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 σ_θ) along the Hokkaido coast. JCOPE2M outputs also show a subsequent increase in denser water (26.6–26.8 σ_θ) 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.