Abstract
Abstract. We analyze extreme sea levels (ESLs) and related uncertainty in an ensemble of regional climate change scenarios for the Baltic Sea. The ERA-40 reanalysis and five Coupled Model Intercomparison Project phase 5 (CMIP5) global general circulation models (GCMs) have been dynamically downscaled with the coupled atmosphere–ice–ocean model RCA4-NEMO (Rossby Centre regional atmospheric model version 4 – Nucleus for European Modelling of the Ocean). The 100-year return levels along the Swedish coast in the ERA-40 hindcast are within the 95 % confidence limits of the observational estimates, except those on the west coast. The ensemble mean of the 100-year return levels averaged over the five GCMs shows biases of less than 10 cm. 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 ERA-40 shows the highest uncertainty in ESLs in the southwestern Baltic Sea and in the northeastern part of the Bothnian Bay. Some regions like 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 to 40 cm at different stations and includes the estimates based on observations. It is of similar size to the 95 % confidence limits of 100-year return levels from tide gauge records.
Highlights
The coastal area of the Baltic Sea is home to around 15 million people
The ensemble uses one regional climate models (RCMs) forced with different general circulation models (GCMs)
This allows to assess the uncertainty of 100-year return levels introduced by large-scale circulation patterns represented by the GCMs
Summary
The coastal area of the Baltic Sea is home to around 15 million people. Sea level rise (SLR) and sea level extremes in the densely populated areas are an immediate concern to the public, to authorities and to other stakeholders. Since the beginning of industrialization, the global warming trend has caused an accelerating global mean sea level (GMSL) rise (Church et al, 2013). These authors give an average of 3.2 mm a−1 GMSL rise for the period of 1993 to 2009. To assess possible trajectories of climate change and related GMSL rise, the Coupled Model Intercomparison Project phase 5 (CMIP5) (Taylor et al, 2012) has coordinated an ensemble of model runs with GCMs. To assess possible trajectories of climate change and related GMSL rise, the Coupled Model Intercomparison Project phase 5 (CMIP5) (Taylor et al, 2012) has coordinated an ensemble of model runs with GCMs These models take into account, apart from natural forcing, Representative Concentration Pathways (RCPs) of how much extra warming is projected at the end of the 21st century (van Vuuren et al, 2011). The GMSL rise in the year 2100 relative to the period of 1986 to 2005 ranges from 44 cm (RCP2.6) to 74 cm (RCP8.5), accord-
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