Kuroshio Transports continued

How are these meanders generated? We present two hypotheses. Further study is needed to explore these possibilities.

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)).

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.

NEXT: Conclusions

Abstract

Introduction

Dynamic Heights

Velocities

Transports

Conclusions

References

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