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1.
Transitions between the three typical paths of the Kuroshio south of Japan (the nearshore and offshore non-large-meander paths and the large-meander path) are described using sea level data at Miyake-jima and HachijÔ-jima in the Izu Islands and temperature data at a depth of 200 m observed from 1964 to 1975 and in 1980.In transitions between the nearshore and offshore non-large-meander paths the variation of the Kuroshio path occurs first in the region off Enshû-nada between the Kii Peninsula and the Izu Ridge and subsequently over the ridge. In the nearshore to offshore transition the offshore displacement of the path occurs first off Enshû-nada and then develops southeastwardly in the direction of HachijÔ-jima. In the reverse transition shoreward displacement occurs first off Enshû-nada and then throughout the region west and east of the Izu Ridge. The position of the Kuroshio south of Cape Shiono-misaki (the southernmost tip of the Kii Peninsula) is almost fixed near the coast throughout these transition periods, and significant variations of the Kuroshio path only occur east of the cape. The nearshore to offshore and offshore to nearshore transitions can be estimated to take about 25 and 35 days, respectively, during which the variation of the Kuroshio path over the Izu Ridge occurs for the last 11 and 25 days.The transitions between the non-large-meander and large-meander paths show that the large-meander path is mostly formed from the nearshore non-large-meander path and always changes to the offshore non-large-meander path.  相似文献   

2.
The topographic effect of the Izu Ridge on the horizontal distribution of the North Pacific Intermediate Water (NPIW) south of Japan has been studied using observational data obtained by the Seisui-Maru of Mie University (Mie Univ. data) and those compiled by Japan Oceanographic Data Center (JODC data). Both data sets show that water of salinity less than 34.1 psu on potential density () surface of 26.8 is confined to the eastern side of the Izu Ridge, while water of salinity less than 34.2 psu is confined to the southern area over the Izu Ridge at a depth greater than 2000 m and to the southeastern area in the Shikoku Basin. It is also shown by T-S analysis of Mie Univ. data over the Izu Ridge that water of salinity less than 34.2 psu dominates south of 30°N, where the depth of the Izu Ridge is deeper than 2000 m and NPIW can intrude westward over the Izu Ridge. JODC data reveal that relatively large standard deviations of the salinity on surface of 26.7, 26.8 and 26.9 are detected along the mean current path of the Kuroshio and the Kuroshio Extension. Almost all of the standard deviations are less than 0.05 psu in other area with the NPIW, which shows that the time variation in the salinity can be neglected. This observational evidence shows that the topographic effect of the Izu Ridge on the horizontal distribution of the NPIW, which is formed east of 145°E by the mixing of the Kuroshio water and the Oyashio water, is prominent north of 30°N with a depth shallower than 2000 m.  相似文献   

3.
Current-generated bedforms were found on sandy seafloor at water depths of 200–400 m on the northern Izu Ridge, where the Kuroshio Current encounters and passes over the ridge. The observed bedforms include large dunes and sand ribbons and are interpreted to be products of present-day oceanographic conditions and to indicate intensive flow activity controlled by local topography. A comparison between the surface flow velocity estimated from empirical relationships for dune formation and the observed flow velocity suggests that the dunes are generated when the main axis of the meandering Kuroshio Current passes through this area, and that subsequent current velocities are sufficiently high to maintain the dunes up to the next event.  相似文献   

4.
Differences in daily mean sea level between Kushimoto and Uragami and daily mean sea levels at Miyake-jima and HachijÔ-jima in the Izu Islands are examined during the 1964–1980 period, and characteristics of the typical paths of the Kuroshio corresponding to the dominant sea level states are described.Sea levels at the islands show three dominant states: high and low sea level states (45 % and 31 %) in the non-large-meander period (October 1963 –7 August 1975) and high sea level state (64 %) in the large-meander period (8 August 1975–15 March 1980). This indicates the existence of three typical paths of the Kuroshio, and the states correspond to the nearshore and offshore non-large-meander paths and the typical large-meander path, respectively. The first path is located near the coast throughout the whole southern area off Japan, the second path leaves the coast around the Izu Ridge and passes south of HachijÔ-jima, and the third path is located near the coast over the ridge after meandering far to the south of Enshû-nada.The positions of the three typical paths are almost the same in the farthest upstream and downstream regions south of Japan between 131E and 142E. The nearshore and offshore non-large-meander paths overlap between Kyûshû and the Kii Peninsula, being invariably close to the coast, while the typical large-meander path south of Shikoku is located offshore and changes its position meridionally.At the mid-depth of 400 m the nearshore non-large-meander and typical large-meander paths pass the Izu Ridge through the deep channel between Miyake-jima and HachijÔ-jima, while the offshore non-large-meander path passes through the deep region south of HachijÔjima. The path of the Kuroshio at mid-depth is well constrained by the bottom topography of the Izu Ridge.  相似文献   

5.
The influence of the Kuroshio flow on the horizontal distribution of North Pacific Intermediate Water (NPIW) in the Shikoku Basin is examined based upon observational data collected by the training vessel “Seisui-maru” of Mie University together with oceanographic data compiled by the Japan Oceanographic Data Center (JODC). Although it has been stated that the NPIW with salinity less than 34.2 psu had been confined to the south of the Kuroshio main axis along the PT (KJ) Line on the eastern side of the Izu Ridge, a similar tendency can be detected on the western side of the Izu Ridge. Namely, the NPIW on the southern side of the Kuroshio main axis in the Shihoku Basin does not indicate a tendency to go northward across the Kuroshio main axis without an increase in salinity of more than 34.2 psu. However, the JODC data show that less saline water (<34.2 psu) was present on the northern side of the Kuroshio main axis south of the Kii Peninsula in May 1992. Satellite observed sea surface temperature (SST) data suggested that the Kuroshio approaches the Kii Peninsula after forming a small meander off Kyushu and some intrusions of the NPIW into the northern coastal side of the Kuroshio main axis occurred in this period. It is concluded that intrusion of the NPIW with salinity less than 34.2 psu to the northern coastal side through the Kuroshio main axis occurred during the decay period of the small meander path in May 1992. Based on these observational results, the source of the salinity minimum water on the northern coastal side of the Kuroshio main axis is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

6.
By tracking the locally strongest part of the sea-surface velocity field, which was obtained by integrating data of satellite altimeters and surface drifting buoys, we extracted the Kuroshio axis south of Japan every 10 days from October 1992 to December 2000. The obtained axes clearly express the effect of the bottom topography; three modes were observed when the Kuroshio ran over the Izu Ridge. The axis was very stable to the south of ‘Tosa-bae,’ off the Kii Channel. Mean current speed at the Kuroshio axis gradually increased from 0.65 m/s south of Kyushu to 1.45 m/s off Enshu-nada. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

7.
Since the Intermediate Oyashio Water (IOW) gradually accumulates in Sagami Bay, it can reasonably be supposed that the IOW also flows out from Sagami Bay, even though it may be altered by mixing with other waters. We have occasionally observed a water less than 34.2 psu with a potential density of 26.8 at the southeastern area off Izu Peninsula in July 1993 by the training vessel Seisui-maru of Mie University. Observational data supplied by the Japan Meteorological Agency and the Kanagawa Prefectural Fisheries Experimental Station show that the IOW of less than 34.1 psu was observed at northern stations of the line PT (KJ) off the Boso Peninsula and to the east of Oshima in the late spring 1993. Based upon these observations, it is concluded that the IOW flows out from Sagami Bay into the Shikoku Basin along southeastern area off the Izu Peninsula. The less saline water (<34.2 psu) was also observed to the west of Miyake-jima during the same cruise, and the westward intrusion of IOW from south of the Boso Peninsula to the Shikoku Basin through the gate area of the Kuroshio path over the Izu Ridge was detected. This event indicated that the IOW branched south of the Boso Peninsula and flowed into Sagami Bay and/or into the gate area over the Izu Ridge. The southward intrusion of IOW into the south of the Boso Peninsula is discussed in relation to the latitudinal location of the main axes of the Kuroshio and the Oyashio. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

8.
Direct current velocity measurements in the countercurrent of the Kuroshio, south of Japan, were carried out to investigate the influence of short-period fluctuations in the small-scale meander of the Kuroshio on its countercurrent. When the Kuroshio took a path having a meander west of the Izu Ridge and approaching the Izu Peninsula, the countercurrent freely intruded into coastal seas with a period of 17 d and a phase velocity almost equal to that of the Kuroshio itself. However, when the Kuroshio did not significantly bend and deflect off the Izu Peninsula, even when taking the same path, the velocity of the countercurrent was considerably reduced and the periodic fluctuations propagated into the coastal seas as a continental shelf wave. The results indicate that a small change in the Kuroshio's path can cause a different process of propagation of the small-scale meandering; this difference probably explains why there are two kinds of phase velocity in coastal temperature fluctuations.  相似文献   

9.
日本南部黑潮存在多种路径模态:近岸非大弯曲路径、离岸非大弯曲路径和大弯曲路径。黑潮延伸体的路径存在两种典型模态:收缩态和伸展态。从地理位置看, 日本南部黑潮和黑潮延伸体是相邻的, 但它们的路径状态是否存在关联一直存在争议。本文基于卫星观测的海表高度资料和长期的海洋高分辨率再分析资料, 对日本南部黑潮和黑潮延伸体路径状态之间的关联性进行定量分析, 结果表明:日本南部黑潮和黑潮延伸体的路径状态存在一定的关联。当日本南部黑潮处于近岸非大弯曲和大弯曲路径时, 黑潮延伸体主要处于伸展态; 当日本南部黑潮处于离岸非大弯曲时, 黑潮延伸体处于伸展态和收缩态的比例相当。进一步分析表明, 黑潮流轴处于伊豆海脊的位置部分决定了上述关系, 可能存在其他因素调制了两者的关联性。  相似文献   

10.
Index and Composites of the Kuroshio Meander South of Japan   总被引：1，自引：0，他引：1  
Using the merged NOAA National Oceanographic Data Center (NODC)/Japan Oceanographic Data Center (JODC)/Marine Information Research Center (MIRC) historical hydrographic dataset, a new Kuroshio large meander (LM) index is introduced. This index helps to distinguish between the LM events and other types of Kuroshio Current (KC) variability south of Japan. Observations, re-systematized according to the index, provide composite patterns of typical formation and decay of the LM. The patterns reveal a remarkable similarity between individual LMs and support the deterministic rather than the stochastic model of LM evolution on a time-scale of one year. A “trigger” meander (TM) occurs on composite maps six months prior to the LM formation as a 1° latitude southward shift of the KC axis south of Kyushu. When propagating eastward along the coast of Japan, TM gradually increases in area. In principal the emergence of LM takes only one month. East of TM and LM a remarkable onshore shift of the KC is noticed, supplying the coastal region with warm water. Other warm anomalies are found on the warm side of KC next to the propagating TM and in the larger warm eddy area southeast of Kyushu. Different LMs survive for different times and decay in some months after KC “jumps” across the Izu Ridge. Changes of water properties on isopycnals in the interior of LM can be roughly described by two-layer kinematics with an interface at σθ = 27 which suggests a strong inflow of deep Kuroshio waters into the LM core during the formation of the latter. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

11.
The volume transport of the Kuroshio, the western boundary current of the North Pacific subtropical gyre, varies vigorously due to merging of disturbances propagating from the entire North Pacific. Taking into account the recirculation in the Shikoku Basin by the zonal observation line at 30°N to the west of the Izu–Ogasawara Ridge, we estimated the volume transport in the top 1,000 m layer toward the Kuroshio Extension region. The volume transport of the local recirculation gyre in the Shikoku Basin increases associated with the westward extension of the gyre, particularly in the period of the large meandering path of the Kuroshio south of Japan. Meanwhile, most of the transport variations toward the Kuroshio Extension region correspond to those of the Kuroshio transport on the continental slope south of Japan, which vary independently of those of the recirculation gyre.  相似文献   

12.
The topographic effect of a meridional marine ridge on the spin-down of a cyclonic eddy is discussed by use of a two layer model. It is shown that cyclonic eddies on the eastern side of a marine ridge decay in a shorter time in comparison with those on the western side. The fast decay on the eastern side is mainly due to the planetary effect, which carries the eddy westward to a shallower region on the ridge, where remarkable frictional spin-down is carried out. Conversely, westward shift of the similar cyclonic eddy yields a weak bottom effect in the western side. It is inferred that the resulting remarkable spin-down on the eastern side of a marine ridge is one of the causes of the observed fast decay of the cyclonic cold water mass accompanied by the large meander of the Kuroshio on the eastern side of the Izu Ridge south of Japan.  相似文献   

13.
Conditions for the formation of large meander (LM) of the Kuroshio are inferred from observational data, mainly obtained in the 1990s. Propagation of the small meander of the Kuroshio from south of Kyushu to Cape Shiono-misaki is a prerequisite for LM formation, and three more conditions must be satisfied. (1) The cold eddy carried by small meander interacts with the cold eddy in Enshu-nada east of the cape. During and just after the propagation of small meander, (2) the Kuroshio axis in the Tokara Strait maintains the northern position and small curvature, and (3) current velocity of the Kuroshio is not quite small. If the first condition is not satisfied, the Kuroshio path changes little. If the first condition is satisfied, but the second or third one is not, the Kuroshio transforms to the offshore non-large-meander path, not the LM path. All three conditions must be satisfied to form the large meander. For continuance of the large meander, the Kuroshio must maintain the small curvature of current axis in the Tokara Strait and a medium or large range of velocity and transport. These conditions for formation and continuance may be necessary for the large meander to occur. Moreover, effects of bottom topography on position and structure of the Kuroshio are described. Due to topography, the Kuroshio changes horizontal curvature and vertical inclination of current axis in the Tokara Strait, and is confined into either of two passages over the Izu Ridge at mid-depth. The former contributes to the second condition for the LM formation.  相似文献   

14.
In the previous paper (Toba and Murakami, 1998) we reported on an unusual path of the Kuroshio Current System, which occurred in April 1997 (April 1997 event), using the Ocean Color and Temperature Scanner (OCTS) data of the Advanced Earth Observing Satellite (ADEOS). The April 1997 event was characterized by the flow of the Kuroshio along the western slope (northward) and the eastern slope (southward) of the Izu-Ogasawara Ridge, a very southerly turning point at about 32°N, followed by a straight northward path up to 37°N of the Kuroshio Extension along the eastern flank of the Izu-Ogasawara and the Japan Trenches. Overlaying of depth contours on ADEOS-OCTS chlorophyll-a images at the April 1997 event demonstrates the bottom topography effects on the current paths. A new finding based on TOPEX/Poseidon altimeter data is that the sea-surface gradient across the Kuroshio/Kuroshio Extension diminished greatly in the sea area southeast of the central Japan, as a very temporary phenomenon prior to this event. This temporary diminishing of the upper-ocean current velocity might have caused a stronger bottom effect along the Izu-Ogasawara Ridge, and over the Izu-Ogasawara Trench disclosed a weak background, barotropic trench-flank current pattern, which existed otherwise independently of the Kuroshio Extension. The very southerly path of the Kuroshio Extension from winter 1996 to autumn 1998 corresponded, with a time lag of about 1.5 years, to the previous La Niña tendency with weaker North Equatorial Current. The April 1997 event occurred in accordance with its extreme condition.  相似文献   

15.
Flow patterns and positions of the Kuroshio in the vicinity of the Izu Ridge are clarified by analyzing hydrographic observations and daily mean sea levels at Hachijo-jima, Miyake-jima and Oshima in the period from 1964 to 1981.Correlations are calculated between differences of dynamic depth anomaly at the surface refered to 1,000 db and differences of daily mean sea level between the two islands. The datum line of the tidal station at Hachijo-jima is about 90 cm higher than that at Miyake-jima, and about 20 cm higher than that at Oshima. A clear correlation is found between the cross-section transport of the Kuroshio and the mean sea-level difference between Hachijo-jima and Miyake-jima. The sea-level difference of the flow pattern without meander off Enshu-nada (type N) tends to be larger than that of the flow patterns with meander (type A and type B). This seems to indicate that the volume transport of the Kuroshio in the meandering period is smaller than that during the straight path period. Large sudden increases or decreases in the mean sea-level difference occur when the flow pattern changes and the Kuroshio axis shifts. The frequency of occurrence of quiet periods in the sea-level difference reflect the flow pattern of the Kuroshio.  相似文献   

16.
The morphological characteristics of the segmentation of the Central Indian Ridge (CIR) from the Indian Ocean Triple Junction (25°30S) to the Egeria Transform Fault system (20°30S) are analyzed. The compilation of Sea Beam data from R/VSonne cruises SO43 and SO52, and R/VCharcot cruises Rodriguez 1 and 2 provides an almost continuous bathymetric coverage of a 450-km-long section of the ridge axis. The bathymetric data are combined with a GLORIA side-scan sonar swath to visualize the fabric of the ridge and complement the coverage in some areas. This section of the CIR has a full spreading rate of about 50 mm yr–1, increasing slightly from north to south. The morphology of the CIR is generally similar to that of a slow-spreading center, despite an intermediate spreading rate at these latitudes. The axis is marked by an axial valley 5–35 km wide and 500–1800 m deep, sometimes exhibiting a 100–600 m-high neovolcanic ridge. It is offset by only one 40km offset transform fault (at 22°40S), and by nine second-order discontinuities, with offsets varying from 4 to 21 km, separating segments 28 to 85 km long. The bathymetry analysis and an empirical orthogonal function analysis performed on across-axis profiles reveal morphologic variations in the axis and the second-order discontinuities. The ridge axis deepens and the relief across the axial valley increases from north to south. The discontinuities observed south of 22°S all have morphologies similar to those of the slow-spreading Mid-Atlantic Ridge. North of 22°S, two discontinuities have map geometries that have not been observed previously on slow-spreading ridges. The axial valleys overlap, and their tips curve toward the adjacent segment. The overlap distance is 2 to 4 times greater than the offset. Based on these characteristics, these discontinuities resemble overlapping spreading centers (OSCs) described on the fast-spreading EPR. The evolution of one such discontinuity appears to decapitate a nearby segment, as observed for the evolution of some OSCs on the EPR. These morphological variations of the CIR axis may be explained by an increase in the crustal thickness in the north of the study area relative to the Triple Junction area. Variations in crustal thickness could be related to a broad bathymetric anomaly centered at 19°S, 65°E, which probably reflects the effect of the nearby Réunion hotspot, or an anomaly in the composition of the mantle beneath the ridge near 19°S. Other explanations for the morphological variations include the termination of the CIR at the Rodriguez Triple Junction or the kinematic evolution of the triple junction and its resultant lengthening of the CIR. These latter effects are more likely to account for the axial morphology near the Triple Junction than for the long-wavelength morphological variation.  相似文献   

17.
Variations of current velocity of the Kuroshio are examined using the 1965–1983 sea-level difference between Naze and Nishinoomote, located on the offshore and onshore sides of the Kuroshio in the Tokara Strait south of Kyûshû.Interannual variations of Kuroshio velocity are large, especially at periods longer than five years and around 2.1 years. They are almost determined by those of sea level on the offshore side of the Kuroshio. They are highly coherent with the offshore sea level at periods longer than 1.7 years, and incoherent with the onshore sea level at periods longer than 2.8 years.The mean seasonal variation averaged for 19 years is at its maximum in July and at its minimum in the second half of October, with a sharp decrease in August and September. However, such a variation does not repeat every year. Amplitude, dominant period and phase are greatly different by year, and they can be roughly divided into four groups: small-amplitude group, semiannual-period group, and two annual-period groups with different phases. The only feature found in almost all years is a weak velocity from September to December.The amplitude of seasonal variation tends to be large in the formation years of the large meander (LM) of the Kuroshio and small during the LM period. It is also large in the years preceding El Niño, and diminishes remarkably in El Niño years.Kuroshio velocity in the Tokara Strait is incoherent with position of the Kuroshio axis over the Izu Ridge, but highly coherent with 70-day variations of coastal sea levels which are dominant during the LM period.  相似文献   

18.
A numerical experiment is made with a three-dimensional source-sink flow model in order to study effects of a marine ridge to the western boundary current. The model represents the Kuroshio current crossing the Izu Ridge which has a submarine narrow passage. It is indicated that most of the surface water of the western boundary current crosses the ridge through the narrow passage. A part of the surface water circulates around a warm eddy formed in an upstream region of the ridge. A water below the thermocline makes a long detour around the ridge and forms a subsurface northward current along the eastern slope of the ridge.  相似文献   

19.
The horizontal components from twenty Ocean Bottom Seismometers deployed along three profiles near the Kolbeinsey Ridge, North Atlantic, have been modelled with regard to S-waves, based on P-wave models obtained earlier. Two profiles were acquired parallel to the ridge, and the third profile extended eastwards across the continental Jan Mayen Basin. The modelling requires a thin (few 100 m) layer with very high V p/V s-ratio (3.5–9.5) at the sea-floor in the area lacking sedimentary cover. The obtained V p/V s-ratios for the remaining part of layer 2A, 2B, 3 and upper mantle, correspond to the following lithologies: pillow lavas, sheeted dykes, gabbro and peridotite, respectively. All crustal layers exhibit a decreasing trend in V p/V s-ratio away-from-the-axis, interpreted as decreasing porosity and/or crack density in that direction. A significant S-wave azimuthal anisotropy is observed within the thin uppermost layer of basalt near the ridge. The anisotropy is interpreted as being caused by fluid-filled microcracks aligned along the direction of present-day maximum compressive stress, and indicates crustal extension at the ridge itself and perpendicular-to-the-ridge compression 12 km off axis. Spreading along the Kolbeinsey Ridge has most likely been continuous since its initiation ca. 25 Ma: The data do not suggest the presence of an extinct spreading axis between the Kolbeinsey Ridge and the Aegir Ridge as has been proposed earlier. The V p/V s-ratios found in the Jan Mayen Basin are compatible with continental crust, overlain by a sedimentary section dominated by shale.  相似文献   

20.
Investigated is a possibility of two-dimensional model in the study of the dynamics of the western boundary current by a numerical experiment. Emphasis is laid on the effect of bottom barrier corresponding to the Izu Ridge.The western boundary current in the model is formed by source and sink of the water prescribed at an artificial eastern wall (600 km offshore). The bottom topographyconsists of a continental slope parallel to the straight western coast, and a ridge protruding from the western coast to 500 km offshore (1,500 m deep and 400 km wide). The grid size of 12 km× 25 km (offshore and longshore directions, respectively) resolves both the western boundary current and the bottom topography.The assumption of homogeneity of the water density makes the western boundary current detour along the isobath of the ridge.A steady state solution is obtained under the assumptions that the horizontal velocity does not change direction vertically (equivalent barotropic), and that the geostrophic relationship holds at the bottom. Homogeneity of the water density is not assumed. The solution shows that most of the volume transport of the western boundary current cross the ridge and the current has cyclonic vorticity near the summit of the ridge. It seems to suggest that the investigation by three-dimensional models is neccesary in order to study the complete dynamics of the western boundary current crossing the ridge.  相似文献   

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