CELOS Constraining the EvoLution of the southern Ocean-carbon Sink (CELOS)

The Southern Ocean (SO) plays a key role for climate and climate change. It is the largest oceanic sink of anthropogenic CO2, absorbing each year between 5 and 10 percent of the global CO2 emissions from human activities. The evolution of the Southern Ocean CO2 sink this century will therefore play an important role for modulating the pace of climate change. Several factors can influence the efficiency of the Southern Ocean CO2 sink. These include the rate and level of change of CO2 in the atmosphere, the associated changes in climate including warming and winds, and the changes in marine

ecosystems and biogeochemistry that occur within the ocean in response to anthropogenic drivers. CELOS will contribute to understanding how each of these sets of processes will impact the Southern Ocean CO2 sinks, and to determine how to keep track of its evolution with metrics. In particular, winds in the Southern Ocean have been observed to increase in the past 50 years. This increase has been the increase in greenhouse gases. The relative evolution of greenhouse gases and ozone recovery could therefore be important this century, and this has not yet been explored. CELOS proposes a unique contribution in the form of a limited set of model simulations that go beyond what is already done, and an analysis that explores scientific boundaries in a way never done before.

The new simulations explore three elements in detail.

Simulations will be done with the NEMO global ocean model in two configurations, one of low physical resolution (2 degrees) and one of relatively high resolution (0.25 degrees). The two resolutions mostly differ in their representation of eddies. In the low resolution, eddies are parameterised, and it is hypothesised that this parameterisation dampens the variability of the Southern Ocean CO2 sink. In the high resolution, the parameterisation of eddies is removed, and eddy activity responds explicitly to changes in ocean density gradients. This enables more mechanistic responses to changes in winds.

The role of ozone will be isolated in future projections. The UKESM will pursue simulations that they have already done, turning off interactive ozone up to year 2015, to the end of this century. They will also do one further simulation, fixing interactive ozone at its 1985 level, which assumes no recovery in ozone. Therefore, we will have three set of forcing, one with no ozone, one with ozone but no ozone recovery, and one with best estimated ozone recovery. This set will nicely border the possible evolution of ozone this century.

Simulations will be done to test the ecosystem and biogeochemical processes identified in CUSTARD and PICCOLO. Five variants are proposed initially, covering iron availability and how changes in surface ecosystems translate into changes in carbon export to the deep ocean. However, there is potential for additional simulations through the interactions with the RoSES and ORCHESTRA communities during annual meetings. Using these new simulations and existing model results from the UKESM, we will then proceed into the development of metrics to keep track of future changes. The metrics work will build on the sink rate as a measure of the efficiency of the carbon sinks in the future. It will use the identification of fingerprints of changes to identify how to keep track of the evolution of specific processes this century. It will pay particular attention to the overall efficiency of the sink and to the changes mediated by marine ecosystems and resulting carbon export.

Finally, the new insights of CELOS will be used to make recommendations for future developments, use and analysis of the UKESM.

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