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 realistic topography into account, we successfully reproduce the observed transient response of the Kuroshio south of Japan during the transition from the 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 vorticity through vertical stretching caused by the interaction between the Kuroshio and the anticyclonic mesoscale eddy approaching the Tokara Strait. The 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 that the Kuroshio loops back west of the Izu-Ogasawara Ridge. Then the sharpness of the meander trough gradually relaxes, and the large meander path is attained. During the rapid amplification of the trigger meander off Cape Shiono-misaki the abyssal anticyclone develops while being trapped by the local topographic feature, Koshu Seamount, located similar to 200 km to the south of Cape Shiono-misaki. This abyssal anticyclone plays a crucial role in intensifying the trigger meander trough in the upper ocean via cross-frontal advection; the intensified trigger meander trough then further amplifies the abyssal anticyclone over Koshu Seamount. This joint evolution of the upper ocean meander trough and the abyssal anticyclone suggests that baroclinic instability enhanced by Koshu Seamount is the dominant mechanism for the rapid amplification of the trigger meander leading to the large meander formation south of Japan.