Variations of the Kuroshio with 1-3 month period are often observed in the East China Sea and in the Tokara Strait. These variations are indicated to be propagated downstream along the Kuroshio axis in the East China Sea to reach the Tokara Strait, from all the related data sets; namely, the sea surface dynamic height (SSDT) determined from the T/P and ERS-1/2 altimetry data, time series of the Kuroshio Position Index (KPI) estimated from tide gauge records in the Tokara Strait, and records of the current meters moored on the Continental Shelf in the East China Sea at approximately 500m depth in 1989. In addition, by taking temporally-lagged cross correlation between the SSDT and the major axis component of the deep current variation, the surface signals southwest of Kyushu are revealed to be ahead in phase by a nearly 1/4 cycle than ones at 500m depth.
The long-term modulation of the amplitude of these variations can also be confirmed in all those data sets. In the present study, 8-year-long SSDT variation averaged over a 1-degree squared box 29-30N and 128.25-129.25E, where the SSDT is highly correlated to both the current meter records in the upstream East China Sea and the KPI in the downstream Tokara Strait, is used to monitor the Kuroshio variation southwest of Kyushu. The wavelet analysis is then applied to this data to extract variations with periods from 40 to 80 days, accounting their long-term amplitude modulation. The time series of the wavelet spectra of the extracted signals shows that the long-term amplitude modulation consists of various variations with periods from a half to two years, but no dominant signals are found with a specific period. Temporally-lagged correlation between the wavelet spectra and the offshore SSDT variations with periods longer than 90 days indicates that the 1-3 month Kuroshio variations in the East China Sea is enhanced (or weakened) about 100-150 days later when the surface volume transport upstream area of the Kuroshio east of Taiwan at about 20N increases (or decreases). In the downstream of the Tokara Strait, on the contrary, the eastward Kuroshio current tends to be weaker when the 1-3 month variation in the East China Sea becomes significant. Both of these upstream and downstream tendencies are consistent with some instability processes which would be induced by a faster flow, and reduce the kinetic energy of the flow as a result. It is also noticed that the SSDT south of Shikoku and Honshu tends to be lower about 50-100 days later when the 1-3 month Kuroshio variation southwest of Kyushu is stronger, which would suggest that enhanced occurrence of small meanders of the Kuroshio in the East China Sea would trigger a larger meander at the far downstream area 100-150 days later. |
|
|