Hydrographic variations on decadal to longer timescales in the kuroshio extension and kuroshio-oyashio confluence

The variability of the Kuroshio Extension (KE) system, modulated by large-scale atmospheric forcing, critically influences North Pacific climate responses. However, despite the ongoing unprecedented northward KE shift, the low-frequency modulations on hydrography within the KE system and their impacts on the Kuroshio-Oyashio Confluence (KOC) region remain unclear. We analyzed hydrographic properties and mesoscale activities in the KE-KOC system and evaluated the effects of atmospheric forcing using satellite and reanalysis data from 1993 to 2023. Our findings indicate a poleward KE shift (0.45 ± 0.02°/decade) and increases in Sea Level Anomaly (SLA, 8.07 ± 0.30 cm/decade in the KE, 2.45 ± 0.27 cm/decade in the KOC) driven by negative wind stress curl (WSC) trends across the mid-latitude North Pacific. Following 2010, the KE system transitioned to a predominantly stable state, except for 2016–2017. The KOC region experienced enhanced eddy kinetic energy during convoluted KE meander phases (2021–2023), indicating baroclinic instability-driven energy transfer. The enhanced negative WSC anomalies north of 40°N and positive anomalies southward created a meridional dipole pattern in the eastern North Pacific after 2010. Accompanied by this shift, westward Rossby wave propagation was disrupted by some mesoscale signals, altering negative phase relationships between KE dynamics and climate indices to near-zero lags, resulting in the synchronized record-high SLA both in the KE and eastern North Pacific, coinciding with extreme negative Pacific Decadal Oscillation conditions during 2019–2023. These results reveal a new state of the North Pacific climate system and highlight the KE-KOC system’s accelerated adjustment to atmospheric forcing.