Temporal increases of phosphate and apparent oxygen utilization in the subsurface waters of western subarctic Pacific from 1968 to 1998

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 sigma (0) increased by an average of 0.9 +/-0.5 and 0.005 +/-0.003 mu mol/kg/y, respectively. Sali nity on these isopycnals also showed an average linear increase of 0.0008 p su/y. Salinity and density of winter mixed layer, on the other hand, was fo und to have decreased during the observation period. Observed bidecadal osc illation (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 li near increase of subsurface phosphate and AOU, we speculated that vertical water exchange in the upper layers of the subarctic North Pacific might hav e been diminished during this period. A decreasing trend of salinity and de nsity of winter mixed layer in Oyashio that was observed during the same pe riod supported this speculation.