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The Atlantic Multidecadal Oscillation (AMO) is a natural mode of variability of the North Atlantic sea surface temperature. The AMO can be used to describe the complex interaction of the coupled atmosphere-ocean system of the North Atlantic. By analyzing a preindustrial period of 850 years with a regional climate model, we show that the AMO influences the Baltic Sea. AMO-related changes of the atmospheric circulation affect precipitation over the Baltic Sea region, which leads to altered river runoff influencing the salinity of the Baltic Sea. A wavelet coherence analysis reveals a persistent coherence between AMO and salinity for the whole period of 850 years. This suggests that the Baltic Sea is under the constant influence of the AMO. Our results provide strong evidence for long-term changes in the Baltic Sea as a result of changing AMO phases.

Plain Language Summary Coastal seas are of great importance to society. A prominent example of such a coastal sea is the Baltic Sea, since it is strongly impacted by human activities. However, besides the human footprint there are also natural phenomena, that influence the Baltic Sea. Especially climate phenomena over the North Atlantic can have a strong impact on the Baltic Sea. One such phenomenon is the so-called Atlantic Multidecadal Oscillation (AMO), a seesaw between warm and cold sea surface temperatures in the North Atlantic with a period of 60-90 years. Reliable observations only exist for a period of about 150 years, which is too short to study multidecadal time scales. Therefore, we use an 850 years long model simulation. Our results show that changes in North Atlantic sea surface temperature associated with the AMO influence the atmospheric circulation, which impacts the rain and snowfall over the Baltic Sea region. This in turn enhances or decreases the river runoff into the Baltic Sea and thus impacts the Baltic Sea salinity. Thus, the AMO has a strong influence on the Baltic Sea.