Relationship between Age and Oxygen along Line W in the Northwest Atlantic Ocean

Abstract

The relationship between age and oxygen is one that is often assumed in oceanography to be relatively simple. Because oxygen utilization rates are difficult to directly measure in the ocean, it is commonly assumed that the apparent oxygen utilization divided by the water age is a good representation of respiration. Likewise, because of limited transient tracer observations and difficulties constraining water age, it has been suggested that oxygen could be a useful proxy for age. In this paper, we explore the relationship between age and oxygen using observations from Line W, a repeat hydrography cruise track which crosses the Gulf Stream, extending from Cape Cod to Bermuda, and an Earth System Model simulation. In both the observations and the model, the assumed positive linear relationship between apparent oxygen utilization (AOU) and mean age is not found within and directly below the ventilated thermocline at the end of Line W. In the Earth System Model decoupling of age and AOU is found in gyre centers, in some coastal upwelling zones, and downstream of mode water formation regions that show large variability in isopycnal depth. In all three regions, differences in the spatial distribution of sources of age and AOU become critically important, producing offset maxima in the gyre centers, and different time variability of sources in the other two areas.