Comparison between vertical shear mixing and surface wave-induced mixing in the extratropical ocean

Most parameterizations of vertical mixing are associated with local shear instability, which do not explicitly include the effects of surface waves. Here, we compared the performance of vertical mixing induced by vertical shear of the mean current and that by nonbreaking surface waves in the upper ocean through three numerical experiments. The vertical mixing from vertical shear alone was too weak especially in the extratropical ocean, and failed to produce a reasonable mixed layer depth and seasonal thermocline, which resulted in a large cold bias and an unrealistic seasonal cycle in the subsurface. Surface waves can enhance the vertical mixing in the upper ocean, and induce vertical mixing to sustain a reasonable upper ocean temperature structure especially in the extratropical ocean. Both the temperature structure and seasonal cycle were significantly improved by including the nonbreaking surface wave-induced vertical mixing, no matter whether shear effect was included or not. These results indicate that the vertical mixing from surface waves is more important than that associated with velocity shear of the mean current for the upper ocean especially in the extratropics.