Abstract The Tsugaru Warm Current shows a remarkable seasonal change in its outflow pattern. Field data and rotating hydraulic model experiments were used to investigate the effect of external parameters on this change. When the Tsugaru Warm Current is assumed as a two-layered, density-driven flow, Rossby number and Ekman number become the most important parameters. These parameters are expressed by the volume transport through the Tsugaru Strait, density difference between upper and lower layers, and upper layer depth. The volume transport plays an important role in the transition of flow pattern in the experiment, but the density difference has a significant effect upon the wane of the Tsugaru Warm gyre in the field data. Monitoring of the upper layer depth in the upstream is the most suitable means for predicting the time of the degeneration of the warm gyre.