Climate variability in the North Pacific thermocline diagnosed from oxygen measurements: An update based on the U.S. CLIVAR/CO2 Repeat Hydrography cruises

Abstract

New observations of oxygen variability in the North Pacific Ocean are reported on the basis of comparison of the U.S. Climate Variability and Predictability and Carbon (CLIVAR/CO2) Repeat Hydrography sections conducted along 30°N (2004) and 152°W (2006) with the earlier World Ocean Circulation Experiment (WOCE) data and other cruises along these sections. The largest changes in apparent oxygen utilization (AOU) continue to occur, as found in earlier North Pacific repeat section analyses, within the thermocline on σθ = 26.6 kg m−3, which is the densest isopycnal to outcrop in the open North Pacific in climatological data. In the northeastern North Pacific along 152°W, where a total of five cruises (1980, 1984, 1991, 1997, and 2006) spanning a period of 26 years are available, the AOU changes correspond to an overall increase in AOU on σθ = 26.6 kg m−3 from the 1980s/early 1990s to 2006. However, from 1997 to 2006 a decrease in AOU is observed within the boundary region between the subtropical and subpolar gyres at 40°–45°N. Along the center axis of the subtropical gyre at 30°N, where two cruises are available (1994 and 2004), AOU has also substantially increased on σθ = 26.6 kg m−3 from 1994 to 2004 in the eastern part of the section. The repeat section data along 152°W and 30°N are consistent with a pattern of decadal‐scale ventilation anomalies that originate in the northwestern Pacific, possibly through variability (including cessation) of the σθ = 26.6 kg m−3 outcrop, travel eastward along the subtropical‐subpolar gyre boundary, and enter the northern portion of the subtropical gyre along the way. For the 152°W AOU data within the gyre boundary region (40°–45°N), good agreement exists with the close‐by time series data from Ocean Station P (50°N, 145°W) where a bidecadal cycle in AOU has been observed. In contrast, a sensible correlation with the Pacific Decadal Oscillation could not be found.