Isopycnal shoaling causes interannual variability in oxygen on isopycnals in the subarctic Northeast Pacific




Cervania, Ahron

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Over sixty years of oceanographic observations from Ocean Station Papa (OSP) in the northeast Pacific indicate the region is losing dissolved oxygen faster than the average global rate. The greatest negative trends in oxygen concentration occur on isopycnals in the upper water column (σθ = 26.1–26.8 kg m−3) but have considerable uncertainty due to natural variability near the surface. In this thesis, I use eight Argo profiling floats equipped with oxygen optode sensors to assess the 2008—2016 interannual variability of subsurface dissolved oxygen near OSP. I developed a method to implement a time-lag correction to the optode profiles using high frequency CTD data and used reference profiles from the OSP time series to calibrate the dissolved oxygen observations. The time-lag correction markedly improves subsurface bias caused by slow optode response time. The analysis of isopycnal properties indicates that episodic shoaling of the isopycnals can cause rapid reduction of the dissolved oxygen concentration. Changes in ventilation, horizontal mixing, and water mass age were assessed and deemed unlikely drivers for the rapid O2 loss events examined. The dissolved oxygen loss during shoaling events is linked to organic matter export, due to higher concentrations of organic matter and greater respiration rates at shallower depths. Reduced net community production during the “Blob” marine heatwave may have reduced the impact of the second shoaling event examined. Studying the natural variability of dissolved oxygen in these layers can provide context for the uncertainty in the long-term trends, as well as provide insight towards the future potential for extreme oxygen minima from the combined impacts of the long-term trend and natural variability.



ocean deoxygenation, ocean oxygen, biogeochemical-Argo