Seasonal pathways of the Tsugaru Warm Current revealed by high-frequency ocean radars

Seasonal pathway of the Tsugaru Warm Current (TWC), coastal boundary current flowing eastward from the Sea of Japan to the North Pacific Ocean, is investigated using data of surface currents measured by high-frequency ocean radars for the period 2014–2020. The TWC takes a pathway along the center of Tsugaru strait during summer season (41.64ºN), while during winter season the TWC moves to south to flow along the southern coast (41.55ºN). These pathways are associated with seasonal outflow modes known as “Gyre mode” and “Coastal mode,” respectively. Seasonal changes in current velocity, characterized by intensified flow in summer and autumn seasons and weakened flow in winter season, are well correlated with seasonal changes of sea level difference between Fukaura and Hakodate. However, this clear relationship is not seen at the eastern exit of the strait (141.5ºE). There is a shallow sill that extends northward from the southern coast. HFR further reveals that Coastal mode is locally intensified and widened when passing through this sill. These TWC’s responses are qualitatively explained by mass conservation for barotropic flow that flows in a horizontal two-dimensional channel with sea floor depth change. These responses over the sill are not seen during Gyre mode because the northward shift keeps the pathway away from the sill. During the Coastal mode, there are some cases in which the TWC does not ride on the sill but takes a pathway around the sill. Such patterns are seen when the TWC is weak, thus a weak Coastal mode.