The Role of an Intense Jet in the Tsugaru Strait in the Formation of the Outflow Gyre Revealed Using High‐Frequency Radar Data

The seasonal features of the eastward jet through the Tsugaru Strait was investigated using surface velocity data between 2014 and 2019 obtained from high‐frequency radar located in the eastern part of the strait, as well as data obtained from repeated shipboard observations from 2009 to 2019. The potential‐vorticity‐front‐model was used, and the volume transport of low potential vorticity (PV) water and the intensity of the PV gap at the front in the eastern part of the strait were estimated from these data. The influence of these parameters on the seasonal flow pattern of the Tsugaru Warm Current out to the Pacific Ocean (the summer/autumn “gyre mode” or winter/spring “coastal mode”) was examined. The distribution of the parameters was consistent with the theoretical understanding, showing that in addition to the volume transport, an increase in the local PV gap can also contribute to the development of the gyre. Plain Language Summary The Tsugaru Warm Current forms part of the coastal flow around Japan and shows remarkable seasonal changes in its flow pattern, which are similar to the Kuroshio meandering. The current flows along the coast from winter to spring, but demonstrates large anticyclonic meandering in the offshore direction, known as “gyre mode,” from summer to autumn. The cause of this change in the flow pattern has been thought to be the seasonal change in volume transport. In this study, we used high‐resolution temporal and spatial data obtained from a high‐frequency radar system, as well as shipboard hydrographic data collected over a period of 10 years, combined with a theory based on the relationship between coastal waves and offshore jets with fronts to identify another important factor that affects the seasonal flow pattern of the current, namely, “potential vorticity.” This study is important because it helps our knowledge of the behavior of fluids flowing out of channels and straits as well as that in relation to the connection between local phenomena and larger phenomena. Key Points Observational verification of the seasonal flow pattern theory of the Tsugaru Warm Current based on velocity data from high‐frequency radar Importance of potential vorticity gap at the front recognized as well as the volume transport of the current on the flow pattern change Integration and improvement of our understanding about the behavior of fluids flowing out from channels and straits