University of Cambridge > > British Antarctic Survey - Polar Oceans seminar series > Contemporary Southern Ocean CO2 flux variability in the UKESM1 and ocean-only simulations.

Contemporary Southern Ocean CO2 flux variability in the UKESM1 and ocean-only simulations.

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The Southern Ocean is a strong contemporary sink for atmospheric CO2 due to the interaction of various drivers, including strong winds, biological activity, or the overturning of water masses and their carbon content. A fundamental question is whether the Southern Ocean will continue taking up similar proportions of CO2 in the future. However, future projections are hampered by the large uncertainty in contemporary CO2 fluxes in Earth System Models (ESMs) and result from differences in individual model set-ups and their representation of CO2 flux drivers, historically sparse observations, and gaps in the understanding of the regional distribution and natural variability of air-sea CO2 flux.

In this talk, I will first introduce some of the open questions and challenges for both observations and models with regard to the CO2 flux in the Southern Ocean. I then explore the CO2 flux and its drivers in the Southern Ocean for the fully-coupled historical UK Earth System Model (UKESM1) simulation, compared to simulations using the UKESM1 ’s ocean component forced with reanalysis data. The comparison highlights several shortcomings in the UKESM1 simulations. Differences in the phase of the seasonal cycle of net CO2 flux appear between the simulations: while the seasonal cycle is out of phase with observations in the UKESM1 , the ocean-only simulation is in phase. This phase change is related to differences in the underlying physical processes, namely upper ocean stratification, winter mixed layer depth in the sub-Antarctic region, and circulation, which occur in response to the different atmospheric forcing. Disagreements in the timing of the seasonal cycle are not uncommon for ESMs, as are biases in the named physical processes. The results described here may therefore be valid for a larger group of ESMs. However, there are features of the CO2 flux which do not improve or even exacerbate despite improved physics representation in the ocean-only run, such as the magnitude of the CO2 flux or the decadal variability. The results thus illustrate the complexity of simulating the Southern Ocean CO2 flux, current limitations and opportunities for future model developments.

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

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