Ventilation Pathways for the North Pacific Oxygen Deficient Zone

Abstract

Oxygen deficient zones (ODZs) in the tropical ocean exert a profound influence on global biogeochemical cycles, but the factors that regulate their long‐term structure and sensitivity to oceanic change remain poorly understood. We analyzed hydrographic observations and a high‐resolution physical/biogeochemical model to diagnose the primary pathways that ventilate the tropical Pacific ODZs. Historical and recent autonomous observations reveal pronounced and widespread O2 peaks, termed secondary oxygen maxima (SOMs), within the depths of the broader O2 minimum layer, especially at the equatorward edge of both northern and southern ODZs. In the northern ODZ, Lagrangian particle tracking in an eddy‐permitting numerical model simulation attributes these features to intrusions of the Northern Subsurface Countercurrent along the equatorial edge of the ODZ. Zonal subsurface jets also ventilate the poleward edge of the northern ODZ but induce a smaller O2 flux and do not yield detectable SOMs. Along the ODZ's eastern boundary, oxygenation is achieved by the seasonal cycle of upwelling of low‐O2 water onto the continental shelf, followed by downwelling of O2‐replenished near‐surface waters back into the ODZ. Waters entering the northern Pacific ODZ originate from the extratropics in both hemispheres, but two thirds are from the Southern Hemisphere and arrive later and with a wider range of transit times. These results suggest that predicting future changes in the large Pacific ODZs will require a better understanding of the climate sensitivity of the narrow zonal jets and seasonal dynamics of coastal upwelling that supply their O2.

Is deoxygenation detectable before warming in the thermocline?

“…A limitation of our study is that all the Earth system models included have a relatively coarse resolution for simulating the complex processes in the O 2 minimum zones (Margolskee et al, 2019). Earth system models diverge in projecting physical and biogeochemical changes in these regions (Brandt et al, 2015;Cabré et al, 2015).…”

“…Carbon emissions from human activities are causing ocean warming (Rhein et al, 2013) and ocean deoxygenation, i.e. a decrease in the oceanic oxygen (O 2 ) concentration (Sarmiento et al, 1998;Bopp et al, 2002;Matear and Hirst, 2003;Battaglia and Joos, 2018). Both warming and deoxygenation adversely affect marine organisms, ecosystems and the services they provide (e.g.…”

Assessing Marine Nitrogen Cycle Rates and Process Sensitivities With a Global 3‐D Inverse Model

“…The dominant processes in the ODZ are driven in part by the availability of O 2 , and in this model we have used monthly O 2 data products to parameterize these rates. The O 2 cycle in ODZs is dynamic and changeable on short time and length scales, driven in part by eddies and niche microenvironments that may allow for particular O 2 -sensitive N cycle processes to occur in areas that are not well represented by the coarse model grid and constant O 2 presented here (Bianchi et al, 2018;Gnanadesikan et al, 2013;Margolskee et al, 2019). A more nuanced and computationally intensive approach to modeling O 2 in the ODZs is likely to produce a complex picture of N cycle processes using the biogeochemical frameworks described here, particularly with the possibility of increased NO 2 − oxidation with increased O 2 injection.…”

Is deoxygenation detectable before warming in the thermocline?

“…A short-coming of our study is that all the Earth system models included have a relatively coarse resolution for simulating the complex processes in the O 2 minimum zones (Margolskee et al, 2019). Perhaps not surprising, Earth System models diverge in projecting physical and biogeochemical changes in these regions (Brandt et al 2015;Cabré et al 2015).…”