Tidal Exchange Through the Kuril Straits
Total Page:16
File Type:pdf, Size:1020Kb
1622 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 30 Tidal Exchange through the Kuril Straits TOMOHIRO NAKAMURA,TOSHIYUKI AWAJI,TAKAKI HATAYAMA, AND KAZUNORI AKITOMO Department of Geophysics, Kyoto University, Kyoto, Japan TAKATOSHI TAKIZAWA Japan Marine Science and Technology Center, Yokosuka, Japan (Manuscript received 11 August 1998, in ®nal form 19 July 1999) ABSTRACT The tidal exchange between the Okhotsk Sea and the North Paci®c Ocean is studied numerically with particular emphasis on the predominant K1 barotropic component. The calculated harmonic constants of the K1 tide in and around the Okhotsk Sea agree well with those obtained from extensive tide gauge observations. The features of the simulated tidal ®elds are similar to those reported in the literature. Since the K1 tide is subinertial in the Okhotsk Sea, topographically trapped waves are effectively generated, contributing to strong tidal currents with a maximum amplitude of over 1.5 m s21 in the Kuril Straits. The structures of tide-induced mean ¯ows in most passages of the straits are characterized by ``bidirectional currents'' (in which the mean ¯ow exhibits a reversal in direction across the passages). This feature is clearly indicated in NOAA infrared imagery. The mean transport shows signi®cant net exchange of water via several straits in the Kuril Islands. A transport of about 5.0 Sv (1 6 3 21 Sv [ 10 m s ) toward the North Paci®c is produced by the K1 tide, primarily through the Bussol, Kruzenshterna, and Chetverty Straits. Analysis reveals that the bidirectional mean currents at shallow passages are produced through the well-known process of tidal recti®cation over variable bottom topography, whereas in deep passages such as Bussol Strait, propagating trapped waves along the islands are essential for generating the bidirectional mean currents. Particle tracking clearly demonstrates these features. The tidal current is therefore thought to play a major role in water exchange processes between the Okhotsk Sea and the North Paci®c. 1. Introduction from the Kuril Straits and that the con¯uence of the East Kamchatka Current and the discharge from the Okhotsk The Okhotsk Sea (Fig. 1) is separated from the north- Sea forms both the Oyashio current, which ¯ows along western Paci®c Ocean by the Kuril Islands and the sills the Kuril Islands as part of the western boundary current in the straits, almost all of which are shallower than 600 of the subarctic gyre, and the Subarctic Current to the m except for the Bussol (;2200 m) and Kruzenshterna east, thus indicating that the Okhotsk Sea is the most (;1500 m) Straits. However, the transport process be- likely source of low salinity water in the North Paci®c tween the Okhotsk Sea and the North Paci®c Ocean (WuÈst 1930; Favorite et al. 1976; Talley 1991). Kitani plays an important role not only in the local environment (1973) showed that in the northern Okhotsk Sea, the but also in determining the water properties of the North freezing of fresh surface waters can form cold saline Paci®c. The North Paci®c is the source of most of the waters as dense as 27.05 su, which still have lower water in the Okhotsk Sea. This in¯ow is thought to take salinity than the ambient waters at the same density. By place mainly through the northern part of the Kuril relating an analysis of historical hydrographic data to Straits, whose neighborhood remains ice free even in Kitani's (1973) result, Talley (1991) proposed that the the most severe winters (Reid 1965; Kawasaki and Kono North Paci®c at mid depth is ventilated through sea ice 1994). formation in the innermost part of the Okhotsk Sea and The Okhotsk Sea, on the other hand, is thought to local vertical mixing in the Kuril Straits. From potential have a signi®cant impact upon the North Paci®c. Pre- vorticity maps, Talley (1993) and Yasuda et al. (1996) vious studies in the northwest Paci®c found that a plume commented that the Okhotsk Sea supplies low salinity of low salinity water at 27.0 s extended to the south u water to the North Paci®c Intermediate Water (NPIW) characterized by a salinity minimum centered at 26.8 su. These conclusions were also con®rmed by recent Corresponding author address: Dr. Tomohiro Nakamura, Depart- studies. Warner et al. (1996) showed that the Okhotsk ment of Geophysics, Kyoto University, Kyoto 606 8502, Japan. Sea is an important location for the ventilation of the E-mail: [email protected] intermediate water of the North Paci®c based on chlo- q 2000 American Meteorological Society Unauthenticated | Downloaded 09/30/21 05:31 PM UTC JULY 2000 NAKAMURA ET AL. 1623 FIG. 1. Model domain and topography. Contours are at 500, 1000, 3000, and 5000 m. Symbols denote tide gauge locations for the International Hydrographic Of®ce (IHO) tidal harmonic constants edited by the Canadian Department of Fisheries and Oceans. ro¯uorocarbons (CFCs) observations. Yasuda (1997) Straits. In fact, Stabeno et al. (1994) reported that only showed the presence of a pycnostad (i.e., a region of 1 of the 14 buoys that were transported along the Kuril low potential vorticity) at 26.8 su in the Okhotsk Sea Islands entered the Okhotsk Sea. This indicates that the and concluded that the low potential vorticity water Oyashio does not directly ¯ow into and out of the ¯owing out into the North Paci®c determines even the Okhotsk Sea. Moreover, Leonov (1960) and Moroshkin density of the NPIW. Thus, a clari®cation of the ex- (1966) mentioned that bidirectional currents occur in change processes through the Kuril Straits is indis- most passages. This picture is clearly seen in the NOAA- pensable for a better understanding of water mass for- 12 advanced very high-resolution radiometer imagery mation in the North Paci®c (such as the NPIW). (Fig. 2). Such current structure cannot be explained by Past studies regarded the current in the Okhotsk Sea large-scale wind-driven geostrophic ¯ow. as part of the cyclonic subarctic circulation, which pre- These observational results strongly suggest the im- dominantly ¯ows into the Okhotsk Sea through Kruz- portance of nongeostrophic components. In fact, tidal enshterna Strait and out through Bussol Strait. However, currents, especially diurnal, are dominant in and around recent observational studies suggest that there is great the Kuril Straits (e.g., Thomson et al. 1996), and their dif®culty in explaining this in/out¯ow in terms of geo- speeds reach a few knots. According to previous studies strophic balance. One reason for this is that the overall on tidal exchange (e.g., Huthnance 1973; Zimmerman surface dynamic height is higher in the Okhotsk Sea 1978; Awaji et al. 1980; Robinson 1981), such strong than in the northwestern North Paci®c (Kawasaki and tidal currents are expected to induce signi®cant mean Kono 1992). Another reason is that due to the weak transport, even if there are sills in the straits. Moreover, strati®cation, the Oyashio extends to such depths that tide-induced mean currents allow us to explain the bi- it tends to ¯ow along the continental shelf slope along directional structure of the (local) mean currents in the the Kuril Islands (2000;3000 m deep) and is most like- passages. ly unable to pass over the shallow sills in the Kuril Suzuki and Kanari (1986) performed a simulation of Unauthenticated | Downloaded 09/30/21 05:31 PM UTC 1624 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 30 FIG. 2. Infrared image of the Kuril Straits taken from the NOAA-12 satellite on 21 September 1994. (Figure courtesy of Prof. S. Saitoh) the tidal ®eld in the Okhotsk Sea and showed that in diurnal tides are considerably smaller than the diurnal the Okhotsk Sea, 1) the K1 tide is dominant, M 2 is sec- tides in current speed, we describe the M2 tide only. ond, and the O1 component the third largest in elevation Section 2 of this paper describes the numerical model. amplitude; 2) diurnal tides propagate from east to west Section 3 is devoted to a description of the character- and have two amphidromic like points near the Soya istics of the calculated K1 and M2 tidal ®elds and their and Nemuro Straits; and 3) the semidiurnal tides are comparison with tide gauge observations. In section 4, propagating waves with two amphidromic points. tidal exchange is estimated from the Eulerian point of Though they provided many interesting results, the tidal view and the mechanism causing the transport is in- currents around the straits were not reproduced in suf- vestigated in section 5. The results are discussed in sec- ®ciently detailed structure or accuracy since Suzuki and tion 6 and are summarized in section 7. Kanari (1986) did not aim at estimating the transport through the Kuril Straits. 2. Model Thus, as a ®rst step toward understanding the trans- port process and estimating its magnitude, we have in- The model region shown in Fig. 1 covers the entire vestigated numerically the characteristics of the baro- Okhotsk Sea and part of the North Paci®c. The open tropic tides and tidal currents around the Okhotsk Sea. boundaries are set away from the Okhotsk Sea in order Investigation of barotropic tides may be a meaningful to reproduce tidal wave propagation from the North Pa- ®rst step since strati®cation in and around the Okhotsk ci®c, which is one of the major forcing factors for the Sea is weak. We have then estimated the mean transport tidal motion around the Okhotsk Sea, and to prevent the through the Kuril Straits and identi®ed the mechanism in¯uence of arti®cial disturbances generated at the open of tidal exchange.