Decadal-scale variability of the core layer temperature of North Pacific Transition Region Mode Water: Influence of the quasi-stationary jet

Repeated hydrographic surveys at 155°E conducted by the T/S Oshoro-maru of Hokkaido University of Japan in spring (May/June) of 1989-2010 reveal a dominant decadal-scale ( 10 years) variation of core-layer temperature and salinity of the North Pacific Transition Region Mode Water (TRMW), a water mass with core characterized by potential temperature of 4-8°C, salinity of 33.5-34.0, and potential density of 26.4-26.7 kg m-3. An ocean heat content budget analysis based on historical temperature profiles and atmospheric reanalysis data shows that the observed decadal-scale variation of the core-layer temperature is not explained by variations in air-sea heat exchange and Ekman heat advection. The satellite-derived dataset and historical temperature-salinity profiles reveal that changes in the Kuroshio Extension (KE) path state, that is, stable state with the two quasi-stationary meanders and unstable state characterized by a convoluted path, are responsible for the formation of temperature-salinity anomalies in the TRMW formation region (40-44°N, 153-162°E). Meso-scale eddies that detach northward from the KE in the unstable path state form warmer-saltier conditions in the Kuroshio-Oyashio Confluence region. The warm-salty water is transported northward into the transition region by the quasi-stationary jet flowing from the subtropics to the subarctic, which induces an increase in temperature and salinity in the TRMW formation region, and this results in a peak in temperature and salinity a few years after the arrival of the warm-salty water in the formation region.