Mesoscale current fluctuations observed in the deep western North Pacific

A set of simultaneous long-term, deep current measurements was taken using a moored array in the mid-ocean of the western North Pacific near 30°N, 146°E. Five current meters at three stations provided good quality records over 84 days. Low-frequency current fluctuations with meridional dominance are clearly seen in the deep layer records. They are consistent with signals of a mesoscale current fluctuation which has a period of about 100 days, an east-west wave length of about 200 km, and a westward phase propagation with a speed of about 2 cm sec^–1. Bottom intensification of the east component of low-frequency current fluctuations is also observed.     

Continental shelf waves off the Fukushima coast

Numerical experiments are performed on shelf waves forced by wind stress with a spectral peak around a period of 100 hr. Water depth in the numerical model is a function of offshore distance only and resembles a bathymetric profile off the Fukushima coast. A pair of vortices alined in the offshore direction and a large vortex are reproduced and they propagate southward outside the forced region. Judging from the propagation speed, the former corresponds to the second-mode and the latter to the first-mode shelf waves. In the forced region, the propagation speed of a trough and a ridge is slow, 3–5km hr^–1. These propagation characteristics reproduce those observed along the Fukushima coast and this propagation speed corresponds to that of second-and third-mode shelf waves. Thus, it is concluded that the periodical current fluctuations observed in the inshore region along the Fukushima coast are due to motions associated with the second-and third-mode shelf waves.     

On vertical velocity fluctuations and internal tides in an upwelling region off the west coast of India

Hourly fluctuations of vertical velocity in relation to components of flow and wind and temperature oscillations at a morring site in the shelf waters off the west coast of India are discussed. The vertical velocities were computed from a time series of vertical temperature profiles assuming that horizontal advection of temperature is negligible. The computed values at a depth of 40 m during the 72-h period of observation were of the order of 10⁻¹ to 10⁻²cm s⁻¹, with a mean value of −2·77 × 10⁻² cm s⁻¹ indicating a net upward movement of water. The computed vertical velocity showed fluctuations of about 2–3 h, in addition to weaker signals of about 12 h. Based on the spectral estimates, we speculate that these fluctuations of 2–3 h in the vertical velocity may be caused by the fluctuations in the along-shore wind. The oscillations of isotherms found in the temperaturedepth time series and the spectral estimates of temperature and cross-shore flow component showed a periodicity of about 12 h, which indicated the presence of semi-diurnal internal waves. The fact that these internal wave troughs were associated with the measured onshore flow suggested that the waves were propagating offshore. The computed stability parameters showed little evidence of instability or mixing. It was found that the isotherm troughs in the temperaturedepth time series at about 12-h period coincided with high vertical shear in the cross-shore direction and low values of Brunt Vaisälä frequency.     

Coastal-Trapped Waves with Several-Day Period Caused by Wind along the Southeast Coast of Honshu, Japan

Generation and propagation of several-day period fluctuations along the southeast coast of Honshu, Japan, were investigated by analyzing sea level data and by using a numerical model. The sea level data obtained at twelve stations from Choshi to Omaezaki in fall in 1991, showed energy peaks at the 3–6 day period at the eastern stations in this coast. Time lags of the 3–6 day period fluctuations between station and station indicate westward propagation along the coast. However, the energy level of the 3–6 day period fluctuations suddenly decreased south of the Izu Peninsula. Numerical experiments using a two-layer model were performed to clarify the generation and propagation mechanism of the several-day period fluctuations by periodical wind in fall. The amplitude distributions of observed sea level were qualitatively explained by a coastal-trapped wave (CTW) in the numerical experiment. From the discussions on propagation of a free wave, CTW with the characteristics of a shelf wave generated by the wind along the northeast of the Boso Peninsula was separated into two types of wave at the southeast of the peninsula. One is an internal Kelvin wave with large interface displacement and the other is the shelf wave propagating over the northern part of the Izu Ridge. The sudden decrease in the surface displacement with the 3–6 day period observed at the western stations is considered to be due to the local effect of the wind and phase relation between the internal Kelvin wave and shelf wave.     

The structure of the nearshore branch of the Tsushima current on the shelf off the San'in coast in summer

Current measurements were made at five moored stations over the continental shelf off the San'in coast of the Japan Sea for a month in the summer of 1980 to study the vertical structure of the nearshore branch of the Tsushima Current. The time-mean current for the observational period is 20 to 25 cm sec^–1 eastward near the surface and about 10 cm sec^–1 westward near the sea bottom except at the shallowest station. The time-mean current,i.e. the nearshore branch of the Tsushima Current is mainly due to the baroclinic modes. The currents are less variable in the first half of the observational period, but fluctuate with a several-day period in the latter half. The obtained current data were decomposed into barotropic and baroclinic modes to investigate the detailed characteristics of the fluctuations. In the latter half, the current fluctuations of the two modes with about a 5-day period are well correlated with each other, as the baroclinic mode lagging behind the barotropic mode by 12 hr. The barotropic current fluctuation is correlated to the sea level, with the former leading the latter by about 12 hr. The baroclinic current is correlated to the temperature at the subsurface layer with a shorter time lag.     

Observations of the countercurrent on the inshore side of the Kuroshio northeast of Taiwan

Intensive current measurements in the area northeast of Taiwan indicate subsurface, southwestward flow existed between the inshore edge of the Kuroshio and the East China Sea continental slope. At 70 km away from Taiwan, this countercurrent has a mean speed about 30 cm s^–1 at mid-depth. Closer to Taiwan, the flow turns along with the topography, and subjects to sidewall and bottom friction. Both the magnitude and the vertical shear of this countercurrent are comparable with that inferred from hydrographic survey. The wind field features short-period (a few days) fluctuations associated with the cold front passages, however, this is not reflected on the current records. It appears that the countercurrent is fairly steady. Together with similar reversing flow found at places much further to the north, the overall pattern seems to be a general quasi-steady feature along most part of the shelf edge of the East China Sea.     

Interannual Variations of Sea Level at the Nansei Islands and Volume Transport of the Kuroshio Due to Wind Changes

Interannual variations of sea level at the Nansei Islands and volume transport of the Kuroshio during 1967–95 are calculated by integrating variations carried by windforced Rossby waves. Effects of eddy dissipation and ocean ridges are considered. Ridge effect is inferred by comparing between the calculated and observed sea levels. The calculation is satisfactory to sea levels and Kuroshio transport for the whole period. They are mostly caused by Rossby waves forced by wind and modified by the ridges, and are due to barotropic wave primarily and the first baroclinic wave secondly. The calculated Kuroshio transport well represents variations of several-year scales with maximums in respective duration of the large meander (LM) of the Kuroshio, as well as bi-decadal variation that transport was small during the non-LM period of 1967–75 and large during the LM-dominant period of 1975–91. Mean volume transport of the subtropical gyre is estimated at 57 Sv (1 Sv = 10⁶ m³s^–1) and divided by the Nansei Shoto Ridge into those of the Kuroshio in the East China Sea (25.5 Sv) and a subsurface current east of this ridge (31.5 Sv). The Subtropical Countercurrent and a southward deep current east of the Izu-Ogasawara Ridge are estimated at 16 Sv and 7 Sv, respectively. The calculated transports of the Kuroshio and other subtropical currents reach maximums at every El Niño event due to strong excitement of upwelling barotropic Rossby wave.     

Bottom currents in Nankai Trough and Sagami Trough

Mean flows and velocity fluctuations are described from direct measurements of bottom currents made at three stations across Nankai Trough and two stations in Sagami Trough from May 1982 to May 1984. Aanderaa current meters were moored 7 m above the bottom. The observed mean flows indicate a counter-clockwise circulation in Nankai Trough with current speeds of 0.9–2.1 cm sec^–1. The mean flows were larger on the slopes than on the flat bottom of the trough. The mean flows observed in Sagami Trough show an inflow into Sagami Bay which is considered to be a part of the Oyashio undercurrent from the north that flows along the eastern coast of Honshu. Velocity fluctuations with periods greater than 100 hr were less energetic in the troughs than those at a station west of Hachijo-jima Island. A highly energetic fluctuation with a period of 66.7 hr was observed on the northern slope of Sagami Trough in the velocity component parallel to the trough axis. A maximum current speed of 49 cm sec^–1 was observed in Sagami Trough.This study was sponsored by the Ministry of Education, Science and Culture, Japan.     

Measurements of the water-following capability of holey-sock and TRISTAR drifters

Since 1985, a number of measurements have been made in deep water to determine the water-following characteristics of mixed layer drifters with both holey-sock and TRISTAR drogues at 15 m depth. The measurements were done by attaching two neutrally buoyant vector measuring current meters (VMCMs) to the top and the bottom of the drogues and deploying the drifters in different wind and upper ocean shear conditions for periods of 2–4 h. The average velocity of the VMCM records was taken to be a quantitative measure of the slip of the drogue through the water, observed to be 0.5-3.5 cm s⁻¹. The most important hydrodynamic design parameter which influenced the slip of the drogue was the ratio of the drag area of the drogue to the sum of the drag areas of the tether and surface floats: the drag area ratio R. The most important environmental parameters which affected the slip were the wind and the measured velocity difference across the vertical extent of the drogue. A model of the vector slip as a function of R, vector wind and velocity difference across the drogue was developed and a least squares fit accounts for 85% of the variance of the slip measurements. These measurements indicated that to reduce the wind produced slip below 1 cm s⁻¹ in 10 m s⁻¹ wind speed, R > 40. Conversely, if the daily average wind is known to 5 m s⁻¹ accuracy, the displacement of the R = 40 drifter can be corrected to an accuracy of 0.5 km day⁻¹.     

On the variability of the high-frequency part of the spectrum of wind-forced sea waves

Data on the temporal variability of sea wave spectral components in the frequency range 1–8 Hz, collected by a drifting vessel in the Pacific ocean (wind speed 1–10 m/s), are discussed in this paper. For the frequency range 3–6 Hz (wind speed 5–8 m/s), a weak variability of the ripples is observed, synchronous with long waves; in the remaining part of the spectral range studied the fluctuations are fortuitous. It is concluded that the wind plays a crucial role in forming the ripples' fluctuation characteristics in the high-frequency part of the spectrum.Translated by Vladimir A. Puchkin.     

Measurement of skin friction distribution along the surface of wind waves

Measurements of local values of the skin friction have been made at many points along the surface of representative wind wave crests in a wind wave tunnel, by use of the distortion of hydrogen-bubble lines. The results obtained at 2.85-m fetch under 6.2 m s^–1 mean wind speed show that the intensity of the skin friction varies greatly along the surface of wind waves as a function of the phase angle. It increases rather continuously at the windward surface toward the crest, attains a value of about 12 dyn cm^–2 near the crest, decreases suddenly just past the crest, and the value at the lee surface is substantially zero Values of the skin friction thus determined along the representative wind waves give an average value of 3.6 dyn cm^–2, rather exceeding the overall stress value of 3.0 dyn cm^–2, which has been estimated from the wind profile. The results are interpreted as that the skin friction bears most of the shearing stress of wind, and that it exerts most intensively around the representative wave crests at their windward faces.     

Coastal upwelling and downwelling events in the Seto Inland Sea

Prominent coastal upwelling and downwelling events due to Ekman transport were observed during the period from 14 to 18 August 1983 along the Misaki Peninsula in the Seto Inland Sea, Japan. The coastline of the Misaki Peninsula is aligned approximately in an ENE-WSW direction. When an ENE wind continued blowing for about two days, the warm water in the upper layer was pushed offshore and cold water in the lower layer upwelled along the peninsula. The estimated upwelling speed 3 m below the sea surface was 0.032 cm sec^–1. On the other hand, when a WSW wind continued blowing for about two days the warm water in the upper layer sank into the lower layer along the peninsula. The estimated downwelling speed 3 m below the sea surface was 0.080 cm sec^–1. The time lag between the variations of the alongshore wind and offshore current was about 0.5 days.     

Characteristics of the eddy caused by Izu-Oshima Island and the Kuroshio branch current in Sagami Bay,Japan

Direct current measurements of the branch current of the Kuroshio intruding into Sagani Bay were carried out during 1989–1990 in order to clarify the frequency characteristics of the eddies in the lee of Izu-Oshima Island, which are well recognized as cold water mass produced by upwelling. Satellite and ADCP (Acoustic Doppler Current Profiler) data indicated that current velocity in the eddy fluctuates with periods of 2–4 days and 6–8 days.When the Kuroshio branch current intruding into Sagami Bay from the western channel is weak and its velocity at the depth of 400 m is approximately 10 cm s^–1, the 6–8 day period fluctuation is dominant. On the other hand, when the branch current strongly intrudes from the western channel with a velocity of approximately 20 cm s^–1, the 2–4 day period fluctuation dominates. The relationship between the periods and velocities agrees well with theory based on laboratory experiments for a flow of a homogeneous fluid past a circular obstacle. These periods correspond to the time scale of appearance of the eddy caused by the intrusion of the Kuroshio branch current into Sagami Bay and Izu-Oshima Island.     

Tidal current fluctuations in the Soya Current

Long-term current measurements were carried out near the Soya Strait in the Okhotsk Sea during a period from February 1980 to September 1982. The data were divided into five segments, each being 150 days long, and the tidal ellipse parameters of major axis, minor axis, orientation, and phase for the four major constituents (M2, S2, K1 and O1 tides) were calculated at each segment. The major axis of the mean tidal ellipse averaged over five segments was 29.9 cm sec^–1 for O1 tide, 28.3 cm sec^–1 for K1 tide, 10.4 cm sec^–1 for M2 tide, and 3.7 cm sec^–1 for S2 tide. The phase and orientation of the tidal ellipse were much stable. But, the root mean square deviations of the major axis reached 20% of the mean values for all four constituents. The tidal currents estimated from the sea level records at Wakkanai and Esashi along the Hokkaido coast in the Okhotsk Sea show that their amplitudes and phases are in good agreement with the observed ones for all four constituents.     

Contrasts in sedimentation flux below the southern California Current in late 1996 and during the El Niño event of 1997–1998

The vertical flux of particulate matter at 330 m depth in San Lázaro Basin off southern Baja California ranged from 63 to 587 mg m⁻² d⁻¹ between August and November 1996. Organic carbon contents were between 5.6 and 14.8%, yielding flux rates of 9–40 mgC m⁻² d⁻¹. In December 1997 and January 1998, at the height of the strong El Niño event, the respective fluxes (47–202 mg m⁻² d⁻¹ and 3–8 mgC m⁻² d⁻¹) were comparable. The February–June 1998 records, however, revealed sharply reduced mass (1–6 mg m⁻² d⁻¹) and organic carbon (0.2–0.8 mgC m⁻² d⁻¹) fluxes. The organics collected in 1996 were predominantly autochthonous (δ¹³C=−22‰; C/N=8). The variations in δ¹⁵N (8.3–11.0‰) suggest an alternation of new and regenerated production, possibly associated with fluctuations in the intensity of deep mixing during that autumn. The relatively high organic matter fluxes in December 1997 appear to be associated with regenerated production. The average composition from February to June 1998 (δ¹³C=−23.6‰; ¹⁵N=11.7‰; C/N=10.5) indicates degraded material of marine origin. The maximum δ¹⁵N value found (14‰) suggests that deeper, denitrified waters were brought to the surface and possibly advected laterally. Regime changes in the waters of the basin occur at 6–10 week intervals, evidenced by concurrent shifts in most of the measured parameters, including fecal pellet types and metal chemistry. The marine snow-dominated detritus collected showed a shift from a mixed diatom-rich-radiolarian-coccolith assemblage in late 1996 to a coccolith-dominated assemblage, including the contents of fecal pellets, during the 1997–1998 El-Niño period. T–S profiles, plankton analysis and chlorophyll contents of the upper water column indicated that the strong phytoplankton bloom, normally associated with seasonal upwelling along the Pacific coast of Baja, did not occur during the spring of 1998. The persistence of oligotrophic conditions during the 1997–1998 El Niño event favored the dominance of nanoplankton and reduced the vertical flux of particles.     

Evaluation of Vector Winds Observed by NSCAT in the Seas around Japan

In order to validate wind vectors derived from the NASA Scatterometer (NSCAT), two NSCAT wind products of different spatial resolutions are compared with observations by buoys and research vessels in the seas around Japan. In general, the NSCAT winds agree well with the wind data from the buoys and vessels. It is shown that the root-mean-square (rms) difference between NSCAT-derived wind speeds and the buoy observations is 1.7 ms^–1, which satisfies the mission requirement of accuracy, 2 ms^–1. However, the rms difference of wind directions is slightly larger than the mission requirement, 20°. This result does not agree with those of previous studies on validation of the NSCAT-derived wind vectors using buoy observations, and is considered to be due to differences in the buoy observation systems. It is also shown that there are no significant systematic trends of the NSCAT wind speed and direction depending on the wind speed and incidence angle. Comparison with ship winds shows that the NSCAT wind speeds are lower than those observed by the research vessels by about 0.7 ms^–1 and this bias is twice as large for data observed by moving ships than by stationary ships. This result suggests that the ship winds may be influenced by errors caused by ship's motion, such as pitching and rolling.     

Local balance in the air-sea boundary processes

A combination of the three-second power law, presented in part I for wind waves of simple spectrum, and the similarity of the spectral form of wind waves, leads to a new concept on the energy spectrum of wind waves. It is well substantiated by data from a wind-wave tunnel experiment.In the gravity wave range, the gross form of the high frequency side of the spectrum is proportional tog u _* ^–4, whereg represents the acceleration of gravity,u _* the friction velocity, the angular frequency, and the factor of proportionality is 2.0×l0^–2. The wind waves grow in such a way that the spectrum slides up, keeping its similar form, along the line of the gross form, on the logarithmic diagram of the spectral density,, versus. Also, the terminal value of, at the peak frequency of the fully developed sea, is along a line of the gradient ofg ^{2 –5}.The fine structure of the spectrum from the wind-wave tunnel experiment shows a characteristic form oscillating around the ^–4-line. The excess of the energy density concentrates around the peak frequency and the second- and the third-order harmonics, and the deficit occurs in the middle of these frequencies. This form of the fine structure is always similar in the gravity wave range, in purely controlled conditions such as in a wind-wave tunnel. Moving averages of these spectra tend very close to the form proportional to ^–5.As the wave number becomes large, the effect of surface tension is incorporated, and the ^–4-line in the gravity wave range gradually continues to a ^–8/3-line in the capillary wave range, in accordance with the wind-wave tunnel data. Likewise, the ^–5-line gradually continues to a ^–7/3-line.Also, through a discussion on these results, is suggested the existence of a kind of general similarity in the structure of wind wave field.     

Large-amplitude internal Kelvin waves trapped off Split (Middle Adriatic Sea)

The paper documents the occurrence of long-period internal Kelvin waves in Split Channel in spring 2002. The analyses were performed on thermohaline and current data measured at three moorings and one hydrographic section. The internal oscillation had a period of 5–6 days, being larger just after the generation which was probably excited by the alongshore Sirocco wind. The recorded current amplitude was up to 0.3 m s⁻¹ in the surface layer, while the observed pycnocline displacement was 10–15 m. The oscillation was reproduced by one-dimensional two-layered model of a channel, imposing nodal lines at its entrances. Cross-shore properties of the oscillation, such as observed offshore decrease in pycnocline amplitude, are explained by the dynamics of an internal Kelvin wave propagating along channel boundaries, because the internal Rossby radius is smaller than the width of the channel. Conclusively, the observed oscillation probably represents the fundamental mode of internal waves trapped in the channel complex off Split.     

A dispersion equation and dispersion coefficient for material transport in Inland Seas

To analyse material transport in inland seas, a horizontal two-dimensional dispersion equation is derived, and the dispersion coefficient due to the combined effect of vertical turbulent mixing and vertical shear of both a steady current and a tidal current is studied. In the present study, the assumption that velocity is uniform in horizontal planes is not necessary, and velocity has a free vertical profile; thus the dispersion coefficient formulated is general, and is represented by a tensor of the second order. The properties of the dispersion coefficient in the horizontal two-dimensional dispersion model are also investigated, and it is shown that the time-averaged dispersion coefficient due to the tidal current over a tidal period is approximately half that due to the steady current, if the velocity amplitude and the vertical profile of the tidal current are the same as those of the steady current (a similar result was presented byBowden (1965) for horizontal one-dimensional models). Finally, the dispersion coefficient in Hiuchi-Nada (Hiuchi Sound) in the central part of the Seto Inland Sea is evaluated by using the model. The values of the dispersion coefficient in that region range from 10³ cm² s^–1 to 10⁵ cm² s^–1 when vertical turbulent diffusivity is taken to be 50 cm² s^–1.