The seasonal pattern of the eastward jet through the Tsugaru Strait between 2014 and 2019 was investigated using surface velocity data obtained from high-frequency radar located in the eastern part of the strait. The vorticity-front-model was used to estimate the volume transport of low-vorticity water and the intensity of the vorticity gap at the front using the climatological mean zonal velocity distribution. The flow mode at the outlet was then defined as either the summer/autumn “gyre mode” or winter/spring “coastal mode”. The distribution of the parameters was consistent with the theoretical understanding, showing that in addition to the volume transport, an increase in the vorticity gap can also contribute to the development of the gyre. The results also suggest an impact from the jet in the strait on the coastal flow along the coast of Japan.

Understanding of the spatial distribution of near-surface oceanic chlorophyll a (Chl-a) in relation to eddy activity is important because Chl-a is critical to biological production and carbon absorption, and its distribution is closely related to meso-scale eddies and Rossby waves. Chl-a distributions based on satellite observations from 2001 to 2018 were analyzed in association with eddy activity in mid-latitude regions of the five ocean basins (North/South Pacific, North/South Atlantic, and South Indian oceans) and related to physical eddy scales including the internal Rossby deformation radius (LD) and Rhines scale (LR). In the open ocean, the horizontal scale of Chl-a (D) decreased with increasing eddy kinetic energy (EKE), converging to πLD in the high-eddy-activity region. The ratio of zonal (Dz) to meridional (Dm) scales, RD (= Dz/Dm), converged to 1 with increasing EKE, implying isotropic uniformization with meso-scale eddies. In regions of low EKE (especially the central subtropical and eastern parts of each basin), Dm was larger than LD, with a scale similar to πLR. In these regions, Dz values greater than Dm were found (with scales of 500–1000 km), indicating the effects of planetary phenomena on Chl-a. In such regions the sensitivity of D to EKE tended to be higher than in western boundary regions, indicating the influence of meso-scale eddy activity on the Chl-a distribution, even with low EKE.

The eastern Tsugaru Strait is an important area because it is located at the southwestern boundary of the subarctic gyre in the North Pacific, and it connects the Pacific Ocean and the Sea of Japan. Combining continuous monitoring (including fixed-point temperature measurements, shipboard observations, and high-frequency radar system observations) with a data-assimilated numerical model (JCOPE2M), we investigated the processes associated with an event characterized by extremely cold surface water observed in winter of 2014 at the southeastern side of the Tsugaru Strait. JCOPE2M outputs reproduce the event closely and reveal a low-temperature, low-salinity region of water generated from around Cape Erimo (probably by wind associated with migrating lows passing across Japan) and propagating from east to west over multiple timescales. This pattern is interpreted as representing the spreading of Coastal Oyashio Water (COW; with a density of 26.2 σθ) along the Hokkaido coast. JCOPE2M outputs also show a subsequent increase in denser water (26.6–26.8 σθ) inflowing from the Sea of Japan into the bottom of the strait. The JCOPE2M results also indicate baroclinic instability in the eastern part of the strait following the inflow of dense water. The confluence of COW and denser water is presumed to lead to horizontal exchange, with cold COW riding up on the denser water, in turn generating the pronounced cold-surface-water event. Some past cold-water events correspond in timing to negative anomalies of the West Pacific pattern, which suggests the important influence of southward shifts in storm-track latitude of migrating atmospheric lows across Japan.