Teleconnection Driven Winter Sea Surface Temperature regime shift and Ecosystem reorganization in the Western marginal Sea of the Northwest Pacific

The western marginal sea of the northwest Pacific has warmed rapidly in recent decades, yet the mechanisms of its recent reorganization remain unclear. Using 1990–2024 observations, we identify a winter sea-surface-temperature (SST) regime shift around 2015, supported by non-parametric change-point tests and the persistence of positive anomalies thereafter. Before 2015, SST variability reflected a balance between atmospheric forcing and East Korea Warm Current (EKWC) transport; afterward, oceanic processes EKWC intensification and a northward Kuroshio displacement—became dominant, concurrent with a meridional reorganization of the Aleutian Low. Despite stable chlorophyll-a, phytoplankton size structure shifted toward pico-dominance at the expense of nano-classes, consistent with enhanced stratification and altered nutrient pathways. These results show that teleconnection-driven boundary-current variability now governs both physical and biological states in this marginal sea, highlighting its role as a sentinel of basin-scale Pacific decadal variability and its relevance for inter-basin climate linkages and decadal prediction.