Quantifying the contribution of ocean mesoscale eddies to low oxygen extreme events.

Abstract

Oceanic dissolved oxygen has declined in recent decades (ocean deoxygenation) and represents a major stressor on marine life and biogeochemistry. Yet, the future extent and distribution of oxygen decline in the global ocean is uncertain. Mesoscale eddies, which are abundant in the global ocean, have a potentially large influence on the redistribution of dissolved oxygen in low oxygen zones. However, eddies and their biogeochemical consequences are understudied owing to the difficulty in resolving these fine-scale features in coarse ocean climate model simulations. Moreover, adequately tracking eddy features, and isolating their biogeochemical signals through the water column is challenging. Here, a global physical-biogeochemical ocean model at eddy-permitting resolution is employed to conduct an eddy-oxygen census of features originating in and around four Eastern Boundary Upwelling Systems (EBUS), located adjacent to Oxygen Minimum Zones (OMZs). Eddies with a surface signature, of both cyclonic and anticyclonic nature, are tracked over the period 1992–2018, and their subsurface biogeochemical signatures are isolated. Strongly deoxygenating eddies are characterised and their contribution to low oxygen extreme events (< 1st percentile) is quantified. It is found that eddies, in particular cyclones, are associated with intense negative oxygen anomalies in all focus regions, likely due to the trapping of low oxygen source waters and potential oxygen depletion mechanisms across eddy lifetimes. For the first time, the enhanced frequency of low oxygen extreme events within an eddy relative to non-eddying locations in EBUS locations is demonstrated (consistently 2-7 times higher in eddies vs. non-eddying locations). Finally, results of this investigation also suggest that eddies have substantially contributed to low oxygen extreme events across large areas outside of the permanent OMZs over the past decades.

Date of Award	22 Mar 2022
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Oliver D Andrews (Supervisor) & Dann M Mitchell (Supervisor)