Kuroshio Transports continued |
How are these meanders generated? We present two hypotheses. Further
study is needed to explore these possibilities.
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In over 5 years of ferry-boat ADCP data and 7 years of
TOPEX/POSEIDON satellite-altimeter data, Ebuchi and Hanawa
(2000) observed a wave-like train of cyclonic and anticyclonic eddies
moving 6 km/day westward through the region at about 27-30°N.
These features were associated with elevation of the
sea-surface-height (SSH) spectrum at periods from 45 to 180 days and a
main spectral peak with period a little longer than 100 days.
Moreover the spatial distribution of SSH variability exhibited a
high-energy ridge extending from the Kuroshio Extension east of
145°E through 28.5°N on the ASUKA line; so it appears
these disturbances originate from meanders of the Kuroshio Extension
in the vicinity of the Emperor Seamounts. According to Nitani (1975)
there is a 4-month-period peak in the spectrum of highest Kuroshio
velocities south of Japan, and from his Figure 12 we can deduce that
these 4-month period disturbances propagate eastward at 15 km/day.
Perhaps a Kuroshio small meander forms east of Kyushu and propagates
into the Kuroshio Extension. There it grows and pinches off as a
cyclonic eddy. This eddy then moves westward until it rejoins the
Kuroshio off Kyushu, triggering another meander. Depending on the
exact longitudes of the two ends of this circuit, the cycle time would
be about 8-12 months; so we would expect 2-3 meander/eddy features
to be propagating in the circuit at any given time. Phase-locking to
the seasonal cycle could result from sensitivity of the meander- or
eddy-formation processes to a seasonally varying parameter such as
wind-induced Sverdrup transport (maximum in February (Kutsuwada and
Termoto 1987)).
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Alternatively, eddies impinging on the Kuroshio off eastern Taiwan
could be responsible. For 20 months (including 14 overlapped with our
ASUKA deployment time), Johns et al. (2000) measured currents flowing
across the PCM-1 line at 24-24.5°N, between Taiwan and the
southern tip of the Ryukyu Island chain, into the East China Sea.
Their horizontally averaged 0307° True current at 400 m depth
showed event-like drops (even going negative in one event). The five
lowest-current events all occurred in February/March, June, and
October, similar to the phasing of the ASUKA small meanders. Yang et
al. (1999) showed low surface flow into the East China Sea was
associated with the arrival of cyclonic eddies from the east or
southeast into the region. It is thought these eddies change the
proportions of Kuroshio transport flowing into the East China Sea and
to the southeast of the Ryukyu Island chain. This in turn could
affect the path of the Kuroshio off Kyushu.
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