Abstract

To assess accurately the effect of tidal mixing in the Kuril Straits on the formation of the North Pacific Intermediate Water (NPIW), the spatial distribution of diapycnal diffusivity recently obtained by the present authors is incorporated into an eddy-permitting OGCM. It is shown that the NPIW is successfully reproduced, although the diapycnal diffusivity averaged over the entire Kuril Straits is an order of magnitude less than has previously been assumed as a tuning parameter to reproduce the NPIW in low-resolution OGCMs. This strongly suggests that the effect of tidal mixing in the Kuril Straits on the formation of the NPIW is relatively minor and that the physical processes omitted by the low-resolution OGCMs, such as isopycnal mixing along the Kuroshio Extension region, are much more important. This suggestion gives warning of the danger that some misleading conclusions might be derived from OGCMs that employ diapycnal diffusivity just as a tuning parameter to reproduce the observed features.

1. Introduction

The Kuril Straits separating the Okhotsk Sea from the North Pacific Ocean are characterized by vigorous diapycnal mixing resulting from breaking of internal waves generated by strong tidal flow over prominent bathymetric features (Kowalik and Polyakov 1998; Rabinovich and Thomson 2001; Ohshima et al. 2002; Katsumata et al. 2004; Nakamura and Awaji 2004; Ono et al. 2007; Itoh et al. 2010). Tanaka et al. (2007) parameterized the baroclinic energy conversion in the Kuril Straits in the form of bottom stress terms in a barotropic tide model; they then adjusted the amount of energy subtracted from the barotropic tide in the Kuril Straits so as to reproduce the satellite-observed tidal elevation field in the Okhotsk Sea. Furthermore, Tanaka et al. (2010) carried out three-dimensional numerical experiments showing that most of the generated diurnal internal tide energy was lost to dissipation processes within the straits. On the basis of these results, they estimated the diapycnal diffusivity averaged over the entire Kuril Straits to be ∼25 × 10−4 m2 s−1.

Over a broad area of the subtropical North Pacific, there exists a water mass termed North Pacific Intermediate Water (NPIW) that is characterized by a salinity minimum centered around 26.8 σθ (Sverdrup et al. 1942; Reid 1965). The tide-induced diapycnal mixing in the Kuril Straits has been regarded as one of the essential factors responsible for the formation of the NPIW, that is, the formation of the salinity minimum in the North Pacific.

One of the main sources of the NPIW is the Okhotsk Sea Mode Water (OSMW), which is a mixture of the dense shelf water produced through sea ice formation over the shelf region and the East Kamchatka Current Water (EKCW) entering through the Kuril Straits (Itoh et al. 2003). The OSMW then flows through the Kuril Straits to the North Pacific and forms the Oyashio water by mixing with the EKCW (Yasuda et al. 1996; Yasuda 1997; Yasuda et al. 2002). The source water for the NPIW thus flows in and out through strong tidal mixing areas in the Kuril Straits where both the salinity and the potential vorticity are expected to be significantly decreased (Talley 1991; Yasuda 2004; Nakamura and Awaji 2004). The strong tidal mixing in the Kuril Straits is also expected to induce local upwelling from the deep to intermediate layers that enhances the southward transport of the Oyashio as part of the thermohaline circulation in the North Pacific (Tatebe and Yasuda 2004; Kawasaki and Hasumi 2010). Part of the Oyashio water then reaches the Kuroshio Extension region and finally produces the NPIW by isopycnally mixing with the Kuroshio water (Talley 1993; Talley et al. 1995; Talley 1997).

Using an ocean general circulation model (OGCM) with a horizontal grid resolution of 1°, Nakamura et al. (2006a) demonstrated that realistic NPIW can be reproduced by tuning the diapycnal diffusivity in the entire Kuril Straits so as to reach ∼200 × 10−4 m2 s−1, an order of magnitude larger than that estimated by Tanaka et al. (2007, 2010). On the other hand, Ishikawa and Ishizaki (2009) successfully reproduced the NPIW using an eddy-resolving OGCM, although the diapycnal diffusivity in the Kuril Straits parameterized following St. Laurent et al. (2002) seems to be much less than that assumed in Nakamura et al. (2006a).

In this study, we examine whether an eddy-permitting OGCM incorporating the spatial distribution of diapycnal diffusivity obtained by Tanaka et al. (2007, 2010) can reproduce the NPIW. Based on the calculated results, we try to make an accurate assessment of the effect of tidal mixing in the Kuril Straits on the formation of the NPIW.

2. Numerical experiments

The numerical experiments are done using the Massachusetts Institute of Technology General Circulation Model (MITgcm; Marshall et al. 1997) under the hydrostatic approximation. The model includes most of the North Pacific and part of the South Pacific, covering the area from 8°S to 64°N and from 120°E to 104°W. The horizontal grid intervals are 0.25° in longitude and 0.2° in latitude, and 36 vertical levels are assumed, with variable grid intervals from 10 m near the ocean surface to 500 m in the abyss. The model topography is constructed using the General Bathymetric Chart of the Oceans (GEBCO) dataset.

We employ the reanalyzed monthly climatological data of the European Centre for Medium-Range Weather Forecasts (ECMWF) averaged from September 1957 through August 2002 to obtain the surface wind stress and to calculate both the surface heat flux and the surface freshwater flux using the bulk formula. Sea surface salinity is restored to the monthly climatological data of the World Ocean Atlas 2001 dataset (WOA01; Conkright et al. 2002) with a relaxation time of 15 days. Potential temperature and salinity at depths deeper than 2200 m are restored to the annual mean climatological data of the WOA01 with a relaxation time of 1 yr. In addition, potential temperature and salinity over the shelf region in the Okhotsk Sea, in particular, are restored to the monthly climatological data of the WOA01 with a relaxation time of 1 month to represent water mass transformation through sea ice formation.

In this study, background horizontal diffusive–dissipative processes are parameterized with a biharmonic operator assuming constant coefficients KH = 108 m4 s−1 and AH = 1010 m4 s−1, respectively, whereas background vertical diffusive–dissipative processes are parameterized with a Laplacian operator assuming constant coefficients KV = 0.1 × 10−4 m2 s−1 and AV = 10−4 m2 s−1, respectively. The K-profile parameterization mixing scheme (Large et al. 1994) is applied to reproduce the surface mixed layer. We perform two numerical experiments for different spatial distributions of diapycnal diffusivity in the Kuril Straits. The first experiment (hereinafter referred to as MIX) assumes the one proposed by Tanaka et al. (2007, 2010) in which the diapycnal diffusivity ranges from ∼0.1 × 10−4 m2 s−1 (away from the straits) to ∼500 × 10−4 m2 s−1 (within narrow straits) with an area average of ∼25 × 10−4 m2 s−1 (Fig. 1). In the second experiment (hereinafter referred to as NOMIX), such mixing hot spots are removed completely by assuming the diapycnal diffusivity in and around the Kuril Straits to be the same as the background value of 0.1 × 10−4 m2 s−1.

Fig. 1.

Map view of the depth-averaged diapycnal diffusivity obtained by Tanaka et al. (2007, 2010). Superimposed are contours of bottom topography.

Fig. 1.

Map view of the depth-averaged diapycnal diffusivity obtained by Tanaka et al. (2007, 2010). Superimposed are contours of bottom topography.

Initial potential temperature and salinity are taken from the annual mean climatological data of the WOA01. The model is driven for 30 yr from an initial state of rest, and the data averaged over the final 5 yr are used for the analysis herein.

3. Results

MIX shows that the amount of water exchange between the Okhotsk Sea and the North Pacific through the Kuril Straits is ∼4.2 Sv (1 Sv ≡ 106 m3 s−1) with the outflow (inflow) mainly through the Bussol Strait (the Kruzenshtern Strait and the Bussol Strait; see Fig. 1 for locations), which is consistent with the hydrographic and current measurements along the Kuril Islands (Yasuda et al. 2002). The energy dissipation rate needed to sustain diapycnal diffusivity in the Kuril Straits employed in MIX is estimated as ∼22.1 GW using

 
formula

where ρ0 is the reference water density, γ = 0.2 is the mixing efficiency, and N is the local buoyancy frequency reproduced in the numerical experiment. This estimate is very close to the value of ∼29.7 GW obtained by Tanaka et al. (2007, 2010), indicating that the realistic effect of diapycnal mixing in the Kuril Straits is well reflected in MIX.

Figure 2b shows the calculated salinity along a vertical cross section at 165°E for MIX, which is found to agree very well with the observed one shown in Fig. 2a. Furthermore, Figs. 3a and 3b show that the calculated salinity on the 26.8-σθ isopycnal surface for MIX differs from the observed one by at most ∼0.15 psu, which is small enough to judge that the observed NPIW is reproduced very well by MIX.

Fig. 2.

Vertical cross sections at 165°E for the salinity obtained from (a) WOA01, (b) MIX, and (c) NOMIX and for (d) the difference of the salinity between MIX and NOMIX. Superimposed are contours of isopycnal surfaces.

Fig. 2.

Vertical cross sections at 165°E for the salinity obtained from (a) WOA01, (b) MIX, and (c) NOMIX and for (d) the difference of the salinity between MIX and NOMIX. Superimposed are contours of isopycnal surfaces.

Fig. 3.

Similar to Fig. 2, but for the salinity mapped on the 26.8-σθ isopycnal surface. Note that the shelf region in the Okhotsk Sea at which both the potential temperature and the salinity are restored to the monthly climatological data of WOA01 is shaded in (b)–(d).

Fig. 3.

Similar to Fig. 2, but for the salinity mapped on the 26.8-σθ isopycnal surface. Note that the shelf region in the Okhotsk Sea at which both the potential temperature and the salinity are restored to the monthly climatological data of WOA01 is shaded in (b)–(d).

This good agreement motivates us to assess the effect of tidal mixing in the Kuril Straits on the formation of the NPIW through a comparison of the calculated results from MIX and NOMIX. As mentioned already, the previous works using low-resolution OGCMs insisted that the increase of thermohaline circulation in the North Pacific and the decrease of salinity in the North Pacific, both resulting from the strong tidal mixing in the Kuril Straits, lead to the formation of the NPIW. Figure 4 shows the upward volume transport at each depth integrated over the entire Kuril Straits region obtained from MIX and NOMIX, respectively. Although tidal mixing in the Kuril Straits enhances upward volume transport and hence the associated thermohaline circulation in the North Pacific by up to ∼0.5 Sv, it is an order of magnitude less than has previously been attributed to the effect of tidal mixing in the Kuril Straits using low-resolution OGCMs (Nakamura et al. 2006a; Kawasaki and Hasumi 2010).

Fig. 4.

Upward volume transport at each depth integrated over the entire Kuril Straits region. The solid and dashed lines correspond to the results from MIX and NOMIX, respectively.

Fig. 4.

Upward volume transport at each depth integrated over the entire Kuril Straits region. The solid and dashed lines correspond to the results from MIX and NOMIX, respectively.

Figures 2d and 3d show the difference of the calculated salinity for MIX and NOMIX on a vertical cross section at 165°E (cf. Fig. 2b vs Fig. 2c) and on a map view on the 26.8-σθ isopycnal surface (cf. Fig. 3b vs Fig. 3c), respectively. We can see that, although the increase of tidal mixing in the Kuril Straits causes the decrease of salinity over a broad area including the Okhotsk Sea and the subarctic and subtropical North Pacific, it is limited to at most ∼0.06 psu, again an order of magnitude less than has previously been attributed to the effect of tidal mixing in the Kuril Straits in low-resolution OGCMs (Nakamura et al. 2006a). We can obtain nearly the same result for the spatial distribution of potential vorticity (figures are not shown), which suggests that the effect of tidal mixing in the Kuril Straits on the formation of the NPIW is much less than has been previously thought.

4. Summary and discussion

In this study, we have shown that the North Pacific Intermediate Water (NPIW) can be successfully reproduced by incorporating the spatial distribution of diapycnal diffusivity obtained by Tanaka et al. (2007, 2010) into an eddy-permitting OGCM. Of special note is the fact that the diapycnal diffusivity averaged over the entire Kuril Straits is an order of magnitude less than has previously been assumed in low-resolution OGCMs (Nakamura et al. 2006a) so that both the increase of thermohaline circulation in the North Pacific and the decrease of salinity in the North Pacific, which lead to the formation of the NPIW in low-resolution OGCMs, are no longer evident. This result strongly suggests that the effect of tidal mixing in the Kuril Straits on the formation of the NPIW is relatively minor and that the physical processes omitted by the low-resolution OGCMs, such as isopycnal mixing along the Kuroshio Extension region, are much more important (Ishizaki and Ishikawa 2004; Ishikawa and Ishizaki 2009). All of the previous OGCMs that employ exaggerated diapycnal diffusivity in the Kuril Straits by just following Nakamura et al. (2006a) (e.g., Hasumi et al. 2008; Matsuda et al. 2009; Kawasaki and Hasumi 2010; Sasajima et al. 2010), therefore, may involve the danger of simulating various phenomena under physical mechanisms that are very different from the ones that are actually at work in the real ocean.

In this study, however, potential temperature and salinity over the shelf region in the Okhotsk Sea, where the source water for the NPIW is created through sea ice formation, are restored to the climatological values. We cannot deny, therefore, the possibility that tidal mixing in the Kuril Straits strongly affects the production rate of the dense shelf water and hence the formation of the NPIW by changing the properties of the water mass flowing from the North Pacific into the Okhotsk Sea [the East Kamchatka Current Water (EKCW); Nakamura et al. 2006b]. To clarify this point, we are planning to perform numerical experiments with a high-resolution OGCM coupled with a sea ice model incorporating accurate spatial distributions of diapycnal diffusivity over the shelf region of the Okhotsk Sea, wind stress, heat flux, and freshwater flux (Matsuda et al. 2009; Sasajima et al. 2010). The results of these numerical experiments will be reported elsewhere.

Acknowledgments

This work was supported by the Research Fellowship of the Japan Society for the Promotion of Science. Figures were produced using the GFD-DENNOU library.

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Footnotes

* Current affiliation: Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Japan

Corresponding author address: Toshiyuki Hibiya, Dept. of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan. Email: hibiya@eps.s.u-tokyo.ac.jp