Transition processes between the three typical paths of the Kuroshio

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.     

Sea level changes south of Japan associated with the non-large-meander path of the Kuroshio

Sea levels south of Japan from 1964 to 1975 are examined in terms of the nearshore and offshore non-large-meander (NLM) paths of the Kuroshio and the transitions between them.The sea-level anomalies from the annual variations on the south coast of Japan are much larger during the transition from the nearshore to offshore NLM paths than during the reverse transition by 9 cm on average. This characteristic can be seen only in the coastal region of the Kuroshio-flowing area, so that the sea-level difference of Naze minus Nishinoomote (indicator of Kuroshio velocity) during the offshore to nearshore transition is larger by 15 cm than during the reverse transition.The transition from the offshore to nearshore NLM paths occurs when the velocity of the Kuroshio is large or increasing, while the nearshore to offshore transition occurs when it is small or decreasing. The former transition occurs whenever the velocity increases greatly, whereas the latter one does not always occur even though the velocity decreases.The sea-level difference between Kushimoto and Uragami is highly coherent with the alternate appearance of the nearshore and offshore NLM paths. Offshore NLM paths longer than 2.5 months appear during large falls of the sea-level difference of Kushimoto minus Uragami, while large rises of the sea-level difference correspond to long-lasting nearshore NLM paths. The mean sea-level difference during the nearshore NLM path is larger by 4 cm than that during the offshore NLM path.     

Variations of the Kuroshio in the Southern Region of Japan: Conditions for Large Meander of the Kuroshio

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.     

日本南部黑潮与黑潮延伸体路径状态关联性的定量分析

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。     

Sea level variations at the Izu Islands and typical stable paths of the Kuroshio

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.     

Numerical Study of the Generation and Propagation of Trigger Meanders of the Kuroshio South of Japan

We examine the processes underlying the generation and propagation of the small meander of the Kuroshio south of Japan which occurs prior to the transition from the non-large meander path to the large meander path. The study proceeds numerically by using a two-layer, flat-bottom, quasi-geostrophic inflow-outflow model which takes account of the coastal geometries of Kyushu, Nansei Islands, part of the East China Sea, and the Izu Ridge. The model successfully reproduces the observed generation and propagation features of what is called "trigger meander" until it passes by Cape Shiono-misaki; presumably because of the absence of the bottom topography, the applicability of the present numerical model becomes questionable after the trigger meander passes by Cape Shiono-misaki. The generation of the trigger meander off the south-eastern coast of Kyushu is shown to be associated with the increase in the supply of cyclonic vorticity by the enhanced current velocity in the upper layer along the southern coast of Kyushu where the no-slip boundary condition is employed. Thereafter, the trigger meander propagates eastward while inducing an anticyclone-cyclone-cyclone pair in the lower layer. The lower-layer cyclone induced in this way, in particular, plays a crucial role in intensifying the trigger meander trough via cross-stream advection in the upper layer; the intensified trigger meander trough then further amplifies the lower-layer cyclone. This joint evolution of the upper-layer meander trough and the lower-layer cyclone indicates that baroclinic instability is the dominant mechanism underlying the rapid amplification of the eastward propagating trigger meander.     

Water and flow variations in Sagami Bay under the influence of the Kuroshio path

Variations of water and flow in Sagami Bay in relation to the Kuroshio path variations are examined by using 100m-depth temperature and salinity data from 25 stations as well as sea level data from five stations (Minami-Izu, ItÔ, Ôshima, Aburatsubo, Mera). In regard to temperature, anomalies from the mean seasonal variations are used. Results show that water properties are clearly different between the three typical paths of the Kuroshio. The difference is more remarkable in temperature than in salinity; temperature is higher during the typical large-eander (LM) path, and lower during the offshore non-large-meander (NLM) path, compared with the nearshore NLM path. Temperature anomaly and salinity distributions, as well as the Ôshima minus Minami-Izu and Ôshima minus Mera sea-level differences strongly suggest that the flows during the typical LM path are distributed as hitherto described in past studies, that is, water in the mouth region of the bay flows clockwise around Ôshima from the west channel to the east channel, and a counterclockwise eddy exists in the interior. On the other hand, flows during the nearshore and offshore NLM paths seem to be quite different from those during the typical LM path; velocities are very weak, and the directions of circulation is frequently reversed. This tendency also can be seen during parts of LM period in which the Kuroshio takes a non-typical LM path.Water properties in Sagami Bay are most characteristic during transitions between nearshore and offshore NLM paths. During transitions from nearshore to offshore NLM paths, temperatures are extremely high as a whole in the bay, while during reverse transitions, both temperatures and salinities are very low in the entire region.     

Study on the Kuroshio and the Tsushima Current

I summarize the variations of the path of the Kuroshio and of the Tsushima Current mainly based on the results of my studies. The Tsushima Current forms three branches just after it enters the Japan Sea through the Tsushima Strait. The first and third branch currents flow along the Japanese and Korean coasts, respectively, and the second branch current flows from the western channel of the Tsushima Strait to the west of the Oki Islands only in summer from June to August. Properties of the topographic waves which are thought to work on the formation of the second branch are described mainly in terms of the dispersion relations. The Kuroshio has three typical paths,i.e., the nearshore and offshore non-large-meander paths and the typical large-meander path. The Kuroshio alternately takes the nearshore and offshore paths in the non-large-meander period, occasionally changes from the nearshore nonlarge-meander path to the large-meander path and, after having taken the large-meander path for several years, changes to the offshore non-large-meander path. Sea levels south of Japan are clearly different between the non-large-meander and large-meander periods, while they are not different between the periods of the nearshore and offshore non-large-meander paths. But, sea level and water properties in the coastal region show remarkable features during short periods of transitions between the typical non-large-meander paths. Future problems and subjects of studies on these currents are indicated. Especially, importance of velocity monitoring of the Kuroshio is emphasized, and a design of the observation across the Tokara Strait is proposed.     

Two processes by which short-period fluctuations in the meander of the Kuroshio affect its countercurrent

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.     

Spectral properties of sea level and time scales of Kuroshio path variations

Spectral properties of sea levels at Naze, Nishinoomote, Kushimoto, Uragami, Miyake-jima and HachijÔ-jima are examined for the non-large-meander (February 1964 – May 1975) and large-meander (October 1975 – December 1979) periods, and the periodicity of variation of the Kuroshio path is clarified.The large meander of the Kuroshio occurs with a primary period of about 20 years and secondary period of 7 to 8. 5 years. During the non-large-meander period, the Kuroshio alternately takes the nearshore and offshore non-large-meander paths with a primary period of 1. 6–1. 8 years. This variation is moreover composed of 110-day, around 195-day and annual periods. The 110-day variation of the Kuroshio path appears to have influence on the coastal sea levels between the Kii Peninsula and the Izu Ridge;i. e., the coastal sea levels rise and fall with one-month time lag after the Kuroshio has begun to approach and leave the Japanese coast. During the large-meander period, the 70 and 110-day variations are remarkable in sea levels south of Japan except Miyake-jima and HachijÔ-jima. The 70-day variation is highly coherent throughout the south coast of Japan; the coherent area of the 110-day variation seems to be smaller.The sea-level variations at Naze and Nishinoomote are not significantly coherent for any of the periods except for annual and semiannual cycles during both the non-large-meander and large-meander periods. That is, the sea-level variations are incoherent between the onshore and offshore sides of the Kuroshio, except for seasonal variation.     

Current Nature of the Kuroshio in the Vicinity of the Kii Peninsula

The Kuroshio flows very close to Cape Shionomisaki when it takes a straight path. The detailed observations of the Kuroshio were made both on board the R/V Seisui-maru of Mie University and on board the R/V Wakayama of the Wakayama Prefectural Fisheries Experimental Station on June 11–14, 1996. It was confirmed that the current zone of the Kuroshio touches the coast and bottom slope just off Cape Shionomiaki, and that the coastal water to the east of the cape was completely separated from that to the west. The relatively high sea level difference between Kushimoto and Uragami could be caused by this separation of the coastal waters when the Kuroshio takes a straight path. This flow is rather curious, as the geostrophic flow, which has a barotropic nature and touches the bottom, would be constrained to follow bottom contours due to the vorticity conservation law. The reason why the Kuroshio leaves the bottom slope to the east of Cape Shionomisaki is attributed to the high curvature of the bottom contours there: if the current were to follow the contours, the centrifugal term in the equation of motion would become large and comparablee to the Coriolis (or pressure gradient) term, and the geostrophic balance would be destroyed. This creates a current-shadow zone just to the east of the cape. As the reason why the current zone of the Kuroshio intrudes into the coastal region to the west of the cape, it is suggested that the Kii Bifurcation Current off the southwest coast of the Kii Peninsula, which is usually found when the Kuroshio takes the straight path, has the effect of drawing the Kuroshio water into the coastal region. The sea level difference between Kushimoto and Uragami is often used to monitor the flow pattern of the Kuroshio near the Kii Peninsula. It should be noted that Uragami is located in the current shadow zone, while Kushimoto lies in the region where the offshore Kuroshio water intrudes into the coastal region. The resulting large sea level difference indicates that the Kuroshio is flowing along the straight path.     

Abyssal current along the northern periphery of Shikoku Basin

Abyssal currents along the northern periphery of the Shikoku Basin south of Japan were measured by current meters moored off Cape Daio-zaki, Cape Shiono-misaki and Cape Ashizuri-misaki and on the eastern foot of the northernmost part of the Kyushu-Palau Ridge. Total length of observation off Cape Shiono-misaki was about five years including the periods of the Kuroshio large meander and no meander. Analyses of current data show:

1. Mean currents with a magnitude of 5–10 cm sec^?1 were observed during the whole observation period at all of current meters which were set 400 m above the sea bottom that was deeper than 4,500 m. The mean current for each current meter was parallel to the local bottom contour arond each station and was toward a direction looking the Nankai Trough (a trough located along the northern end of the Shikoku Basin) to the left.
2. At each station located above the shelf toe off Cape Daio-zaki and off Cape Shiono-misaki and on the foot of the Kyushu-Palau Ridge, the mean current increases with depth (a bottomward intensification of the mean current), and the vertical extent of the mean current is estimated to be about 2,000 m above the sea bottom.
3. At a station located at 2,600 m depth on the continental slope off Cape Shiono-misaki, no bottom-ward intensification of currents was observed.

These results strongly suggest that a steady abyssal flow exists in the depths deeper than about 3,000 m along the northern and northwestern peripheries of the Shikoku Basin. Existence of the abyssal circulation system is also suggested, at least, in the north of the Shikoku Basin.     

POM模式在日本南部黑潮路径变异研究中的应用

日本南部黑潮路径变异对北太平洋地区的气候和环境具有显著的影响,对黑潮路径变异的研究具有重要的意义。本文利用POM(Princeton Ocean Model)数值模式模拟了日本南部黑潮的路径变异情况,分析了黑潮大弯曲路径形成的可能机制。研究结果表明,当黑潮处于非大弯曲路径时,相对位势涡度的平均值呈现递减趋势,说明日本南部低位势涡度水在不断积累,这样会使得四国再循环流的强度增强,迫使黑潮保持平直路径,同时,近岸黑潮垂直流速剪切增大,斜压不稳定性的作用也逐渐增大;当黑潮从非大弯曲路径向大弯曲路径过渡时,再循环流强度的减弱会导致黑潮的流速剪切减小。根据海表高度异常场以及海洋上层流场信息发现,近岸黑潮附近的气旋涡会随着再循环流区域反气旋涡的东侧向南运动,最终导致黑潮大弯曲的发生。分析涡流的能量,结果显示,黑潮大弯曲路径的形成与斜压不稳定性密切相关。     

Intraannual variability of sea level around Tosa Bay

Through analysis of monthly in situ hydrographic, tide gauge, altimetry and Kuroshio axis data for the years 1993–2001, the intraannual variability of sea level around Tosa Bay, Japan, with periods of 2–12 months is examined together with the intraannual variability of the Kuroshio south of the bay. It is shown that the intraannual variation of steric height on the slope in Tosa Bay can account for that of sea level at the coast around the bay as well as on this slope. It is found that the steric height (or sea level) variation on the slope in this bay is mainly controlled by the subsurface thermal variation correlated with the Kuroshio variation off Cape Ashizuri, the western edge of Tosa Bay. That is, when the nearshore Kuroshio velocity south of the cape is intensified [weakened] concurrently with the northward [southward] displacement of the current axis, temperature in an entire water column decreases [increases] simultaneously, mainly due to the upward [downward] displacement of isotherms, coincident with that of the main thermocline. It follows that the steric height (or sea level) decreases [increases].     

Relationship between the path of the Kuroshio in the south of Japan and the path of the Kuroshio Extension in the east

A relationship between paths of the Kuroshio and Kuroshio Extension (KE) is investigated, using the satellite-derived altimetry dataset of 1993–2008. When the Kuroshio takes the nearshore nonlarge meander path or typical large meander path and resultantly goes through the deeper channel (about 2500 m) of the Izu-Ogasawara Ridge, the KE path adopts a relatively stable state with the two quasi-stationary meanders. On the other hand, when the Kuroshio takes the offshore nonlarge meander path and then passes over the shallower part of the Ridge (about 1000 m), the KE path tends to be convoluted, i.e., an unstable state.     

Topographic Effect of Izu Ridge on the Distribution of the North Pacific Intermediate Water South of Japan

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.     

Path and volume transport of the Kuroshio current in Sagami Bay and their relationship to cold water masses near Izu Peninsula

The path of the Kuroshio in Sagami Bay was surveyed through drifter tracking from Oshima-West Channel to Oshima-East Channel. A subsurface drifter with a drogue at 300 m depth flowed around Oshima from Oshima-West Channel to Oshima-East Channel. A difference in flow directions between the upper and lower layers was apparent in the northwest of Oshima. Flow directions there were shown to change from north in the surface layer to east in the bottom layer, and this was confirmed with moored currentmeters.A profile of northward current velocity was estimated from measurements in six layers with currentmeters deployed in the Oshima-West Channel. The profile shows a core of northward flow along the eastern bottom slope and a weak southward flow along the western bottom slope. Volume transport of the Kuroshio into Sagami Bay was estimated to be 1.8×10⁶m³sec^–1 from the profile.Long-term current measurement showed that southward flows were observed in Oshima-West Channel in July 1977, May 1978 and April 1979. Cold or warm water masses appearing south of the Izu Peninsula are suggested to have caused the changes.Displacement of the cold water mass in July 1977 is discussed on the basis of current measurements and offshore oceanographic conditions.     

Temporal-spatial oceanic variation in relation with the three typical Kuroshio paths south of Japan

Empirical orthogonal function(EOF) analysis was applied to a 50-year long time series of monthly mean positions of the Kuroshio path south of Japan from a regional reanalysis. Three leading EOF modes characterize the contributions from three typical paths of the Kuroshio meander: the typical large meander path, the offshore nonlarge meander path, and the nearshore non-large meander path, respectively. Accordingly, the spatial variation characteristics of oceanic anomaly fields can be depicted by...     

Variability of Kuroshio velocity assessed from the sea-level difference between Naze and Nishinoomote

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.     

Monitoring of generation and propagation of the Kuroshio small meander using sea level data along the southern coast of Japan

The occurrence of the small meander of the Kuroshio, generated south of Kyushu and propagating eastward, was examined using sea level data collected during 1961–1995 along the south coast of Japan. Intra-annual variation of the sea level was expanded by the frequency domain empirical orthogonal function (FDEOF) modes, and it was found that the second and third modes are useful for monitoring the generation and propagation of the small meander. The third FDEOF for periods of 10–100 days has a phase reversal between Hosojima and Tosa-shimizu with significant amplitude west of Kushimoto, and the amplitude of its time coefficient is large during the non-large-meander (NLM) period and has a significant peak when the small meander exists southeast of Kyushu. The second FDEOF for periods of 20–80 days has a phase reversal between Kushimoto and Uragami, and the amplitude of its time coefficient is large when the small meander propagates to the south of Shikoku. The third FDEOF mode allowed us to conclude that the small meander occurred 42 times from July 1961 to May 1995, most of them (38) occurring during the NLM periods. The second FDEOF mode permits the conclusion that half of the 38 small meanders reached south of Shikoku. Of these, five small meanders influenced transitions of the Kuroshio path from the nearshore NLM path; one caused the offshore NLM path and four brought about the large meander. About one-tenth of the total number of small meanders are related to the formation of the large meander.