Effects of temporal variation in tide-induced vertical mixing on thermohaline circulation: Numerical experiments for the case of the Okhotsk Sea

Tidally-induced vertical mixing is important for thermohaline circulation. Previous estimations of tidal mixing have aimed to obtain time-averaged values, and ocean general circulation models (OGCMs) typically parameterize such mixing using a temporally constant diffusivity coefficient. However, tidal mixing varies temporally during tidal or spring--neap cycles. Here, we investigate the effects of temporal change in tidally induced vertical diffusivity (κ) in the Kuril Straits using an OGCM. The results show that variations of vertical mixing on diurnal, 2-week, and 1/2-year timescales induce significant differences in the net effect of mixing and, therefore, in the thermohaline circulation originating in the Okhotsk Sea. For diurnal and 2-week variations, the net effect of tidal mixing depends on (1) the period and length of the duration over which κ is larger than the temporal average and (2) the amplitude of the temporal variation of κ, even if the time-averaged values are the same. This is explained by the relative importance of two states. In a quasi-equilibrium state, a larger κ results in weaker stratification and vice versa, and thus the net tidal mixing effect is weaker when κ is variable. Conversely, in an adjustment stage just after an increase in κ, a larger κ acts on stronger stratification and vice versa, resulting in a stronger net effect. For a 1/2-year variation, the net effect of tidal mixing also depends on the phase relationship with seasonal variation in stratification. These results imply the necessity of considering temporal change when estimating tidal mixing from observations and understanding its net effects.