The oceanic uptake of anthropogenic CO2 has resulted in acidification in surface and subsurface waters throughout the global ocean. We initiated acidification monitoring in the eastern part of the Tsugaru Strait, which connects the Sea of Japan and the North Pacific. Annual mean pH and CaCO3 saturation state decreased considerably throughout all depths at rates of 0.0030−0.0051 yr−1 and 0.017−0.036 yr−1 during 2012-2019, respectively. These rates of decrease are faster than those caused by increasing atmospheric CO2, and faster than those observed during previous research in the Sea of Japan and the North Pacific. The accelerated acidification is attributed to enhanced increase in dissolved inorganic carbon caused by elevated mixing of the upper and deeper waters from the Sea of Japan at the western part of the strait by the strengthening of the Tsugaru Warm Current.

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.