Turbulent mixing by non-breaking surface waves in the ocean: theory, observation and modeling

Most ocean general-circulation models face the persistent challenge that the simulated summer ocean mixed layers are too shallow, indicating a lack of mixing; climate models coupled to them simulate too hot sea-surface temperatures (SSTs). Global wind energy input to the oceans supplies a great amount of energy to the surface waves. It is commonly thought that wave-induced turbulence mixing occurs only through wave breaking, but that turbulence, though intense, is injected only to a depth of the order of a wave height. Here we lay the theoretical foundation that non-breaking ocean surface waves can enhance background turbulence and increase irreversible mixing. Such turbulence dissipation and mixing extend to about a horizontal wavelength in depth, comparable to the mixed-layer depth. The direct wave-to-turbulence transfer of energy occurs without needing to be linked to the second-order Stokes drift, and mixing is thus more effective. When introduced into ocean models, such mixing effectively reduces model biases, producing deeper summer mixed layers and cooler SSTs where waves are stronger, as in observations. The predicted wave-turbulence interactions have characteristics which are consistent with laboratory and field observations, including the wave-modulation of turbulence as an energy transfer mechanism, and absence of phase locking to the wave.