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a core project of
2018WCRPspon col July2018 01 1

Quantifying the influence of sea ice internal variability in CMIP5 models

Dirk Olonscheck (dirk.olonscheck[@]mpimet.mpg.de)

1) Changes in internal variability under global warming (completed)

Scientific questions:
1. How much does the manifestation of sea ice internal variability differ for global climate models?
2. How do unforced climate variability through time and the ensemble spread compare to each other?
3. How does internal variability of climate variables change under strong CO2 forcing?
4. How plausible are CMIP5 sea-ice simulations when consistently taking model-specific estimates of internal variability into account?

To answer these questions, we developed a simple method that regresses each model’s estimate of internal variability from the pre-industrial control simulation to the variability derived from a model’s ensemble simulations. We find that this method is well suited to test for changes in internal variability across multiple models for any given forcing.

For details, see
Olonscheck D., and D. Notz (2017), "Consistently Estimating Internal Climate Variability from Climate Model Simulations", J. Climate 30, 9555–9573, doi:10.1175/JCLI-D-16-0428.1

2) Drivers of sea-ice variability (completed)

Scientific questions:
1. Which processes cause sea-ice variability?
2. How much do these drivers contribute to the total variability?

To answer these questions, we continued to develop the method of “feedback locking” within MPI-ESM. We quantified how much of the variability of sea ice arises from climate feedbacks and forcings such as the surface-albedo, cloud, and water-vapour feedback, as well as surface winds and fluctuations in oceanic heat transport. Based on the minor contributions from these processes, we show that Arctic sea-ice variability is primarily caused by fluctuations in advected atmospheric temperature, which is consistent across global climate models and observations.

For details, see
Olonscheck D., T. Mauritsen, and D. Notz (2019), "Arctic sea-ice variability is primarily driven by atmospheric temperature fluctuations", Nat. Geoscience 12, 430–434, doi:10.1038/s41561-019-0363-1

3) Progress in representing the sea-ice evolution with climate models (open)

Scientific questions:
1. How much does the representation of the observed Arctic sea-ice evolution change for the inter-comparison projects CMIP3 to CMIP6? Which role plays internal variability?
2. How much do the estimates of sea-ice sensitivity change from CMIP3 to CMIP6?

To answer these questions, we analyse sea ice, surface air temperature and emission data from observations, CMIP5, CMIP6, and large ensembles. This is collaborative work with other project partners.