University of Cambridge > > British Antarctic Survey - Polar Oceans seminar series > Challenges in Understanding Decadal Trends and Variability of Oceanic Dissolved Oxygen: Perspectives from MPI-ESM's Large Ensemble Simulations

Challenges in Understanding Decadal Trends and Variability of Oceanic Dissolved Oxygen: Perspectives from MPI-ESM's Large Ensemble Simulations

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Ocean deoxygenation is known to be one of the major marine ecosystem stressors along with the warming and ocean acidification under future climate change [Gruber, 2011]. The observational studies [e.g., Ito et al., (2017);Schmidtko et al., (2017)] indicate that the ocean deoxygenation might already have started to emerge in the recent decades. However, it is challenging to detect the emergence of ocean deoxygenation from the observations because of the limited sample size in both space and time. The existence of internal/natural variability rise additional uncertainties on interpreting the observed dissolved oxygen variability and trend along with future projections from the Earth System Models (ESMs).

In this talk, I will address the potential role of the internal variability in driving the observed decadal trend and variability of oceanic oxygen in the upper 300m. To disentangle the internal variability and forced trend (i.e. the long-term trend (response) rising from external forcing such as greenhouse gases and volcanic eruptions), we use so-called “Large Ensemble Simulations” from the Max-Planck Institute for Meteorology Earth System Model (MPI- ESM ). The Large Ensembles are powerful tool for quantifying the internal variability of the climate system and are widely used in the climate science community [e.g.,Deser et al., (2012)] We analysed the globally compiled dissolved oxygen observations from the World Ocean Database 2013 [Boyer et al ., (2013); Ito et al., (2017)] within the context of the MPI -ESM’s Large Ensemble Simulations and discuss the possible role of internal variability in driving the decadal trends.

The results indicate that most of the decadal trends in global and regional oceanic oxygen are driven by internal variability. Internal variability could change the decadal trends in a diverse way, causing differences in magnitudes and even the sign of the decadal trends. The effect of ventilation and biological consumption play a dominant role regulating the decadal trends rather than the thermodynamically driven solubility effect for both observations and Large Ensembles. However, the observed decadal trend is larger than the range of internal variability simulated from Large Ensembles. It is likely that both model limitations (in simulating unresolved features such as mesoscale eddies) and the nature of chaotic spatial patterns of internal variability of oceanic oxygen could both introduce major challenges in the interpretation of the observations within the context of the Large Ensembles. I will further discuss how limitations in both observations and modelling could cause additional uncertainties when interpreting the past observations and future projections.

This talk is part of the British Antarctic Survey - Polar Oceans seminar series series.

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