Journal of Oceanography, Vol. 53 (No. 2), pp. 129-142, 1997
Kazunori Akitomo, Shuhei Masuda and Toshiyuki Awaji
Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto 606-01, Japan
(Received 26 December 1995; in revised form 26 August 1996; accepted 28 August 1996)
Abstract: Executing numerical experiments with a barotropic inflow-outflow model with Hachijo Island embodied explicitly, we have re-examined the dependence of the Kuroshio path variation south of Japan on its velocity, Vmax, and investigated the stability of a straight and a meandering path in the multiple equilibrium regime to a short-term (pulse-like) variation of Vmax. The dependence of path selection on Vmax is the same as in previous studies: the Kuroshio takes a straight path at low Vmax (regime I), a meandering path at high Vmax(regime II) and two different paths at intermediate Vmax(regime III; multiple equilibrium regime). However, the modeled Hachijo Island causes two differences. First, the Kuroshio takes a detouring path around the island (a quasi-steady C-type path) at the lower end of regime III. Secondly, a transient C-type path always appears in the course of the transition from a meandering to a straight path when Vmax decreases from regime III (II) to I. It takes one to several years for this transition to be completed, almost consistent with observations. With the short-term increase in Vmax imposed in regime III, a straight path develops into a meandering path when its amplification rate Amp and duration Texceed 1.75 and 30 days, respectively, or when Tranges from 45 to 75 days with Amp of 1.5. The transition occurs in the same manner as observed, strongly suggesting that such a short-term increase in the current velocity can cause the transition from a straight to a meandering path in the actual situation. On the contrary, a meandering path does not turn into a straight path with a similar increase in Vmax. Instead, a short-term decrease in Vmax causes the transition from a meandering to a straight path in some cases with Amp0.5. Whether or not the transitions occur between a straight and a meandering path largely depends on variability of the Kuroshio position south of Kyushu against short-term changes of Vmax, showing that it is a key factor controlling the path transition south of Japan.