(ONR Sponsored Project)

Zygmunt Kowalik and Igor Polyakov

Institute of Marine Science, University of Alaska Fairbanks

The Sea of Okhotsk (SO) is a region of large tidal sea level oscillations and strong tidal currents. For example, in shallow Penzhinskaya Guba, total tidal sea level oscillations reach 13 m. Because of the strong currents and sea level changes, the tides significantly influence water mass formation in the SO. Total tidal currents of up to 4 knots occur in the Kuril straits. These large currents cause mixing of the upper ocean layer around the Kuril Islands, generating a front between the Sea of Okhotsk and the Pacific Ocean. An almost homogeneous vertical structure of the temperature, salinity and dissolved oxygen has been found over Kashevarov Bank and on the shelf in the mouth of Penzhinskaya Guba. During winter, strong vertical mixing sustains a polynya over Kashevarov Bank. One source of this vertical mixing is a strong tidal current.

This research seeks to establish the role of the tides in the oceanography of the Sea of Okhotsk through application of 2-D and 3-D models. (Kowalik and Polyakov, "Tides in the Sea of Okhotsk," J. Phys. Oceanogr., 1998; and Kowalik and Polyakov, "Diurnal tides over Kashevarov Bank, Okhotsk Sea," J. Geophys. Res., submitted).

The area of investigation is shown in Figure 1, where the box denotes the Kashevarov Bank region used for 3-D modeling.

The cotidal charts for two major constituents, K1 and M2, are given in Figure 2. Co-amplitudes (cm) and co-phases (degrees) referred to Greenwich are shown by solid and dashed lines, respectively. The S2, N2, and K2 constituent charts (not shown) generally repeat the pattern of the dominant M2 tidal wave, whereas the diurnal O1, P1, and Q1 constituents (not shown) repeat the pattern of K1, the dominant diurnal wave. The amplitudes of the diurnal waves are large in the northeastern part of the Sea of Okhotsk. The K1 and O1 constituents have maximum amplitudes of about 2.5 and 1.5 m, respectively, in the Penzhinskaya Guba. The largest calculated amplitudes for the semidiurnal band occur in Udskaya Guba of the Sea of Okhotsk. For example, amplitude of the M2 constituent is 1.81 m there.

Both the diurnal and semidiurnal currents increase in the shallow areas and in the straits. Udskaya Guba, Penzhinskaya Guba, and the Kuril and Soya straits are areas of M2 and K1 current enhancement. Amplification of the diurnal currents is not, however, limited to these regions. Additional regions occur in proximity to Sakhalin Island, off Hokkaido, over Kashevarov Bank, over escarpments located between Kashevarov Bank and Kamchatka, and in proximity to Kamchatka.

In Figure 3 the energy flux for the K1 and M2 waves is depicted (big arrows show the energy fluxes crossing several transects including open boundaries). One can discern a general pattern of a progressive wave with the flux of energy from the open boundary in the Pacific through the Kuril Islands towards the region of high frictional dissipation. These are the areas of Shelikhov Bay and Penzhinskaya Guba for diurnal waves, and Shelikhov Bay, Penzhinskaya Guba, and the northwestern portion of the Sea of Okhotsk for semidiurnal waves. The general pattern of the K1 energy flow is broken by larger and smaller domains of a circular or semicircular flux of energy. These are regions of trapped tidal energy and enhanced flux. The areas around the Kuril Islands (especially on the Pacific side), Kashevarov Bank, and the entrance to Shelikhov Bay are major domains of trapped energy. Lesser domains are located at the escarpments between Kashevarov Bank and the entrance to Shelikhov Bay, along Kamchatka, and along Sakhalin. According to our results, more than 60% of the K1 tidal energy dissipates in Shelikhov Bay and Penzhinskaya Guba. This fact shows that rather small basins may play a very important role in the balance of the tidal energy. No energy trapping occurs for semidiurnal waves.

The nonlinear interaction of the diurnal tidal constituents above seamounts is an important element of tidal dynamics. Diurnal K1 and O1 currents generate, through nonlinear interaction, new oscillations at semidiurnal M2 and fortnightly (13.66-day) periods, residual circulation, and shorter minor periodicities. The upper and lower envelopes of the tidal currents over the Kashevarov Bank summit are asymmetrical (Figure 4). The values of the upper envelope of the north-south (V) velocity component are greater than those of the lower envelope, resulting in a 13-14 cm/s residual current.

For additional analysis we used a digital filter tuned to the narrow frequency band around K1 and O1 constituents. To achieve proper resolution we used a three-month time series of hourly values and a filter with 501 weights. This filter loses 501 hours (close to 21 days) from each end of the original time series. Some results of filtration for the east-west current component over Kashevarov Bank are given in Figure 5. The upper panel of Figure 5 depicts results of filtration at the K1 period, and the bottom panel shows a 13.66-day period and mean residual current derived by low-pass filter. The filtration of the computed time series reveals that the K1 tidal current amplitude over Kashevarov Bank is not constant in time but is modulated by a fortnightly period. It can be deduced from Figure 5 that the time variations of the diurnal and fortnightly oscillations are reciprocally connected, i.e., diminished amplitude in the diurnal band is linked to increased amplitude in the fortnightly band.

The power spectrum of the east-west component of the tidal current over the bank summit is given in Figure 6. The major maxima occur at the semidiurnal, diurnal, and fortnightly periods. Minor maxima are located close to 8 hours and 6 hours. The major maxima can be explained by nonlinear interaction of two original tidal constituents (the so-called compound tides). Moreover, each basic constituent (K1 or O1) produces, through the nonlinear terms, overtides represented in the power spectra as both major and minor maxima.

The 3-D numerical model domain (box in Figure 1) incorporates a uniform horizontal grid step of 5.29 km. The number of grid points in the vertical direction is 45 with a variable grid spacing which allows a 5 m resolution at the summit of the bank. The model is forced by the barotropic K1 + O1 tide specified as sea level oscillations at the open boundaries taken from a 2-D tidal model. Initial temperature and salinity distributions are specified to verify the hypothesis that the observed homogeneous vertical distribution of the temperature and salinity over the bank summit is due to tidal mixing. To be certain that this is the case, initial temperature and salinity are taken as horizontally uniform, and their vertical profiles are based on observations made at areas surrounding Kashevarov Bank. For these profiles, a rather sharp pycnocline is typical. Thus, when the wind stress, and surface and bottom heat and salt fluxes are omitted, tides may be the only force causing the mixing and homogeneous structure over the bank.

The maximum total current occurs above the bank top and attains 264.3 cm/s at the surface and 177.4 cm/s at the 80 m depth. Maximum K1 and O1 tidal current amplitudes are 93.9 and 75.6 cm/s. The vertical structure of the predicted along- and cross-isobath components of the tidal flow reflects the formation of the current maximum, due to stratification at an intermediate depth on the flanks of the bank. The circular shape of the tidal current ellipses, above the bank, changes to rectilinear oscillations at the steepest southwestern and southern slopes. The trapping of the tidal energy is well pronounced: at 2.5 m depth the K1 current is amplified to the maximum of 93.9 cm/s, i.e., 20-30 times relative to off-bank values of 2.5-10 cm/s. Nonlinear interactions of the original tidal constituents K1 and O1 generate M2, M4, and fortnightly periodicities and residual current. The maximum of the residual flow is 32.6 and 0.067 cm/s for the horizontal and vertical components, respectively, with several vertical and horizontal cells on the bank top (Figure 7). The cross-section in Figure 7 (top) depicts upwelling and downwelling cells over the bank's summit whereas in the horizontal plane (Figure 7, bottom) a well-developed clockwise eddy over the bank top extends from the surface to the bottom.

Strong periodical and residual tidal currents cause an intense mixing so that after only 360 hours the initially stratified water becomes almost homogeneous there (Figure 8). This validates the hypothesis that tides are responsible for the homogeneous structure over Kashevarov Bank.

Time sequences of the temperature for two different locations are shown in Figure 9. Over the bank summit (Figure 9, top), where the water is well mixed, the internal wave amplitudes reach almost half the depth. The fortnightly and M2 periodicities are well developed in the internal tidal waves. Off the bank the internal wave amplitudes decrease substantially (Figure 9, bottom). At the eastern flank of the bank, a soliton-like wave is generated in the pycnocline in the upper 20-30 m layer.

Download the tidal data now. Also it is available via anonymous ftp:

        % ftp ftp.ims.uaf.edu
        username: ftp or anonymous
        password: [your email address]
        ftp> cd okhotsk
        ftp> binary
        ftp> get okhotsk.tar.gz
        ftp> bye
The file okhotsk.tar.gz is compressed. To uncompress it, use the following command:

        gunzip okhotsk.tar.gz
After this procedure the name of the file will become "okhotsk.tar". This file is a converted form of a directory named "tide-web". The command

        tar xvf okhotsk.tar
will transfer this file to the directory "tide-web". This directory includes a README file containing instructions for reading data and a description of file structure. The directory "tide-web" also includes 16 files with tidal data. Each tidal constituent (K1, O1, Q1, P1, M2, S2, K2, and N2) uses two files: one is for the sea level amplitudes and phases, and the other is for the tidal current data. For more information read the file "README" in "tide-web".