Characterizing Subsurface Oxygen Variability in the California Current System (CCS) and Its Links to Water Mass Distribution.

The california current system (CCS) supports a wide array of ecosystem services with hypoxia historically occurring in near‐bottom waters. Limited open ocean data coverage hinders the mechanistic understanding of CCS oxygen variability. By comparing three different models with varying horizontal resolutions, we found that dissolved oxygen (DO) anomalies in the CCS are propagated from shallower coastal areas to the deeper open ocean, where they are advected at a density and velocity consistent with basin‐scale circulation. Since DO decreases have been linked to water mass redistribution in the CCS, we conduct a water mass analysis on two of the models and on biogeochemical Argo floats that sampled multiple seasonal cycles. We found that high variability in biogeochemical variables (DO and nutrients) seen in regions of low variability of temperature and salinity could be linked to water mass mixing, as some of the water masses considered had higher gradients in biogeochemical variables compared to physical variables. Additional DO observations are needed, therefore, to further understand circulation changes in the CCS. We suggest that increased DO sampling north of 35˚N and near the shelf break would benefit model initialization and skill assessment, as well as allow for better assessment of the role of equatorial waters in driving DO in the northern CCS. Plain Language Summary: The California Current System (CCS) is an important region for fisheries and recreation that has historically experienced with episodes of low dissolved oxygen (DO), which is problematic for marine organisms that depend on this oxygen for breathing. Since there are not many observations in the Northeast Pacific Ocean, it is hard to fully understand what causes DO variations in this region. We use a combination of models and observations to study current DO variations, and to provide guidance into how to improve our ability to predict future changes in DO. We find that coastal variations in DO are transported into the open ocean. Since ocean circulation brings water from different places and with a different DO signature, we use temperature, salinity, and nutrients to understand where the water in the CCS comes from, and what the role of different locations is in determining DO in the CCS. Increasing observations north of 35˚N and near the continental shelf break would allow us to refine the precision of our calculations. Key Points: Oxygen anomalies observed near the coast are propagated to the offshore region and incorporated into large‐scale circulationWater mass mixing could explain high oxygen variability in regions of low temperature and salinity variabilityIncreasing biogeochemical Argo sampling near shelf break could help to further understand circulation in the northeast Pacific [ABSTRACT FROM AUTHOR]