Effects of tidal mixing at the Kuril Straits on North Pacific ventilation: Adjustment of the intermediate layer revealed from numerical experiments

Abstract

The effects of tidal mixing at the Kuril Straits on the North Pacific intermediate layer are investigated using an ocean general circulation model. A comparison of numerical experiments with and without a tidal mixing effect suggests that tidal mixing at the Kuril Straits enhances the ventilation of the North Pacific intermediate layer. The enhanced ventilation results in both freshening and cooling down to ∼27.6 σθ. In particular, the simulated North Pacific Intermediate Water (NPIW) becomes fresher and denser (by 0.3 psu and 0.1 σθ at the maximum) and hence more realistic. The enhanced ventilation is caused both through the supply of the ventilated water from the Kuril Straits, which subsequently spreads along the subarctic and subtropical gyres, and through a modification of the circulation there. The ventilation of the supplied water in turn originates from a combination of tidally enhanced convection in the Okhotsk Sea and downward diffusion at the Kuril Straits as discussed in a previous paper. The former affects the upper part of the NPIW, while the latter is dominant in the denser layers. The circulation is modified through a dynamical adjustment to the mass input into an intermediate layer that is produced by the convergence of diapycnal transport due to the tidally enhanced convection and diffusion. The dynamical adjustment is conducted mainly through Kelvin waves which have the ability to induce intergyre flow along the western boundary and also by eastward moving long Rossby waves. The latter can be present under the influence of the wind‐driven gyres for the second and higher baroclinic modes and act to spread information into the interior directly from the western boundary. As a direct consequence of this adjustment, transport of the ventilated water from the Kuril Straits to the subtropical gyre is enhanced by the intergyre flow along the western boundary, which appears as the southward intrusion of the Oyashio Current. The transported water leaves the coast to encircle the interior, and returns to the western boundary, eventually flowing into the equatorial region. Such equatorward transport associated with mass convergence in the intermediate layer is compensated by transport toward the Kuril Straits in the shallower and deeper layers, thereby enhancing both shallow and deep meridional overturning cells by 2–3 Sv. The above dynamical adjustment is thus central to our understanding of the ventilation of the intermediate layer and provides a basis for the analytical model developed in an accompanying paper.