TIDES IN THE SEA OF OKHOTSK
TIDES IN THE SEA OF OKHOTSK
(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
MODELING OF TIDES IN THE SEA OF OKHOTSK
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 , 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 . 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.
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 . 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
The upper panel of
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
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 . 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.
INTERNAL TIDES OVER KASHEVAROV BANK
The 3-D numerical model domain (box in ) 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 . The cross-section
(top) depicts upwelling and downwelling cells over the bank's
summit whereas in the horizontal plane (, 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 . 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 . Over the bank summit (, 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
(, bottom). At the eastern flank of the bank, a soliton-like wave is
generated in the pycnocline in the upper 20-30 m layer.
. 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
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". 上传我的文档
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