EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

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

Eike Eduard Köhn1, Matthias Münnich1, Meike Vogt1, and Nicolas Gruber1,2
Eike Eduard Köhn et al.
  • 1Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Zürich, Switzerland (
  • 2Center for Climate Systems Modeling, ETH Zürich, Zürich, Switzerland
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.

How to cite: Köhn, E. E., Münnich, M., Vogt, M., and Gruber, N.: Characterisation of low oxygen extreme events in the Eastern Tropical Pacific between 1979 and 2016, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11803,, 2020

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