Numerical simulation of the transient response of the Kuroshio leading to the large meander formation south of Japan

Using a three-dimensional, primitive equation numerical model that takes re alistic topography into account, we successfully reproduce the observed tra nsient response of the Kuroshio south of Japan during the transition from t he nonlarge meander path to the large meander path. The transient response is triggered by the generation of what is called the "trigger meander" off the southeastern coast of Kyushu resulting from the supply of cyclonic vort icity through vertical stretching caused by the interaction between the Kur oshio and the anticyclonic mesoscale eddy approaching the Tokara Strait. Th e trigger meander thus generated propagates eastward south of Shikoku while inducing an anticyclone-cyclone pair in the lower ocean. After the trigger meander passes Cape Shiono-misaki it slows down and rapidly amplifies so t hat the Kuroshio loops back west of the Izu-Ogasawara Ridge. Then the sharp ness of the meander trough gradually relaxes, and the large meander path is attained. During the rapid amplification of the trigger meander off Cape S hiono-misaki the abyssal anticyclone develops while being trapped by the lo cal topographic feature, Koshu Seamount, located similar to 200 km to the s outh of Cape Shiono-misaki. This abyssal anticyclone plays a crucial role i n intensifying the trigger meander trough in the upper ocean via cross-fron tal advection; the intensified trigger meander trough then further amplifie s the abyssal anticyclone over Koshu Seamount. This joint evolution of the upper ocean meander trough and the abyssal anticyclone suggests that barocl inic instability enhanced by Koshu Seamount is the dominant mechanism for t he rapid amplification of the trigger meander leading to the large meander formation south of Japan.