Teleconnection-driven winter sea surface temperature regime shift and ecosystem reorganization in the western part of the East/Japan Sea.

The Western part of the East/Japan Sea (WES) has warmed rapidly in recent decades, yet the mechanisms of its recent reorganization remain unclear. Using 1990-2024 observations, we identify a likely winter sea surface temperature (SST) regime shift around 2015, supported by complementary non-parametric change-point and persistence diagnostics and by the persistence of positive anomalies thereafter. Before 2015, SST variability reflected a balance between atmospheric forcing and East Korea Warm Current (EKWC) transport; afterward, EKWC-related variability and a northward Kuroshio displacement became more prominent, 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 potentially altered nutrient supply pathways. These results suggest that teleconnection-driven boundary-current variability may increasingly shape both physical and biological variability 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.