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
The amount of penetration of a western boundary current into a marginal sea which is connected to an open ocean by two narrow straits is estimated from a linear, steady and barotropic theoretical model. In this model the western boundary current in the open ocean is driven by a wind stress imposed at the sea surface. The inflow of the water of the open ocean into the marginal sea is caused by the pressure difference between two straits produced by the wind-driven circulation in the open ocean.Main external parameters are combined into two non-dimensional parameters;
and (the ratio of the depth of the marginal sea to that of the open ocean), whereb is the distance between north and south boundaries of the ocean,D0 is the depth of the open ocean, is the latitudinal variation of the Coriolis parameter andR is the coefficient of friction. The friction is assumed to be proportional to the flow velocity.In the limit of infinite the volume transport into the marginal sea is not affected by the width of two straits and . It is mainly controlled by the wind stress and the positions of two straits. For finite values of , however, the volume transport depends considerably on and the width of the straits.Guided by both this model and physical considerations, we obtained a relation between the volume transport into the marginal sea and the external parameters. This relation predicts that about 2 % of the volume transport of the Kuroshio penetrates into the Japan Sea. 相似文献
In the near future, a new generation of sample return missions (Hayabusa2, OSIRIS‐REx, MMX, etc.) will collect samples from small solar system bodies. To maximize the scientific outcome of laboratory studies and minimize the loss of precious extraterrestrial samples, an analytical sequence from less destructive to more destructive techniques needs to be established. In this work, we present a combined X‐ray and IR microtomography applied to five Itokawa particles and one fragment of the primitive carbonaceous chondrite Paris. We show that this analytical approach is able to provide a 3‐D physical and chemical characterization of individual extraterrestrial particles, using the measurement of their 3‐D structure and porosity, and the detection of mineral and organic phases, and their spatial co‐localization in 3‐D. We propose these techniques as an efficient first step in a multitechnique analytical sequence on microscopic samples collected by space missions. 相似文献
To understand the preservation of coesite inclusions in ultrahigh‐pressure (UHP) metamorphic rocks, an integrated petrological, Raman spectroscopic and focussed ion beam (FIB) system–transmission electron microscope (TEM) study was performed on a UHP kyanite eclogite from the Sulu belt in eastern China. Coesite grains have been observed only as rare inclusions in kyanite from the outer segment of garnet and in the matrix. Raman mapping analysis shows that a coesite inclusion in kyanite from the garnet rim records an anisotropic residual stress and retains a maximum residual pressure of ~0.35 GPa. TEM observations show quartz is absent from the coesite inclusion–host kyanite grain boundaries. Numerous dislocations and sub‐grain boundaries are present in the kyanite, but dislocations are not confirmed in the coesite. In particular, dislocations concentrate in the kyanite adjacent to the boundary with the coesite inclusion, and they form a dislocation concentration zone with a dislocation density of ~109 cm?2. A high‐resolution TEM image and a fast Fourier transform‐filtered image reveal that a tiny dislocation in the dislocation concentration zone is composed of multiple edge dislocations. The estimated dislocation density in most of the kyanite away from the coesite inclusion–host kyanite grain boundaries is ~108 cm?2, being lower than that in kyanite adjacent to the coesite. In the case of a coesite inclusion in a matrix kyanite, using Raman and TEM analyses, we could not identify any quartz at the grain boundaries. Dislocations are not observed in the coesite, but numerous dislocations and stacking faults are developed in the kyanite. The estimated overall dislocation density in the coesite‐bearing matrix kyanite is ~108 cm?2, but a high dislocation density region of ~109 cm?2 is also present near the coesite inclusion–host kyanite grain boundaries. Inclusion and matrix kyanite grains with no coesite have dislocation densities of ≤108 cm?2. Dislocation density is generally reduced during an annealing process, but our results show that not all dislocations in the kyanite have recovered uniformly during exhumation of the UHP rocks. Hence, one of the key factors acting as a buffer to inhibit the coesite to quartz transformation is the mechanical interaction between the host and the inclusion that lead to the formation of dislocations in the kyanite. The kyanite acts as an excellent pressure container that can preserve coesite during the decompression of rocks from UHP conditions. The search for and study of inclusions in kyanite may be a more suitable approach for tracing the spatial distribution of UHP metamorphic rocks. 相似文献
The quantitative evaluation of the effects of bedrock groundwater discharge on spatial variability of stream dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) concentrations has still been insufficient. We examined the relationships between stream DOC, DIN and DIP concentrations and bedrock groundwater contribution to stream water in forest headwater catchments in warm-humid climate zones. We sampled stream water and bedrock springs at multiple points in September and December 2013 in a 5 km2 forest headwater catchment in Japan and sampled groundwater in soil layer in small hillslopes. We assumed that stream water consisted of four end members, groundwater in soil layer and three types of bedrock groundwater, and calculated the contributions of each end member to stream water from mineral-derived solute concentrations. DOC, DIN and DIP concentrations in stream water were compared with the calculated bedrock groundwater contribution. The bedrock groundwater contribution had significant negative linear correlation with stream DOC concentration, no significant correlation with stream DIN concentration, and significant positive linear correlation with stream DIP concentration. These results highlighted the importance of bedrock groundwater discharge in establishing stream DOC and DIP concentrations. In addition, stream DOC and DIP concentrations were higher and lower, respectively, than those expected from end member mixing of groundwater in soil layer and bedrock springs. Spatial heterogeneity of DOC and DIP concentrations in groundwater and/or in-stream DOC production and DIP uptake were the probable reasons for these discrepancies. Our results indicate that the relationships between spatial variability of stream DOC, DIN and DIP concentrations and bedrock groundwater contribution are useful for comparing the processes that affect stream DOC, DIN and DIP concentrations among catchments beyond the spatial heterogeneity of hydrological and biogeochemical processes within a catchment. 相似文献
Ganymede has an intrinsic magnetic field which is generally considered to originate from a self-excited dynamo in the metallic core. Driving of the dynamo depends critically on the satellite's thermal state and internal structure. However, the inferred structure based on gravity data alone has a large uncertainty, and this makes the possibility of dynamo activity unclear; variations in core size and composition significantly change the heat capacity and alter the cooling history of the core. The main objectives of this study is to explore the structural conditions for a currently active dynamo in Ganymede using numerical simulations of the thermal history, and to evaluate under which conditions Ganymede can maintain the dynamo activity at present. We have investigated the satellite's thermal history using various core sizes and compositions satisfying the mean density and moment of inertia of Ganymede, and evaluate the temperature and heat flux at the core-mantle boundary (CMB). Based on the following two conditions, we evaluate the possibility of dynamo activity, thereby reducing the uncertainty of the previously inferred interior structure. The first condition is that the temperature at the CMB must exceed the melting point of a metallic core, and the second is that the heat flux through the CMB must exceed the adiabatic temperature gradient. The mantle temperature starts to increase because of the decay of long-lived radiogenic elements in the rocky mantle. After a few Gyr, radiogenic elements are exhausted and temperature starts to decrease. As the rocky mantle cools, the heat flux at the CMB steadily increases. If the temperature and heat flux at the CMB satisfy these conditions simultaneously, we consider the case as capable of driving a dynamo. Finally, we identify the Dynamo Regime, which is the specific range of internal structures capable of driving the dynamo, based on the results of simulations with various structures. If Ganymede's self-sustained magnetic field were maintained by thermal convection, the satellite's metallic core would be relatively large and, in comparison to other terrestrial-type planetary cores, strongly enriched in sulfur. The dynamo activity and the generation of the magnetic field of Ganymede should start from a much later stage, possibly close to the present. 相似文献
