Numerical study of K₁ internal tides in the Kuril straits

Abstract

Tide‐induced strong diapycnal mixing in the Kuril straits is thought to be one of the essential processes controlling water mass formation in the North Pacific. In order to make a definite quantification of diapycnal diffusivity in the Kuril straits, we drive a three‐dimensional numerical model and examine the generation, propagation, and dissipation features of internal waves which play an essential role in transferring energy from the predominant K1 barotropic tide to diapycnal mixing processes. It is shown that most of the internal wave energy subtracted from the K1 barotropic tide is dissipated within the Kuril straits such that the local dissipation efficiency becomes 0.8–1.0, about three times the value previously employed. This is because the K1 tidal frequency is subinertial in this area so that significant amount of K1 tidal energy is fed into coastal trapped waves (CTWs) which stay around each island without propagating away from the straits; CTWs induce strong velocity shear near the ocean bottom causing bottom‐confined intense mixing with a vertical decay scale ∼200 m, less than half the value previously employed, although there remains some uncertainties resulting from the employed parameterizations of viscosity and diffusivity. The average diapycnal diffusivity in the Kuril straits becomes ∼25 × 10−4 m2 s−1, about three times the value previously estimated, although it is still an order of magnitude less than assumed for the Kuril straits in the existing ocean general circulation models.

Tidal forcing, energetics, and mixing near the Yermak Plateau

“…The energy of the topographically trapped waves propagates along the slope, around the topographic feature with a decay scale of Rossby radius of deformation and negligible radiation in the cross-slope direction. Sub-inertial internal wave energy and energy fluxes have been observed and modelled elsewhere (Allen and Thomson, 1993;Tanaka et al, 2010;Johnston and Rudnick, 2014;Robertson, 2001;Kunze and Toole, 1997). The internal Rossby radius, c 1 /f , for the first mode eigenspeed obtained from the modal analysis of our observational data, varies between 3 and 5 km.…”

“…Over the Juan de Fuca Ridge, trapped (laterally and vertically) baroclinic subinertial motions were reported (Allen and Thomson, 1993). In a numerical study, Tanaka et al (2010) show that most of the internal wave energy subtracted from the diurnal barotropic tide is dissipated within the Kuril straits. K 1 tidal frequency is sub-inertial in this area and the tidal energy is fed into topographically trapped waves which propagate along slope around each island with negligible radiation away from the straits.…”

“…Although these studies address the barotropic diurnal currents in neutral stratification (for simplicity), they are applicable to baroclinic solutions (Brink, 1989;Wang and Mooers, 1976). For example, Tanaka et al (2010) uses the baroclinic coastal trapped wave solutions to explain the subinertial diurnal baroclinic tidal energy propagating around the Kuril Islands. On the continental slope of the Laptev Sea, Pnyushkov and Polyakov (2012) reported that the baroclinic solutions of the topographically trapped waves show no significant change of the cross-slope structure of tidal current and sea level amplitudes compared with the barotropic experiment.…”

“…This is analogous to the sub-inertial baroclinic trapped waves propagating around isolated seamounts (Brink, 1989). Trapped tides dissipate their energy locally, or elsewhere along the topography, leading to substantial vertical mixing (Padman and Dillon, 1991;Tanaka et al, 2010;Johnston and Rudnick, 2014).…”

Causes of the Multidecadal-Scale Warming of the Intermediate Water in the Okhotsk Sea and Western Subarctic North Pacific

“…The effective amplification of topographically trapped waves generated by the subinertial diurnal tides cause considerable intensification of the tidal flow around the Kuril Straits, and this strengthened tidal flow generates internal lee waves with large amplitudes over sills. The breaking of the waves thus leads to enhanced vertical mixing (Nakamura and Awaji 2004), and the topographically trapped waves also induce a strong velocity shear near the sills, which results in bottom-confined intense mixing (Tanaka et al 2010). The importance of tidal mixing on the formation of the OSIW was indicated by a numerical study (Nakamura et al 2006) and observational data .…”