). These internal waves are generated at a frequency close to the inertial or Coriolis frequency and are referred to as near-inertial internal waves (NIWs). NIWs have been hypothesized (Garrett, 2001) and observed (Alford, 2003) to propagate predominantly equatorward away from their region of generation. Nonlinear interactions within the internal wave frequency band subsequently transfer energy to smaller and smaller scales, ultimately leading to dissipation as heat on molecular scales (Garrett and Munk, 1972). Alford (2003) calculated that 0.5 TW of power from the wind is injected in the internal wave field, which ultimately contributes to the deep ocean mixing budget, either in the open ocean or near boundaries.