The large meander of the Kuroshio western boundary current is well known to influence the local climate, fisheries, and aquaculture by greatly modulating regional heat transport, but its impacts on biogeochemical processes remain unclear. Using high-resolution numerical ocean modelling and long-term observational datasets, we show that the path of the Kuroshio determines the extent of the shallow nutricline region, where winter convective mixing replenishes nutrient availability for subsequent blooms of phytoplankton during spring. During the large meander phase, this mechanism triggers offshore phytoplankton blooms that are otherwise absent during the non-large meander phase. The large meander also modulates the spatial distributions of primary production, air-sea carbon flux, and export production. These biogeochemical impacts of the Kuroshio large meander exert bottom-up control on regional marine ecosystems that is disproportionate to the thermal effect, and therefore need to be assessed to understand the large meander's overall impacts on fisheries and aquaculture.

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.