Ocean Deoxygenation Conference | Kiel 2018

While observations suggest a long-term expansion of the OMZ in the South Eastern Pacific (SEP) which has been suggested to be due to global warming, the SEP OMZ also apparently exhibits a notable decadal variability in its upper and lower limits. Large uncertainties of the fate of the SEP OMZ in a warmer climate as simulated by Earth system models also questions to which extent natural variability in the OMZ can obscure the detection of externally forced trends. Here we analyze a long-term regional simulation of the OMZ of Peru and Chile and show that a large share of the variability is not linearly related to the equatorial forcing (ENSO), suggesting that it originates from internal dynamics associated to both local non-linear physical and biogeochemical processes. The natural variability in the model is further diagnosed from sensitivity experiments to the ocean boundary forcing of the regional model. The simulations of the Community Earth System Model (CESM) Large Ensemble are also analyzed suggesting that low-resolution coupled models may underestimate natural variability. Implications of our results are discussed in terms of the concept of emergent constraints and of the challenge for observing systems.