[Received 9/7/03; Should be section 5.3]
Interactions of the Kuroshio Current with Bottom Topography East of Japan
Meandering and vorticity in currents near underwater ridges and mountains are correlated
and interrelated processes, and are highly pronounced near the Izu-Ogasawara Ridge off Japan.
A database of vortex activity in this region has been developed through the national Sections
Program implemented by the Far Eastern Hydrometeorological Research Institute (DVNIGMI)
during 1981–91. Such a great number of standard observations, made by 41 surveys over 10
years, has not been taken in any other area of the world ocean with a composite bottom contour.
Here are presented some results of this research on meandering and vortex formation in the
vicinity of underwater ridges and mountains near Japan. We analyze the variability of
geostrophic current vorticity using an integral parameter: the overall vorticity of the currents
with unlike signs in every survey site of the Sections polygon (Darnitsky 1996). The analysis of
all current systems encountered in all surveys is covered in Darnitsky and Pokudov (1995) and
Darnitsky (1996); therefore only the vortex-type flows initiated by the interaction of the
Kuroshio Current with the bottom topography are addressed here.
In analyzing the activity of the Kuroshio south of Honshu, most oceanologists note a
natural obstacle for the current: the Izu–Ogasawara Ridge, oriented along longitude 139E from
Japan to 27 latitude 50°N. In actual fact, underwater topography in this area is more complex.
Here, the current jet crosses three underwater ridges, from west to east: the Izu, Sichito, and
Boninsky ridges, all located between 138E and 142E longitude, and three abyssal troughs
separating these ridges. Furthermore, there are many isolated underwater mountains located both
on the underwater ridges and to either side of them.
The Kuroshio Current’s bimodal character in the region from Shikoku to
Enshunada is unique, compared, for example, to the Gulf Stream. In its alongshore
mode, the Kuroshio flows along the continental flank near the 1000-m isobath, and
at other times it manifests as the Great Kuroshio Meander, which persists for 3–11
years at a time. In his review of theoretical and experimental research on this
phenomenon, V. Kozlov (1986) concluded that the bimodality of the Kuroshio is
the manifestation of alternate steady states caused by the local topography of the
Izu Ridge and Continental Shelf. The system rests in one of two steady states, each
of which presents a characteristic pattern of subtropical circulation rings, until it
moves to the other state. According to Kozlov (1986), the ratio of the durations of the
meander and alongshore modes is on average 4:6.
The meander mode is characterized by the oceanward deflection of the current, such that
its path envelops the Izu Ridge, which forms the meridian for the meander. The meander
wavelength may vary from 500 to 1000 km and its amplitude may be as high as 250 km. Similar
situations have repeatedly been observed by Japanese oceanologists (Ikeda 1979) on expeditions
organized by the Pacific Research Institute of Fisheries and Oceanography and DVNIGMI, and
have been described in special literature (Results …, 1989). The meander has often been
observed near the Izu Ridge, where it may be understood as the Rossby stationary wave
generated by the ridge (White and McCreary 1976). White and McCreary (1976) believed that
meander generation is possible when the Kuroshio discharge rate near the Kii Peninsula
decreases to as low as 40 Sv, but when the discharge rate is high, the current is directed along the
shores. The results of experimental research on the Kuroshio in 1981 (Results …, 1986)
supported this view. For example, at the time that the autumn meander was initiated, maximum
velocities in the Kuroshio were 2–3 times lower than those registered in previous surveys, when
the current was observed in alongshore mode. On average, the discharge rates equally decreased.
The authors of the Sections research advanced a different opinion on the factors involved
in meander formation, and yet other explanations are given in Kozlov (1986) and Sokolovsky
(1985) and the works listed in their bibliographies. Sokolovsky (1985) gives a qualitative
explanation based on nonlinear effects in barotropic currents crossing meridional ridges:
depending on current discharge, either considerable zonal velocities or a weak zonal transport
may occur, the latter accompanied by major meridional deflection of the current. Such an
interpretation is not in conflict with experimental observations. A pronounced meridional current
along the Izu Ridge was noted by A. D. Nelezin (Results …, 1989), who used Levitus’s archive.
This may constitute evidence for more frequent repetition of the meander mode of the Kuroshio
in the corresponding climatic period.
The hypothesis that the Kuroshio meander is due to local topography accords with
observations in other regions of the ocean. A submeridional ridge southeast of Taiwan also
causes meandering and upwelling conditioned by the interaction of the Kuroshio with the
underwater ridge, which is about 500 m deep (Kuang-lung 1991). After flowing around the
ridge, the meander assumes a cyclonic character; its linear characteristics and location change,
depending on the reversal of monsoons.
The Kuroshio Meander was detected in the fall of 1981 by DVNIGMI expeditions and
was registered by every seasonal survey until 1985 (Results …, 1987) and then again in later
surveys of the Sections program up to May 1989, and was recorded in bathymetric observations
throughout the 1980s. However, it should be noted that discrete seasonal observations do not
provide an adequate picture of its dynamics. Instrumental measurements with short sampling
periods (GEK) in 1954–64 and in 1988 (Taft 1970) revealed several features of the meander’s
short-period dynamics. Over 56 days, five paths of the Kuroshio were tracked, three of them
(with a period of two weeks) in the meander mode. One path was observed west of the Izu
Ridge–—a topographical Rossby wave. In another case, the ridge formed the median of the
meander, i.e. there was a 2 longitudinal shift (from 138 to 140 E) of the current. The current
shifted from the Rossby mode to the median mode suddenly, over twenty-four hours, and then
after two weeks, switched just as suddenly to the alongshore mode, again within twenty-four
hours. In other cases the change from the median mode, which had been observed over 20 days,
to the Rossby wave mode took about 19 days and involved a 2 longitudinal shift of the meander
to the west.
The Kuroshio Current in its alongshore mode has been observed to vary in where it
crosses the Izu Ridge and in its direction after crossing. The alongshore mode modifications
changed one another within one week in the end of the measurement time interval. Where it
crossed the Izu Ridge, the axis of the current shifted by one degree of latitude; the current
direction changed by more than 100 after crossing the Ridge. These observations were given by
Isino and Otsuka (1970), who also pointed to the great influence of the changes in current path
on the location of fisheries.
Dramatic changes between the different modes of the Kuroshio may be common to
periods of increased instability in the system as a whole. Sporadic increases of up to 4 knots in
velocity of the Kuroshio were confirmed by instrumental measurements. Some apparent
contradictions in the way different papers have characterized the modes of the Kuroshio may be
related to this. Burkov and Kharlamov (1986) described periods of alongshore mode in 1967–73
and meander mode in 1976–82. The researchers from DVNIGMI (Results…, 1986)
characterized the Kurishio Meander in a slightly different way. This paper and its cited sources
concluded that there was an interruption in the meander, which was observed in July 1980,
disappeared in August–September, and was again generated in the fall of 1981. It is interesting to
note that in these two cases, authoritative researchers and specialists employed original
qualitative observations from different sources. Burkov and Kharlamov (1986) worked with the
data from Japanese observations (to a depth of 1000 m), whereas the researchers from
DVNIGMI used data (to a depth of 1500 m). from expeditions organized by that institute. In
another example, Taft (1970) wrote that both alongshore and meander modes of the Kuroshio
were observed in 1956–64; while Taramoto (
) characterized 1959–63 as a meander period.
Muromtsev (1970) mentioned the summer of 1965 and the winter of 1965–66 as periods of welldeveloped meander, but according to Taramoto (
), the alongshore mode of the Kuroshio was
observed during that time. Following the noted observational series, Pokudov (1978) concluded
that in 1955–57 no meander was observed. We conclude that interpretations of the Kuroshio
during other periods of time could easily be revised, if sufficient observational data were
available.
It is clear that long periods of meander existence or non-existence observed in various
years still do not yield an exhaustive picture of the full variety of circulation forms associated
with this current. First, transient conditions can persist for different lengths of time, sometimes
progressing very quickly, in much the same way as boilovers in hydrodynamic systems (Gledser
et al. 1981), and at other times taking place over an entire season (Results…, 1987).
The migration of marine life in the Kuroshio ecosystem and in the thalassobathyal region
close to the Izu Ridge depends to a great extent on the position of the Kuroshio axis. Hattory
(1970), on the basis of 1967 data, showed that even in the alongshore mode of the Kuroshio, the
distribution of spawn and larvae of pelagic fishes in the spawning zone near Cape Shionomisaki
showed five different subtypes, on the dynamics of siome, horizontal velocity distribution across
the Kuroshio streamflow, and horizontal and vertical structure of the current. The distribution
and quantity of aquatic life formed differentiated bands across the Kuroshio axis that clearly
depended on the carryover velocity of water mass from spawning areas, and aquatic organisms
migrating with it.
On the basis of the published information, we reason that in the periods of alongshore
mode of the Kuroshio, there is a increased eastern transfer of aquatic life, of both pelagic and
thalassobathyal communities, as well as an increase in its transit through the openings of the IzuOgasawara Ridge. During the meander periods of the Kuroshio, its water masses are blocked
either along the axis of the ridge (when the meander envelops the ridge itself) or on its western
flank and in adjacent waters (during the Rossby mode of the meander). As this takes place,
within the meander there is as a rule a cyclonic eddy, which promotes blocking water masses and
aquatic life west of the ridge.
It should be noted that although the Kuroshio bimodality phenomenon is clear in
principle, its separate modes are inconsistently interpreted as a result of shortage of monitoring
information. The development of bimodality may be a more or less evident signal of general
circulation reconstruction in the northwest Pacific and perhaps in the whole subtropical circle.
Kuang et al. (1991) established a statistical correlation between global climatic changes (i.e.
atmospheric temperature) and meandering processes over a long period of time (1803–1975),
with a variation interval of about 7 years (range 6–8) and a correlation index of 0.75.
Burkov and Kharlamov. 1986.
Darnitsky. 1996.
Darnitsky and Pokudov. 1995.
Gledser et al. 1981.
Hattory. 1970.
Ikeda. 1979.
Isino and Otsuka. 1970.
Kozlov, V. 1986.
Kuang et al. 1991.
Kuang-lung. 1991.
Muromtsev. 1970 .
Nelezin, A. D. 1989. Results . . .
Pokudov. 1978.
Sokolovsky. 1985.
Taft, B. 1970.
White and McCreary. 1976.
These all need to be listed by author:
Results…. 1986.
Results …. 1987.
Results …. 1989.