Breakdown of the North Atlantic Age-AOU Relationship

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. We explore the relationship between age and apparent oxygen utilization (AOU) using observations from Line W, a repeat hydrography cruise track which transects the Gulf Stream, extending from Cape Cod to Bermuda, and an Earth System Model simulation. We find that in both the observations and model, the assumed positive linear relationship between AOU and mean age is not found within and directly below the ventilated thermocline at the end of Line W. Model analysis indicates that this phenomenon is relatively localized to Line W due to the combination of relatively weak horizontal gradients in age and oxygen resulting reduced along-isopycnal variability and vertical heave acting on a depth offset between age and AOU extrema.