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Discussion papers
https://doi.org/10.5194/os-2019-65
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2019-65
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 21 Jun 2019

Submitted as: research article | 21 Jun 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

Extreme Sea Levels in the Baltic Sea under Climate Change Scenarios. Part 1: Model Validation and Sensitivity

Christian Dieterich, Matthias Gröger, Lars Arneborg, and Helén C. Andersson Christian Dieterich et al.
  • Swedish Meteorological and Hydrological Institute, Folkborgsvägen 17, 601 76 Norrköping, Sweden

Abstract. An ensemble of regional climate change scenarios for the Baltic Sea is validated and analyzed with respect to extreme sea levels (ESLs) in the recent past. The ERA40 reanalysis and five Coupled Model Intercomparison Project Phase 5 (CMIP5) global general circulation models (GCMs) have been downscaled with the coupled atmosphere-ice-ocean model RCA4-NEMO. Validation of 100-year return levels against observational estimates along the Swedish coast shows that the model estimates are within the 95 % confidence limits for most stations, except those on the west coast. The ensemble mean 100-year return levels turns out to be the best estimator with biases of less than 10 cm. The ensemble spread includes the 100-year return levels based on observations. A series of sensitivity studies explores how the choice of different parameterizations, open boundary conditions and atmospheric forcing affects the estimates of 100-year return levels. A small ensemble of different regional climate models (RCMs) forced with ERA40 shows the highest uncertainty in ESLs in the southwestern Baltic Sea and in the northeastern part of the Bothnian Bay. Also the Skagerrak, Gulf of Finland and Gulf of Riga are sensitive to the choice of the RCM. A second ensemble of one RCM forced with different GCMs uncovers a lower sensitivity of ESLs against the variance introduced by different GCMs. The uncertainty in the estimates of 100-year return levels introduced by GCMs ranges from 20 cm to 40 cm at different stations. It is of similar size as the 95 % confidence limits of 100-year return levels from observational records.

Christian Dieterich et al.
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Christian Dieterich et al.
Model code and software

NEMO-Nordic 3.3.1 for RCA4-NEMO C. Dieterich and NEMO Team https://doi.org/10.5281/zenodo.2643477

Christian Dieterich et al.
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Short summary
We assess storm surges in the Baltic Sea and how they are represented in a regional climate model. We show how well different model formulations agree with each other and how this model uncertainty relates to the observational uncertainty. With an ensemble of model solutions that represent today's climate we show that this uncertainty is of the same size as the observational uncertainty. A second part of this study compares climate uncertainty with scenario uncertainty and natural variability.
We assess storm surges in the Baltic Sea and how they are represented in a regional climate...
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