Temporal changes in pCFC-12 ages and AOU along two hydrographic sections in the eastern subtropical North Pacific

Abstract

Changes in chlorofluorocarbon-12 (CFC-12) derived ventilation ages (pCFC-12 ages) and in apparent oxygen utilization (AOU) are investigated in the eastern subtropical North Pacific, where a portion of a 1991 WOCE cruise along 152°W was repeated in 1997 and a portion of a 1985 WOCE cruise along 24°N was repeated in 2000. The effects of mixing biases and possible gas undersaturations at the isopycnal outcrops are carefully examined during the calculation of pCFC-12 ages and oxygen utilization rates ( OUR = AOU / age ). The most conspicuous and robust change observed in the data is an increase in pCFC-12 ages and in AOU that occurs at the subtropical–subpolar gyre boundary at 152°W and is centered at σ θ = 26.65 kg m - 3 . pCFC-12 ages (including a correction for mixing biases) and AOU increased by as much as 5 years and 40 μ mol kg - 1 from 1991 to 1997 here, which corresponds to a change of about 40%. These data are in agreement with previous studies suggesting a reduction of ventilation in the subpolar gyre. A proposed mechanism for these changes is a reduction or cessation of the outcropping of the core isopycnal ( σ θ = 26.65 kg m - 3 ) that most likely occurred prior to 1991. Since mixing-bias-corrected pCFC-12 ages and AOU changed in approximately equal proportions, OURs close to the core isopycnal remained roughly constant. In contrast, OURs in the 24°N sections appear to have decreased off the coast of California from 1985 to 2000. The average OUR difference here reduces to < 1 μ mol kg - 1 yr - 1 when a mixing-bias correction and gas saturations less than 100% (90% for CFC-12 and 95% for oxygen) are applied, but it remains significant at the 95% confidence level. A robust change in ventilation ages is not observed in the 24°N sections.