Characterisation of low oxygen extreme events in the Eastern Tropical Pacific between 1979 and 2016

The oxygen minimum zones (OMZs) in the Eastern Tropical Pacific (ETP)

have expanded over the past 50 years, likely leading to more frequent and more

intense low oxygen extreme events. This has potentially far-reaching implica-

tions for e.g., the production of the climate-relevant gas nitrous oxide or the

reduction of habitat for fish and zooplankton. Yet, to date our understanding

of the distribution and characteristics of low oxygen extreme events in the ETP

remains limited.

 

To fill this gap, we study low oxygen extremes in the ETP using results from

an eddy-resolution hindcast simulation with the coupled physical-biogeochemical

model ROMS-BEC for the Pacific from 1979 to 2016. Our setup permits us to

simulate oxygen variability in the ETP affected by processes on a broad range

of scales, from climate modes down to mesoscale dynamics. We detect and

track low oxygen extreme events in the upper 500 meters of the ETP, by ap-

plying temporally constant statistical thresholds to the hindcast simulation and

requiring a minimum event duration of 5 days. While most extremes last less

than 10 days and are of small volumetric extent, about 15% of the extremes

exist for over a month. The diversity of the long-lasting extremes is dominated

by westward propagating low oxygen eddies, which are mostly generated in the

near-coastal area. Superimposed inter-annual variability associated with the El

Niño-Southern Oscillation (ENSO) leads to a decrease in mesoscale extremes

during El Niño periods. Along the boundaries of the ETP OMZs transient

shoaling events of the oxycline linked to ENSO dynamics or the seasonal cycle

contribute to the generation of further pronounced low oxygen extreme events.

 

The presented detection and tracking of low oxygen extremes is an important

step towards a better understanding of extreme event occurrences and charac-

teristics and lays the groundwork for further research such as the biogeochemical

impact of such extremes.