Oceanographic regional climate projections for the Baltic Sea until 2100

Abstract

Abstract. Recently performed scenario simulations for the Baltic Sea including marine biogeochemistry were analyzed and compared with earlier published projections. The Baltic Sea, located in northern Europe, is a semi-enclosed, shallow and tide-less sea with seasonal sea ice cover in its northern sub-basins and a long residence time causing oxygen depletion in the bottom water of the southern sub-basins. With the help of dynamical downscaling using a regional coupled atmosphere-ocean climate model, four global Earth System Models were regionalized. As the regional climate model does not include components for the terrestrial and marine biogeochemistry, an additional catchment and coupled physical-biogeochemical model for the Baltic Sea were used. In addition to previous scenario simulations, the impact of various water level scenarios was examined as well. The projections suggest higher water temperatures, a shallower mixed layer with sharper thermocline during summer, reduced sea ice cover and intensified mixing in the northern Baltic Sea during winter compared to present climate. Both frequency and duration of marine heat waves would increase significantly, in particular in the coastal zone of the southern Baltic Sea (except in regions with frequent upwelling). Due to the uncertainties in projections of the regional wind, water cycle and global sea level rise, robust and statistically significant salinity changes cannot be identified. The impact of changing climate on biogeochemical cycling is considerable but in any case smaller than the impact of plausible nutrient input changes. Implementing the proposed Baltic Sea Action Plan, a nutrient input abatement plan for the entire catchment area, would result in a significantly improved ecological status of the Baltic Sea and reduced hypoxic area also in future climate, strengthening the resilience of the Baltic Sea against anticipated future climate change. While our findings about changes in variables of the heat cycle mainly confirm earlier scenario simulations, earlier projections for salinity and biogeochemical cycles differ substantially because of different experimental setups and different bioavailable nutrient input scenarios. During the time in which this paper was prepared, shortly before submission, Christian Dieterich passed away (1964–2021). This sad event marked the end of the life of a distinguished oceanographer and climate scientist who made important contributions to the climate modeling of the Baltic Sea, North Sea and North Atlantic regions.\xa0\n