Sensitivity of the North Pacific oxygen minimum zone to changes in ocean circulation: A simple model calibrated by chlorofluorocarbons

[ 1] Chlorofluorocarbon (CFC) data collected in 1999 at 11 stations along the western margin of Baja California indicate that the oxygen-minimum zone (OMZ) of the area is ventilated from the far North Pacific on decadal timescales. The new data are combined with existing CFC data to constrain a one-dimensional advection-diffusion model that simulates changes in water column properties on the sigma(Theta) = 26.80 density surface along the path of ventilation. The results show that the penetration of CFCs into the OMZ off Baja California can be explained by slow advection and rapid isopycnal mixing from the southern margin of the Alaskan Gyre. The deficit in dissolved oxygen along the same path relative to conservative behavior is modeled with a consumption term that is the product of a single rate constant and the dissolved oxygen concentration. The model is used to show that very different oceanographic conditions must have prevailed in the North Pacific between 15 and 13 kyr ago, when water containing less than 5 mu mol kg(-1) oxygen impinged on a portion of the western margin of North America that was considerably expanded compared to today. To match the distribution of oxygen from the presence of laminations in a series of sediment cores, the least extreme scenario combines a 2.5-fold decrease in advection and diffusion along the current flow path, a 2.5-fold reduction in the oxygen content of ventilated waters of the central North Pacific, and a 2.5-fold increase in the rate constant for oxygen consumption rate, presumably linked to a proportional increase in surface productivity.