Non-large Meander Research Articles

Numerical simulation of the transient response of the Kuroshio leading to the large meander formation south of Japan

Using a three‐dimensional, primitive equation numerical model that takes realistic topography into account, we successfully reproduce the observed transient response of the Kuroshio south of Japan during the transition from the nonlarge meander path to the large meander path. The transient response is triggered by the generation of what is called the “trigger meander” off the southeastern coast of Kyushu resulting from the supply of cyclonic vorticity through vertical stretching caused by the interaction between the Kuroshio and the anticyclonic mesoscale eddy approaching the Tokara Strait. The trigger meander thus generated propagates eastward south of Shikoku while inducing an anticyclone‐cyclone pair in the lower ocean. After the trigger meander passes Cape Shiono‐misaki it slows down and rapidly amplifies so that the Kuroshio loops back west of the Izu‐Ogasawara Ridge. Then the sharpness of the meander trough gradually relaxes, and the large meander path is attained. During the rapid amplification of the trigger meander off Cape Shiono‐misaki the abyssal anticyclone develops while being trapped by the local topographic feature, Koshu Seamount, located ∼200 km to the south of Cape Shiono‐misaki. This abyssal anticyclone plays a crucial role in intensifying the trigger meander trough in the upper ocean via cross‐frontal advection; the intensified trigger meander trough then further amplifies the abyssal anticyclone over Koshu Seamount. This joint evolution of the upper ocean meander trough and the abyssal anticyclone suggests that baroclinic instability enhanced by Koshu Seamount is the dominant mechanism for the rapid amplification of the trigger meander leading to the large meander formation south of Japan.

A Numerical Study on the Coastal Topographic Effect of a Peninsula on the Western Boundary Current

Abstract Coastal topographic effects of a peninsula on the western boundary current are studied numerically with reference to the difference in northern boundary inclination (NBI) from a zonal direction. It is shown from the numerical experiments that the two dominant path patterns of the western boundary current exist: one flow pattern is a large meander path with spinup and spindown of the cyclonic eddy and the other is a nonlarge (straight) path along the northern boundary. If a peninsula is located upstream (west) of the separation point of the mean flow flowing out from the northernmost area of the western boundary region, a large meander path with spinup and spindown of the cyclonic eddy is changed to an irregular oscillation by the topographic effect of the peninsula in the case of no NBI. A nonlarge meander path with a small cyclonic eddy just downstream of the peninsula is formed in the cases with NBI larger than 10°. Formation of the large meander path and the nonlarge meander path agrees with m...

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.

Kyucho induced by intrusion of Kuroshio water in Sagami Bay, Japan

On January 9, 1994, a Kyucho (stormy current) rushed along the Sagami Bay coast and destroyed the fishing set-net at the western side of the bay head. The current record at 10 m depth, at a mooring station near the destroyed set-net, showed that the maximum velocity was 0.65 ms −1 with a steep temperature rise, and the strong current continued for about half a day. The thermal front is estimated to have moved cyclonically at the speed of 0.6–0.7 ms −1 from the temperature records at four stations along the bay coast. The successive satellite images, NOAA-IR, also showed the warm water intrusion of the Kuroshio water through the Oshima west channel and moving cyclonically along the coast. The CTD casts in the bay indicated high temperature and high salinity water with the thickness of 70 m and width of 15 km. The behavior of the Kyucho is similar to that of the coastal density current in a rotating fluid, theoretically derived by Kubokawa and Hanawa (1984b). The temperature variations in the Kuroshio region suggest that the Kyucho occurred during the transition period from the non-meander nearshore path of the Kuroshio to the non-large meander offshore path.

Why does the sea level difference between Kushimoto and Uragami show periods of large meander and non-large meander paths of the Kuroshio south of Japan ?

The sea level difference between Kushimoto and Uragami, located to the west and east of the southern tip of the Kii Peninsula, is relatively large in periods of non-large meander path (nLMP) of the Kuroshio south of Japan in comparison with periods of large meander path (LMP). Based on this clear relationship, the sea level difference between Kushimoto and Uragami has been used as an index showing the periods of nLMP and those of LMP of the Kuroshio south of Japan. It has been pointed out that warm and saline Kuroshio water, separated from the main path of the Kuroshio, has a tendency to approach the western area off Kii Peninsula to off Muroto Peninsula in periods of nLMP, while it approaches the eastern area off Kii Peninsula to Omae-zaki in periods of LMP. On the basis of this observational evidences, the dynamic background underlaying the well-known relationship between the Kuroshio path and the sea level difference between Kushimoto and Uragami is examined in the present study, using the temperature and salinity data observed by Wakayama Prefectural Fisheries Experimental Station and Fisheries Research Institute of Mie. It is shown that deviations in vertically integrated specific volume off Kushimoto and Uragami almost equal deviations in observed sea level at Kushimoto and Uragami, respectively. It is also shown that the difference in vertically integrated specific volume between off Kushimoto and off Uragami almost equals the difference in observed sea level between Kushimoto and Uragami. As for the Kuroshio water, the high-temperature contribution is predominant for its specific volume rather than that of high salinity, which yields thermal expansion in comparison with coastal water. Because the difference in vertically integrated specific volume between off Kushimoto and off Uragami almost equals the difference in observed sea level between Kushimoto and Uragami, it is concluded that the relationship between the Kuroshio path and sea level difference between Kushimoto and Uragami is caused by the different approaching of the warm Kuroshio water between in nLMP periods and in LMP periods.

Monitoring of position of the Kuroshio axis in the Tokara Strait using sea level data

Properties of the index of position of the Kuroshio axis in the Tokara Strait, named the Kuroshio position index (KPI), were examined using sea-level data during 1984–92. The index is KPI=(X+M x )/(Y+M y whereX(Y) is the anomaly of sea-level difference of Nakanoshima (Naze) minus Nishinoomote from the 1984–92 meanM x (M y ). The correlation with the latitude of the Kuroshio axis in the Tokara Strait concluded that the KPI withM x /M y =0.83 and realisticM y (100±40 cm) best indicates the position of the Kuroshio axis in the strait. The KPI withM x =83 cm andM y =100 cm was newly called the KPI as the best index. Using daily values of this KPI, the relation between the position of the Kuroshio in the strait and the large meander of the Kuroshio shown by Kawabe (1995) was confirmed and studied in detail. A large meander forms (ends) 3.3 (5.1) months after a northward (southward) shift of the Kuroshio in the Tokara Strait. Yet, a temporary southward shift with a duration of ten to twenty days does not finish the large-meander (LM) path. At the LM formation, a small meander southeast of Kyushu begins to move eastward associated with the northward shift. The processes of LM formation and decay are started by the meridional move of the Kuroshio axis in the Tokara Strait. The Kuroshio axis at the FES line during the LM path is located farther north by 7′ latitude than that during the non-large-meander (NLM) path. The latitude during the LM formation (decay) stage is a little higher (lower) than that during the LM (NLM) period, though the Kuroshio still takes an NLM (LM) path.

Model Study of Flow Conditions Causing the Large Meander of the Kuroshio South of Japan

The upstream conditions for large-meander (LM) and non-large-meander (NLM) paths of the Kuroshio south of Japan were examined by calculating the current axis with a steady model imposed upstream flow conditions at the origin put in the Tokara Strait south of Kyushu. The calculation in each case, defined by velocity at the origin U0 and volume transport of the Kuroshio, was made with respect to the conditions at the origin, that is, a curvature of current axis κ0, a spatial change of current depth ΔH, and a direction of the current axis. The depth change ΔH accompanies the spatial changes of current velocity and width. The κ0 and ΔH are greatly effective to the downstream path. Nearly realistic κ0 is necessary for producing a realistic path. The spatial changes at the origin, a decrease of velocity and increases of current width and depth (ΔH > 0), are necessary for producing LM paths for realistic κ0 in case of medium and large U0. The LM paths are caused by smaller κ0 than the NLM paths. This is firmly concluded by considering realistic ranges of current width and a realistic relation between κ0 and ΔH. The LM path, therefore, is caused by Small κ0 in a realistic range and a decrease of velocity at the origin. These conditions for the LM path are not thought to be realized in the case of small transport and velocity in which an LM path has never been observed. These conclusions well explain the observation facts and show that the path of the Kuroshio, LM or NLM, is almost determined by the flow conditions in the Tokara Strait south of Kyushu.

Variations of Current Path, Velocity, and Volume Transport of the Kuroshio in Relation with the Large Meander

The variations of the Kuroshio Current path and their relations with current velocity, volume transport, and upstream position of the Kuroshio are examined using sea level data. After the long-time, non-large-meander (NLM) path during 1963–75, the Kuroshio takes a large-meander (LM) path during most of 1975–91: 1975–80, 1981–84, 1986–88, and 1989–91. The study of these meanders confirms the description of the path of the Kuroshio based on the three typical paths and the transitions between them, with an exception being the 1981–84 LM path. The formation and decay of the large meander are associated with velocity and position of the Kuroshio in the Tokara Strait south of Kyushu. A small meander, precursor of the large meander, seems to be formed southeast of Kyushu in relation with a temporary increase of Kuroshio velocity and a northward shift of the Kuroshio in the Tokara Strait. An eastward propagation of the small meander, leading to the LM formation, is associated with remaining in the northern position in the Tokara Strait, but not with large velocity except in 1975. Decay of the large meander occurs associated with large velocity and a return to the southern position of the Kuroshio in the Tokara Strait. Unusual separation of a cold eddy from the LM path is also related with a temporary increase of Kuroshio velocity. Volume transport of the Kuroshio shows an interdecadal variation, small before 1975 and large afterward, that is, the transport during the 1975–91 LM-dominant period is larger on average than that during the 1963–75 NLM path. All the LM paths are formed when the transport increases from about 24 Sv (Sv = 106 m3 s−1), while the Kuroshio takes an NLM path whenever the transport is less than 23.5 Sv. Velocity of the Kuroshio main current is mostly correlated with the transport but is different in the decrease after 1981. This shows that the Kuroshio always takes a NLM path for small transport and velocity, but can take either path for their large and medium ranges. A factor for determining the path in these ranges is the position of the Kuroshio in the Tokara Strait; LM paths begin and terminate about four months after the Kuroshio shifts to the north and returns to the south, respectively. The time lags are for the formation and decay processes. In conclusion, LM paths occur when the Kuroshio has large or medium transport and velocity, and is in a northern position in the Tokara Strait. The northern position implies a small curvature and a large vertical inclination of current axis south of Kyushu, which may be related with the dynamics of a large meander.

Short‐period fluctuations in meander of the Kuroshio's path off Cape Shiono Misaki

Direct current measurements in the strong stream layer of the Kuroshio were carried out to investigate short‐period fluctuations of the small‐scale meander of the Kuroshio at two mooring stations off Cape Shiono Misaki in 1985–1986. The Kuroshio had a stable nonlarge meander path and flowed eastward parallel to the mooring stations during a large part of the observations, which were appropriate conditions for temporal analysis. Temperature and velocity variations reveal the existence of dominant periodic fluctuations in the small‐scale meander of the Kuroshio's path and its frontal disturbances. A fluctuation with a period of 17–19 days and 400‐km wavelength is found in association with small‐scale meander of the Kuroshio's path. Periodic fluctuations of 5–8 days and 10–12 days are also recognized and have 100‐km and 200‐km wavelength, respectively. Satellite imagery shows that the two periodic fluctuations are dominant in frontal regions as plumes from the north wall of the Kuroshio. The fluctuations with the three periodic bands propagating downstream at about 20 cm s−1 and transitions of wavelike disturbances of the Kuroshio front observed by satellite suggest that all of the propagations are subject to the barotropic component of the eastward velocity. The temporal and spatial characteristics of the Kuroshio meander show good correspondence with other observations in upstream regions of the Kuroshio. The phase speed of wavelike perturbations well recognized in the Gulf Stream region is approximately twice as large as that of Kuroshio perturbations. The difference seems to be caused by differences of water depth beneath the two streams.

A steady model of typical non-large-meander paths of the Kuroshio Current

Conditions south of Cape Shiono-misaki for the nearshore and offshore non-large-meander (NLM) paths of the Kuroshio Current are studied using a two-layer reduced gravity model. A steady and non-diffusive state is assumed, and the conservation laws of Bernoulli's function and potential vorticity along the current axis are used. Spatial changes of velocity and depth of the current are imposed as boundary conditions south of Cape Shiono-misaki. These conditions are based on the facts that are ohserved south of the cape: abrupt acceleration of the Kuroshio and the spatial change of sea levels. The current paths east of the cape are computed.

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.

Interannual correlations between sea level difference at the south coast of Japan and wind stress over the north pacific

Relationships of the sea level differences between Naze and Nishinoomote and between Kushimoto and Uragami with wind stress over the North Pacific are examined for interannual variability. These sea level differences are considered to be indications of Kuroshio transport in Tokara Strait and Kuroshio path south of Enshu-nada, respectively. In the sea level difference between Kushimoto and Uragami, dominant variations are found to have periods of about seven years and 3–4 years. The variation of about 7-year period, which corresponds to that in the Kuroshio path between the large meander and non-large meander, is coherent with the variation of the wind stress curl in a region about 2,400 km east of the Kii Peninsula, where negative stress curl weakens about two years before the sea level difference drops (i.e. the large meander path in the Kuroshio generates). The variation of the 3–4 year period is coherent with that of the wind stress in a large area covering the eastern equatorial Pacific, which suggests that it links with global-scale atmospheric variations. Interannual variation in sea level difference between Naze and Nishinoomote is not coherent with that between Kushimoto and Uragami, which suggests that it is not related to the variation of the Kuroshio path south of Enshu-nada, but is coherent with that of the zonally-integrated Sverdrup transport in the latitudinal zone along 30°N. It is suggested that the interannual variation of the Kuroshio transport in Tokara Strait can be explained by the barotropic response to the wind stress.

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, ItO, Oshima, 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 Oshima minus Minami-Izu and Oshima 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 Oshima 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.