Enhanced ventilation in the Okhotsk Sea through tidal mixing at the Kuril Straits

Abstract

The role played by tide-induced diapycnal mixing at the Kuril Straits in the formation of the Okhotsk Sea water is examined using an ocean general circulation model. The result shows that tidal mixing causes a freshening of intermediate water down to ∼ 27.6 σ θ . This implies an enhancement of ventilation by tidal mixing, since a freshening flux can come only from above in the Okhotsk Sea. In fact, when compared with the case without tidal mixing, the production rate of Dense Shelf Water (DSW) increases from 0.01 to 0.13 Sv ( 10 6 m 3 s - 1 ) with maximum DSW density increased from 26.6 to 26.8 σ θ . This enhanced production of DSW allows us to successfully reproduce a layer of local potential vorticity minimum in the Okhotsk Sea, which characterizes the Okhotsk Sea Mode Water. The analysis reveals that tidal mixing at the Kuril Straits has two roles in enhancing ventilation. First, it preconditions the water properties of the DSW because tidal mixing at the Kuril Straits induces an upward salt flux from the saltier lower layer. The subsequent transport of the saline water from the Straits to regions of sea-ice generation increases the density of subducting water, resulting in more realistic densities and production rates of DSW. Second, tidal mixing directly modifies water properties throughout the water column. The ventilation due to this mechanical mixing further enhances the intermediate layer ventilation, and is dominant in the Kuril Straits and in the layer denser than the DSW. Although the effect of double diffusion on ventilation in the Okhotsk Sea is also examined, it is much less effective than the tidal mixing. Since the Okhotsk Sea is a key location for the ventilation of the North Pacific intermediate layer, the important implication here is that tidal mixing at the Kuril Straits enhances the meridional overturning in the entire North Pacific Ocean.