30-years time series of AOU and phosphate in the Oyashio sub-surface domain showed an increasing trend superimposed on bidecadal oscillation. AOU and phosphate trend on isopycnals between 26.7 and 27.2 &sgr;&thgr; increased by an average of 0.9¿0.5 and 0.005¿0.003 &mgr;mol/kg/y, respectively. Salinity on these isopycnals also showed an average linear increase of 0.0008 psu/y. Salinity and density of winter mixed layer, on the other hand, was found to have decreased during the observation period. Observed bidecadal oscillation (average period 20¿1 y) in subsurface AOU negatively correlated with that of North Pacific Index (r=-0.88¿0.06). As the cause of the linear increase of subsurface phosphate and AOU, we speculated that vertical water exchange in the upper layers of the subarctic North Pacific might have been diminished during this period. A decreasing trend of salinity and density of winter mixed layer in Oyashio that was observed during the same period supported this speculation. ¿ 2001 American Geophysical Union