Changes in water properties around North Pacific intermediate water between the 1980s, 1990s and 2000s

Abstract

Abstract We detected temperature (salinity) decreases on the neutral density surfaces above the salinity minimum and increases below throughout the western North Pacific subtropical gyre ( 25 ∘ N – 40 ∘ N , west of 170 ∘ W ) by comparing World Ocean Circulation Experiment (WOCE) hydrographic sections from the 1980s, 1990s, and 2000s. The temperature (salinity) changes around the salinity minimum were associated with an apparent oxygen utilization (AOU) increase, and could not be explained by an increase of the subpolar input. These property changes were due to a density decrease of the salinity minimum. Furthermore, similar changes in temperature (salinity) were detected along 165 ∘ E as a 12-year trend from 1996 to 2007. This suggests that the changes evident in the comparison of the WOCE sections are the representative of changes at the intermediate depths in the last decade. This pattern was not detected in the eastern subtropical gyre ( 25 ∘ N – 40 ∘ N , east of 170 ∘ W ) and along 47 ∘ N , where the AOU increase due to ventilation reduction has been reported. Along 47 ∘ N , changes in temperature, salinity and AOU from 1999 to 2007 were opposite to those from 1985 to 1999.