Role of vertical mixing originating from small vertical scale structures above and within the equatorial thermocline in an OGCM

Recent high vertical resolution measurements show small vertical scale structures (SVSs) are present in the flow above and within the equatorial thermocline and that these structures contribute significantly to ocean mixing. The SVSs are typically unresolved in OGCMs and thus their impact needs to be parameterized. We investigate the impact of the mixing induced by the SVSs on the state of the equatorial Pacific in an ocean general circulation model (OGCM). As a first step to determine the importance of the SVS induced mixing we introduce an enhanced mixing within and above the equatorial thermocline. It is found that this enhanced mixing reduces the stratification above the thermocline, and sharpens the thermocline through the Phillips effect. The sharpened thermocline limits the exchange of heat across the thermocline and traps the surface heating above the thermocline. The reduced stratification leads to less cooling of the mixed layer through entrainment, a reduced annual cycle and an increase in the annual mean of the sea surface temperature (SST) in the eastern equatorial cold tongue. The depth dependency in enhanced SVS mixing is crucial to its impact; when the enhanced mixing is applied throughout the depth of the ocean (as has been done usually in previous studies,) the cold tongue SST is cooled further. In the western equatorial Pacific, where the thermocline is deeper, SVS enhanced mixing induces a colder SST. We also find that the SVS mixing reduces the eddy kinetic energy associated with the tropical instability waves through a reduction of the meridional and vertical shear of the equatorial currents and temperature gradient.

► Vertical mixing induced by small vertical scale structures (SVSs) within and above the equatorial thermocline.
► Parameterization of enhanced vertical mixing induced by SVSs in an ocean general circulation model.
► Importance of the uneven vertical distribution of the enhanced vertical mixing on the model solution.