ENSO-Driven Thermohaline Reorganization of the Shelf–Ocean Boundary in the California Current Transition Zone

Interannual climate variability exerts a strong control on the thermohaline structure of eastern boundary upwelling systems, particularly in transition zones where distinct water masses converge. Seasonal and interannual variability in temperature and salinity were examined in the southern California Current System for the period 2000–2015 using hydrographic observations and satellite altimetry, analyzed by season and ENSO phase. During El Niño, the upper 100 m exhibits positive temperature and salinity anomalies of 1–2 °C and ~0.1–0.2 g kg−1 associated with 50–80 m isopycnal deepening, reduced upwelling-induced ventilation, the expansion of subtropical waters onto the shelf, and enhanced poleward geostrophic transport. In contrast, La Niña conditions shoal isopycnals, enhances upper-layer stratification, and sustains equatorward flow throughout the year. Temperature and salinity anomalies extend below 100 m, suggesting a remote reorganization of the baroclinic structure at the shelf–ocean boundary. Salt fingering is inferred to be the dominant non-conventional mixing process in the region, with peak occurrence in autumn. These results highlight that ENSO confines thermohaline reorganization to the inner continental shelf (~150 km), modulates coastal–ocean density gradients, weakens equatorward geostrophic transport during El Niño, and alters coastal–ocean heat and salt exchanges within the southern CCS transition zone.