Effects of temporal variation in tide-induced vertical mixing in the Kuril Straits on the thermohaline circulation originating in the Okhotsk Sea

Abstract

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 strength. However, tidal mixing is known to vary temporally during tidal or spring–neap cycles. Here, we investigate the effects of temporal change in tidally induced vertical diffusivity (κt) in the Kuril Straits using an OGCM. The results demonstrate 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 strength of the tidal mixing effect depends on (1) the period and length of the duration over which κt is larger than the temporal average and (2) the amplitude of the temporal variation of κt, 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 κt results in weaker stratification and vice versa, and thus the net tidal mixing effect is weaker when κt is variable than when it remains constant. Conversely, in an adjustment stage just after an increase in κt, a larger κt acts on stronger stratification and vice versa, resulting in a stronger mixing effect. For a 1/2-year variation, the strength of the tidal mixing effect 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, specifying it in OGCMs, and understanding its effects.