The treatment of diffusion due to the tidal current in the near-shore ocean, and the similitude in the hydraulic model experiment are studied.In broad and shallow tidal bays and in the coastal seawaters near irregular boundaries, horizontal eddy-currents induced geometrically and turbulence caused by their cascading have predominant effect on dispersion of river and waste waters. These turbulent diffusion processes are similarly reproduced in the Froude models of turbulent resume, by adding the similitude for the self-similar structure of the spectral density of turbulence, or the eddy diffusivity. The similitude means to take the scale ratios of the time and the vertical length as the two-thirds power of the scale ratio of the horizontal length.Similitudes are also derived for the system of the gravitational circulation, the stratification and the salt-mass transport in partially and well-mixed estuaries. Generally, the vertical eddy diffusivity must be exaggerated by a half power of the model distortion externally by some methods of agitation. When the tidal bay is broad and very shallow, the Rayleigh number and the Hansen number are small, and the effect of density current and stratification on the flushing is small. Instead the effect of local eddies, geometrically induced tidal residual circulations become predominant. In this special case, there is no need to satisfy the similitude for density difference and vertical shear effects on dispersion. 相似文献
The effects of the coastal boundary geometries on the tidal currents and the tidal mixing are studied mainly on the basis of hydraulic model experiments. Mizushima-Nada Sea, which is located in the central part of Seto Inland Sea, and the whole Seto Inland Sea were chosen as the prototypes.Currents and eddies geometrically induced in the tidal currents and in the ocean currents have significant effects on the water exchange, from small scales to meso scales, in the bays and near-shore regions of the ocean.As to small scale phenomena near irregular coastal boundaries such as river mouths, headlands and harbors, tidal currents produce organized eddy currents of the width scale. They are important to the temporary flushing and the local redistribution of the river water or the waste water.As to phenomena of larger scales, the tidal currents produce rather steady residual circulations in each part of the Inland Sea, due to the non-linear effects of the oscillating component. They are controlled by the geometry of the sea as separated by a narrow strait. These horizontal circulations of about 20 km in scale become the main mechanism of the water exchange in the Inland Sea. The one-dimensional dispersion coefficient due to these circulations is proportional to the product of the diameter and the current velocity of the circulations. The proportional constant takes the value of 0.40.5. 相似文献
A spread of warm water from the first crest of the Kuroshio Extension is periodically enhanced by northward warm water intrusions from the main current. The water type in the spread area was previously found to be the same as that in the Kuroshio front at depth. In looking for the possible mechanism responsible for the northward warm water intrusions, a dynamic analysis in the Kuroshio front was carried out by using CTD, ADCP, AVHRR and ARGOS buoy data, obtained in 1996 by the R.V. Hakuho Maru. Downstream, cross-stream and vertical velocities in the Kuroshio Extension were found by using a "stream coordinate system". The velocity field in the Kuroshio front at the first crest showed a double structure with two surface velocity maxima. In the inner part of the front, relatively high cross-stream (northward) and vertical (upward) velocities were found. Thus, this study suggests that while water particles flow downstream along the first stationary meander of the Kuroshio Extension, they also experience lateral and vertical movements which allow the deeper water from an upstream location to rise to the surface layer, and in certain locations to deflect northward. By assuming isopycnal movement and conservation of potential vorticity, it was found that in those locations where anticyclonic curvature of the meander increases, warm water is more likely to deflect northward. High ageostrophic components observed in the first 300 m of the water column are probably related to the relatively high cross-stream and vertical velocities in the inner part of the front. 相似文献
In order to examine the responses of primary productivity in the southern coastal sea of Japan to the Pacific Decadal Oscillation
(PDO) in the 20th century, sedimentary records of diatom productivity (diatom valve fluxes) were reconstructed using core
samples from the Bungo Channel (BC) in southwest Japan. The record of the Thalassionema spp. flux—the best index of fall primary productivity in the BC—indicated a multidecadal-scale duration with a low flux (1943–1982)
and those with a high flux (1913–1943 and 1982–2001); apparent shifts were recognized in 1943 and 1982. The shift in 1982
was also recognized in the flux records of other early summer to fall predominant genera in the BC and, previously, in the
biogenic silica records from a broad region of the southeast BC. This indicates that in our records, this shift reflects a
general trend in the primary production in the southeast BC. A comparison among the Thalassionema spp. flux records, meteorological data from an observatory adjacent to the core site, and the PDO index showed that the flux
records were more similar to the PDO index than the other meteorological records, which suggests that the multidecadal-scale
variability of the BC primary productivity may be associated with some marine-derived forcing. The bottom intrusions of nutrient-rich
water that upwelled from the shelf slope into the BC, the axis movement or the transport of the Kuroshio Current off the BC,
and a basin-scale wind stress in the North Pacific might play an important role in this forcing and mediate between the BC
primary productivity and the PDO. 相似文献
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
Features of El Niño events and their biological impacts in the western North Pacific are reviewed, focusing on interactions between ENSO and the East Asian monsoon. Impacts of El Niño on the climate in the Far East become evident as ‘cool summers and warm winters’. Effects of climate regime shift on ENSO activities, western boundary currents and upper-ocean stratification, as well as their biological consequences are summarized. These have been:
1. In the western equatorial Pacific, an eastward extension of the warm pool associated with El Niño events induces an eastward shift of main fishing grounds of skip jack and big eye tunas.
2. The surface salinity front in the North Equatorial Current region retreats southward, associated with El Niño events. This leads to a southward shift of the spawning ground of Japanese eel, which is responsible for a reduction in the transport of the larval eels to the Kuroshio and Japanese coastal region, causing poor recruitment.
3. Intensification of winter cooling and vertical mixing associated with La Niña (El Niño) events in the northern subtropical region of the western (central) North Pacific reduces surface chlorophyll concentration levels and larval feeding condition for both Japanese sardines and the autumn cohort of Neon squid during winter–early spring. The semi-decadal scale calm winter that occurred during the early 1970s triggered the first sharp increase of sardine stock around Japan.
4. A remarkable weakening of southward intrusion of the Oyashio off the east coast of Japan during 1988–91, resulted in a decrease in chlorophyll concentrations and mesozooplankton biomass in late spring–early summer of the Kuroshio-Oyashio transition region. Changes occurred in the dominant species of small pelagic fish, through successive recruitment failures of Japanese sardine.
Article Outline
1. Introduction
2. Linkage between Asian monsoon and ENSO
2.1. Features of Asian monsoon and its role in ENSO
2.2. Influence of ENSO events on summer and winter climate and hydrographic conditions in the western North Pacific
3. Evidence of biotic impacts of ENSO events in the western and central North Pacific
3.1. Eastward shift or spread of fishing grounds of skipjack, bigeye and albacore
3.2. Decrease of recruitment rate of neon squid and Japanese eel
3.3. Increase of plankton biomass in El Niño winters in the northern subtropical gyre south of Japan
3.4. Bleaching phenomena of corals around the Okinawa Islands
4. Discussion
4.1. Modulation of extra-tropical effect of ENSO by inter-decadal variations
4.2. Effects of ENSO and ocean/climate regime shifts on plankton biomass and population variation of small pelagic fish
5. Conclusion
Acknowledgements
References
1. Introduction
During El Niño events the climate in Northeast Asia is generally cool and wet in summer, and warm and calm in winter (Kurihara and Kimura). In the 1998 summer, near the end of 1997/98 El Niño, the East China Sea and southern part of the Japan Sea were covered with abnormally low saline water. This was the result of the huge amounts of fresh water that were discharge from the Yangtze River and caused poor year classes of Japanese common squid.During the recent cold regime that persisted between 1976/77 and 1987/88 in the North Pacific, Japanese sardine, Sardinops melanostictus, maintained a higher stock level, whereas stocks of anchovy, Engraulis spp., remained low (Kasai; Yasuda and Nakata).To clarify the features of this biological response associated with El Niño events and climate regime shifts, in this paper we provide evidence of several environmental and biological responses in the western and central North Pacific. First, we review the linkage between ENSO and the Asian Monsoon. Second, we present data on the extra-tropical effects of El Niño and La Niña on marine ecosystems and the ocean environment. Finally, we describe the modification of extra-tropical effects of ENSO by interdecadal variations in the ocean and the atmosphere.
2. Linkage between Asian monsoon and ENSO
2.1. Features of Asian monsoon and its role in ENSO
Climate of the western North Pacific is dominated by monsoon winds and precipitation. In summer, the southeast monsoon develops between the Tibetan Low and the North Pacific Subtropical High (Fig. 1a). When the summer monsoon encounters the Japanese mountain range, it produces a considerable amount of precipitation on the Pacific side of Japan. In winter, however, the northwesterly monsoon develops between the Siberian High and the Aleutian Low superimposed on the westerly wind (Fig. 1b). 相似文献
Sand banks around straits are used as a commercial fishing ground. In order to clarify the mechanism of sand bank formation, the Lagrangian method was used to measure currents and turbidity around the banks in the Neko Seto Sea in the Seto Inland Sea of Japan. A neutrally buoyant float released in the Neko Seto Strait at the maximum tidal flow stage was engulfed in a pair of tidal vortices and moved around one of the sand banks. The vertical distribution of turbidity, which was measured by the vessel moving with the neutral float, showed an extremely high turbidity in the bottom layer of this bank area. According to the analysis of these observational data, the process of sand bank formation around straits is as follows. The tidal vortex transports water mass with suspended materials (including sand) which are whirled up at the bottom by the tidal jet. In the decaying stage of the vortex, the materials in the bottom layer are gathered in the central part of the vortex by the secondary convergent flow in the vortex. Among these materials, a large-size sand particle with a high critical erosion velocity accumulates at the bottom and forms banks. The distribution of bottom sediment and the thickness of alluvium support this result. 相似文献
Making Upper Cretaceous biostratigraphic correlations between the Northwest Pacific and Tethyan–Atlantic sections have been difficult because of rare frequencies of age-diagnostic macro- and microfossils in the sequences in the Northwest Pacific region. In order to correlate these sections precisely, an integrated planktic foraminiferal and bulk wood carbon-isotope stratigraphy from the upper Cenomanian to the lower Campanian succession (the middle–upper part of the Yezo Group) of Hokkaido, northern Japan is established with an average resolution of 50 k.y. The δ13C curves from bulk wood of the Yezo Group and from bulk carbonate of English Chalk show remarkably similar patterns of isotopic fluctuation, allowing the correlation of 22 carbon isotopic events between these sections. This high-resolution correlation greatly improves the previous micro- and macrofossil biostratigraphic schemes in the Northwest Pacific region, and reveals that global events, such as the oxygen depletion at the OAE 2 horizon, the constant decrease in pCO2 during the Late Cretaceous, and the eustatic sea-level falls in the late middle Turonian, Santonian/Campanian Boundary and early Campanian, are recorded in the Upper Cretaceous sequence of the Northwest Pacific. 相似文献
The path of the Kuroshio Extension describes two stationary meanders with crests at approximately 144°E and 150°E. The short-term
meridional fluctuations of the warm water spreading northward from the first crest at the surface and its vertical structure
were analyzed by using 5-day-mean surface temperature maps published by JAFIC, montly 100-m-depth temperature maps edited
by the JMA, and CTD data obtained by the R.Vs.Kofu-Maru, Hakuho-Maru andTansei-Maru cruises from 1990 to 1994. A Northern Boundary of the Spreak Kuroshio Water (NBSKW) and a Southern Boundary of the Spread
Kuroshio Water (SBSKW) at the surface were defined as the northern and southern boundary of the pronounced meriodional temperature
gradients, respectively. The vertical structure of the Spread Kuroshio Water was analyzed in terms of its T-S properties.
The location of the NBSKW at the surface corresponds well with the northern boundary of the subsurface high salinity water
that represents the Spread Kuroshio Water. The short-term meridional fluctuations of the northern and southern boundary of
the Spread Kuroshio Water at the surface were studied through the spectral analysis of the maximum latitude of the two lines
defined. We obtained the following results: (1) the meridional fluctuations of the NBSKW and SBSKW at the first creast have
major periods between 16 and 38 days; (2) the 50 day running mean of the SBSKW at the first crest, for the purpòse of this
study, can be generally used as indicative of the location of the Kuroshio axis; and (3) the northward extent of the Spread
Kuroshio Water and the velocity of the meridional shift suggest seasonal variability that could be related with their vertical
structure. 相似文献
