Structure and Variability of the Kuroshio Current in Tokara Strait

Four years of mooring array measurements in Tokara Strait, south of Kyushu, Japan, from 1992 to 1996 are used to analyze the structure and temporal variability of the Kuroshio Current. The mean Kuroshio current in Tokara Strait shows a nearly permanent subsurface double-core structure, possibly due to topographic blockage effects. The dominant variations of the Kuroshio in Tokara Strait are separated into long-term variability (typically 100-day period) and short-term variability (10 days to 1 month) according to spectrum and wavelet analysis. The long-term variability has a large horizontal scale across the strait, with a striking twofold banded structure in spatial correlations. This is due to the north–south Kuroshio axis shift that advects the double-core structure of the mean current. The axis shift can be indexed with the northeastward current velocity at the northernmost station; a composite analysis using this index shows well-defined northward and southward axis shift structures of the Kuroshio current. From the composite of the TOPEX/Poseidon sea level anomaly in terms of this index, the Kuroshio axis shift and the current structure change are associated with a dipole-shape sea level anomaly east of Tokara Strait. On the other hand, the short-term variability of high kinetic energy only has a small horizontal scale within the northern part of the current, which is related to frontal variability. There exists a deep southwestward undercurrent below 600 m in the northern part of Tokara Strait, flowing along the isobaths. The undercurrent becomes stronger during the northward shift of the Kuroshio axis, while it almost disappears during the southward shift.