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1.
Relation between internal waves with short time scale and density distribution near the shelf break in the East China Sea is studied utilizing moored current meters, thermometers and conductivity-temperature-depth (CTD) casts. A well developed pycnocline was frequently observed around 150–200 m depth near the shelf break accompanied with the development of internal waves with short time scale. During the cruise in May 1998, the intensified internal wave motion with short time scale and the distinct offshore flow were observed just below the lower pycnocline, which shoaled and extended above the shelf area. It is suggested that vertical mixing generated by amplified internal waves would produce cross-shelf ageostophic density current around the pycnocline. During the cruise in May 1999, on the other hand, the lower pycnocline was located offshore below the shelf break, and the internal wave motion was amplified just above the lower pycnocline. In this case, the offshore flow should be generated above the lower pycnocline, but vertical profiles of current velocity were not obtained because acoustic Doppler current profiler (ADCP) data were not available around the lower pycnocline.  相似文献   

2.
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3.
Internal tides and sediment movement on Horizon Guyot,Mid-Pacific Mountains   总被引：2，自引：0，他引：2  
Internal tidal currents are the likely cause of erosional features such as current ripples, sand waves, and truncated bedding horizons on the sediment cap of Horizon Guyot. Current meter data obtained over a 9 month period in 1983–1984 at about 213 m above the guyot show that the tidal currents are anomalously strong for mid-oceanic depths, probably the result of topographically induced generation of internal tidal waves. An analysis of the initiation of motion of the foraminiferal sand by the internal tidal currents indicates that these currents, particularly during the months of March–May, are likely to transport the surficial sediment and generate the observed bedforms.  相似文献   

4.
A three-dimensional, multi-level model was used to study the energy dissipation of semidiurnal internal Kelvin waves due to their interaction with bottom topography. A simplified topography consisting of a channel with an additional shallow bay was used to clarify the wave’s scattering process. When the first mode semidiurnal internal wave given at an open boundary arrives at the bay mouth, higher-mode internal waves are generated at a step bottom of the bay mouth. As a result, the energy of the first mode internal Kelvin wave is effectively decayed. The decay rate of the internal Kelvin wave depends on both the width and length of the additional bay. The maximum decay rate was found when a resonance condition occurs the bay, that is, the bay length is equal to a quarter of wave length of the first mode internal wave on the shallow region. The decay rate in the wide bay cases is higher than that in a narrow case, due to a contribution from the scattering due to the Poincare wave that emanates from the corners of the bay head. The decay rate with the additional bay is 1.1–1.8 times that of the case without the additional bay. The decay rate due to the scattering process is found to be of the same order as that of the internal and bottom friction.  相似文献   

5.
An erosional channel and upslope-climbing sediment waves have been observed in Ytre Orkdalsfjorden and the marine fjord branch Gaulosen off the mouth of Gaula River in Trondheimsfjorden, central Norway. The submarine channel (up to 100–150 m wide and 12 m deep) is interpreted as the pathway of hyperpycnal flows and turbidity currents. It can be traced for 20 km on the seafloor from the mouth of Gaula River down to 500 m of water depth. Based on swath bathymetry and seismic data, the sediment waves are shown to have an accumulated thickness of 50–60 m. They are up to 8 m high, have up to 1-km-long crests, and wavelengths of 100–900 m. The sediment waves are attributed to hyperpycnal flows and turbidity currents overflowing the banks of the channel. Many of the sediment waves were instigated by pre-existing topography created by mass movements since early Holocene times.  相似文献   

6.
内波场中水平桩柱波阻的实验研究   总被引：3，自引：0，他引：3  
分析水平阻力与内波波要素的关系,采用实验室实验的方法,研究了水平桩柱与内波的相互作用,焦点是研究内波波要素与波阻之间的关系。实验中的密度分层采用典型跃层型剖面,而内波是由自行研制的造波机生成。实验结果表明,当内波的频率不变时,水平桩柱所受到的阻力随着内波的振幅增加而增加,它的量值与分层有关。对于强跃层的密度分布,当桩柱处在跃层中时,受到的水平波阻较小,而在跃层之外则受到较大的水平波阻。  相似文献   

7.
A theory of the coastal Kelvin wave is presented in which the pressure is assumed not to be hydrostatic. The results show that the non-hydrostatic Kelvin wave is dispersive and that the e–1 decay distance of the wave amplitude from the coast decreases as the wave length becomes shorter. Similar conclusions can be drawn on the equatorial Kelvin wave.  相似文献   

8.
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.  相似文献   

9.
A high resolution modeling study is undertaken, with a 2.5-dimensional nonhydrostatic model, of the generation of internal waves induced by tidal motion over the ridges in Luzon Strait. The model is forced by the barotropic tidal components K1, M2, and O1. These tidal components, along with the initial density field, were extracted from data and models. As the barotropic tide moves over the Luzon Strait sills, there is a conversion of barotropic tidal energy into baroclinic tidal energy. Depressions are generated that propagate towards the Asian Seas International Acoustics Experiment (ASIAEX) test site on the Chinese continental shelf. Nonlinear effects steepen the depressions, frequency and amplitude dispersion set in, and disintegration into large amplitude solitary waves occurs. The effects of varying the initial density field, tidal component magnitudes, as well as adding a steady background current to represent the occasional excursions of the Kuroshio Current into the strait, are considered.Depressions are generated at each of the two sills in Luzon Strait which radiate away, steepening and evolving into internal solitary wave trains. Baroclinic fluxes of available potential energy, kinetic energy and linear are calculated for various parameter combinations. The solitary wave trains produced in the simulations generally consist of large amplitude wave trains alternating with small amplitude wave trains. During strong tidal flow, Kelvin–Helmholtz type instabilities can develop over the taller double-humped sill. The solitary waves propagating towards the ASIAEX test site have been observed to reach amplitudes of 120–250 m, depending on the tidal strength. ASIAEX observations indicate amplitudes up to 150 m and the Windy Island Experiment (WISE) measurements contain magnitudes over 200 m. The model results yield solitary wave amplitudes of 70–300 m and half widths of 0.60–3.25 km, depending on parameter values. These are in the range of observations. Measurements by Klymak et al. (2006), in the South China Sea, exhibit amplitudes of 170 m, half widths of 3 km and phase speeds of 2.9 m s?1. Model predictions indicate that the solitary waves making up the wave packet each experience different background currents with strong near surface shear.The energy in the leading soliton of the large amplitude wave trains ranges between 1.8 and 9.0 GJ m?1. The smaller value, produced using barotropic tidal currents based on the Oregon State University data base, is the same as the energy estimated to be in a solitary wave observed by Klymak et al. (2006). Estimates of the conversion of barotropic tidal energy into radiating internal wave energy yield conversion rates ranging between 3.6% and 8.3%.  相似文献   

10.
Current records obtained in the inshore region along the Fukushima coast are analyzed. The existence of periodical current fluctuations whose period is about 100 hours and whose amplitude is as large as 15–25cm s–1 is recognized. Auto-spectral analyses are made also for sea level, atmospheric pressure and wind records. Each spectrum has significant peaks at the similar period to the current spectrum. The wind spectrum has a broad peak compared with the current. The periodical current fluctuations propagate southward with speed of 3–5 km h–1. These propagation speeds seem to correspond to those of the second-and third-mode shelf waves.  相似文献   

11.
Diurnal-period internal waves were observed near Point Conception California, using an array of moorings extending 120 km along the inner shelf. The waves have an along-shelf coherence scale of at least 50 km, and appear to propagate nearly straight onshore. Wave amplitudes vary over time, depending on thermal stratification and the amplitude of the diurnal sea breeze oscillation. Barotropic tides and vorticity over the mid-shelf are not correlated with internal wave amplitude. Large amplitude internal waves, with supercritical Froude numbers, are observed in mid-summer. Although such waves may drive vertical mixing and cross-shelf transport of passive particles, there is no significant correlation between wave amplitude and invertebrate settlement in the Santa Barbara Channel.  相似文献   

12.
Three years of temperature data along two transects extending to 90 m depth, at Palau, Micronesia, show twice-a-day thermocline vertical displacements of commonly 50–100 m, and on one occasion 270 m. The internal wave occurred at a number of frequencies. There were a number of spectral peaks at diurnal and semi-diurnal frequencies, as well as intermediate and sub-inertial frequencies, less so at the inertial frequency. At Palau the waves generally did not travel around the island because there was no coherence between internal waves on either side of the island. The internal waves at a site 30 km offshore were out-of-phase with those on the island slopes, suggesting that the waves were generated on the island slope and then radiated away. Palau Island was thus a source of internal wave energy for the surrounding ocean. A numerical model suggests that the tidal and low-frequency currents flowing around the island form internal waves with maximum wave amplitude on the island slope and that these waves radiate away from the island. The model also suggests that the headland at the southern tip of Palau prevents the internal waves to rotate around the island. The large temperature fluctuations (commonly daily fluctuations ≈10 °C, peaking at 20 °C) appear responsible for generating a thermal stress responsible for a biologically depauperate biological community on the island slopes at depths between 60 and 120 m depth.  相似文献   

13.
Near surface vertical distributions of Japanese anchovy eggs (isolated and pelagic) were studied at western Wakasa Bay, Japan. Samples were collected in horizontal tows with four plankton nets simultaneously operated at different four depths of 0, 0.5, 1.0, and 2.0 m. The egg concentration was found to decrease exponentially with depth. The egg concentration profile can be explained by considering the balance of the eggs' ascent and vertical diffusion. Taking further into account vertical difference in rising/sinking rates of the eggs, a clear accumulation of the eggs on a pycnocline, observed elsewhere, also can be explained. Vertical eddy diffusivity in the surface layer in a calm sea was 1–10 cm2 s–1, as estimated from the egg concentration profile and the velocities of the eggs in the vertical direction.  相似文献   

14.
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.  相似文献   

15.
On the basis of the climatic data array on temperature and salinity including the data of observations carried out at 102,000 oceanographic stations in 1910–1998, we estimate the amount of available potential energy of large-scale processes in the 0–300-m layer of the Black Sea and study its annual variability. The annual average amount of available potential energy in the active 0–300-m layer is equal to 13.5×1014 J, its maximum amount attained in March is 25×1014 J, and its minimum amount attained in August is 8×1014 J. In the autumn–winter period, the density of available potential energy in the 0–50-m layer increases and its maximum is attained in December–January (30 J/m3). Beginning with January, the density of available potential energy increases in the layer of the main pycnocline (50–150 m). Its maximum is attained in March (50 J/m3) and then this quantity decreases till August (down to 14 J/m3). In August, we study the interdecadal variability of the available potential energy in the 50–300-m layer by using the data accumulated for decadal periods with five-year shifts in 1956–1995. The maximum variability is observed at a depth of 100 m. The maximum average (over the sea) amounts of available potential energy were observed in 1961–1970 and 1986–1995. The minimum amounts were recorded in 1976–1985.  相似文献   

16.
Boundary currents and internal waves determine cross-slope zonation of erosion and deposition in the Faeroe-Shetland Channel. Currents were measured at 8 and 34–50 m above the bottom at three mooring sites (502, 595 and 708 m depth) for 14 days. The structure of the water column was evaluated from CTD sections, and included nepheloid layers and particulate matter concentrations. Indicators for recent deposition in the sediment (organic carbon, phytopigments, 210Pb) were measured at eight stations across the slope. Strong near-bottom currents at the upper slope sustain down-slope particle transport in a benthic nepheloid layer, which is eroded under the influence of critically reflecting M2 internal tidal waves at 350–550 m, where the major pycnocline meets the sloping bottom. Beam attenuation profiles confirmed the presence of intermediate nepheloid layers intruding into the Channel along the major pycnocline, and elevated concentrations of particulate matter and chlorophyll-a were measured at this depth. Near-bottom currents decreased with depth, thus allowing particle deposition down the slope. Inventories of excess 210Pb activity in the sediment deeper than 600 m were higher than what was expected on the basis of atmospheric input of 210Pb and production in the water column, thus indicating additional lateral inputs. Simple calculations showed that off-slope input of particles from areas shallower than 600 m may be responsible for the enhanced deposition at greater depths.  相似文献   

17.
The Faroe Bank Channel (FBC) is the deepest passage across the Greenland–Scotland Ridge and there is continuous overflow of dense water through the channel, which provides a substantial contribution to the global thermohaline circulation. Since November 1995, Acoustic Doppler Current Profilers (ADCPs) have been moored at the sill of the channel continuously, except for short annual servicing periods. In addition to this, CTD sections have regularly been occupied across the channel and instruments, recording temperature and salinity, have been moored for short periods. These observations demonstrate that the channel, due to its small width, can be monitored fairly well with only one ADCP mooring and they allow the generation of a time-series of overflow volume flux for the 1995–2005 period. The kinematic overflow flux, defined solely from the velocity field, was found to have an average value of 2.1 ± 0.2 Sv with a seasonal and with inter-annual variations of approximately 10% amplitude, but with no discernible trend for the whole period. The average flux of water with potential density exceeding 27.8 kg m−3 was found to be 1.9 ± 0.3 Sv with average properties: θ = 0.25 °C, S = 34.93, γθ = 28.01 kg m−3 for this period. No evidence was found for a weakening overflow flux, but the salinity of the FBC-overflow, especially the warmer parts, has increased substantially during the period, which implies an increased average density on the order of 0.01 kg m−3. Previous observations of a helical cross-channel circulation are confirmed and may explain the thin pycnocline layer, but shear-instability induces intensive mixing that puts a lower limit on the layer thickness. Critical control at one point of the sill cross-section, rather than friction, seems to govern the overflow dynamics and simple layered models can explain much of the observed features as well as the seasonal variation, but potential vorticity, as defined for a single overflow layer, is not conserved. A previously suggested 1950–2000 weakening of the FBC-overflow, which was partly based on a subset of the data presented here, is not supported by the full dataset.  相似文献   

18.
The collapse of a uniform density fluid (a “mixed region”) into a surrounding ambient fluid with complex stratification is examined by way of laboratory experiments and fully nonlinear numerical simulations. The analysis focuses upon the consequent generation of internal gravity waves and their influence upon the evolution of the collapsing mixed region. In experiments and simulations for which the ambient fluid has uniform density over the vertical extent of the mixed region and is stratified below, we find the mixed region collapses to form an intrusive gravity current and internal waves are excited in the underlying stratified fluid. The amplitude of the waves is weak in the sense that the intrusion is not significantly affected by the waves. However, scaling the results to the surface mixed layer of the ocean we find that the momentum flux associated with the waves can be as large as 1 N/m2. In simulations for which the ambient fluid is stratified everywhere, including over the vertical extent of the mixed region, we find that internal waves are excited with such large amplitude that the collapsing mixed region is distorted through strong interactions with the waves.  相似文献   

19.
Mesoscale eddies and tropical instability waves in the eastern tropical Pacific, first revealed by satellite infrared imagery, play an important role in the dynamics and biology of the region, and in the transfer of mass, energy, heat, and biological constituents from the shelf to the deep ocean and across the equatorial currents.From boreal late autumn to early spring, four to 18 cyclonic or anticyclonic eddies are formed off the coastal region between southern Mexico and Panama. The anticyclonic gyres, which tend to be larger and last longer than the cyclonic ones, are the best studied: they typically are 180–500 km in diameter, depress the pycnocline from 60 to 145 m at the eddy center, have swirl speeds in excess of 1 m s−1, migrate west at velocities ranging from 11 to 19 cm s−1 (with a slight southward component), and maintain a height signature of up to 30 cm. The primary generating agents for these eddies are the strong, intermittent wind jets that blow across the isthmus of Tehuantepec in Mexico, the lake district in Nicaragua and Costa Rica, and the Panama canal. Other proposed eddy-generating mechanisms are the conservation of vorticity as the North Equatorial Counter Current (NECC) turns north on reaching America, and the instability of coastally trapped waves/currents.Tropical Instability Waves (TIWs) are perturbations in the SST fronts on either side of the equatorial cold tongue. They produce SST variations on the order of 1–2 °C, have periods of 20–40 days, wavelengths of 1000–2000 km, phase speeds of around 0.5 m s−1 and propagate westward both north and south of the Equator. The Tropical Instability Vortices (TIVs) are a train of westward-propagating anticyclonic eddies associated with the TIWs. They exhibit eddy currents exceeding 1.3 m s−1, a westward phase propagation speed between 30 and 40 km d−1, a signature above the pycnocline, and eastward energy propagation. Like the TIWs, they result from the latitudinal barotropically unstable shear between the South Equatorial Current (SEC) and the NECC with a potential secondary source of energy from baroclinic instability of the vertical shear with the Equatorial Undercurrent (EUC).This review of mesoscale processes is part of a comprehensive review of the oceanography of the eastern tropical Pacific Ocean.  相似文献   

20.
We analyze the time-longitude structure of composite cases from model-assimilated ocean data in the period 1958–1998, following on from earlier work by Huang and Kinter (J. Geophys. Res. 107(C11) (2002) 3199) that studied east–west thermocline variability in the Indian Ocean. Our analysis focuses on the Rossby wave signal along the thermocline ridge in the tropical SW Indian Ocean (10°S, 60–80°E), where wind stress curl is important. Anomalous winds in the equatorial east Indian Ocean force successive Rossby waves westward at speeds of 0.1 m s−1±30%. With a wavelength of 7000 km, the period of oscillation is in the range 1.9–5.2 years. The Indian Ocean Rossby wave is partially resonant with the global influence of the El Nino–Southern Oscillation, except during quasi-biennial rhythm. The presence of the Rossby wave offers potential predictability for east–west atmospheric circulation systems and climate that affect resources in countries surrounding the Indian Ocean.  相似文献   

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