The index of the North Atlantic Oscillation, the dominant mode of climatic variability in the North Atlantic region, changed in the late 1980s (1987–1989) from a negative to a positive phase. This led to regime shifts in the ecology of the North Sea (NS) and the central Baltic Sea (CBS), which involved all trophic levels in the pelagial of these two neighbouring continental shelf seas. Increasing air and sea surface temperatures, which affected critical physical and biological processes, were the main direct and indirect driving forces. After 1987, phytoplankton biomass in both systems increased and the growing season was extended. The composition of phyto- and zooplankton communities in both seas changed conspicuously, e.g. dinoflagellate abundance increased and diatom abundance decreased in the CBS. Key copepod species that are essential in fish diets experienced pronounced changes in biomass. Abundance of Calanus finmarchicus (NS) and Pseudocalanus sp. (CBS) fell to low levels, whereas C. helgolandicus (NS) and Temora longicornis and Acartia spp. (CBS) were persistently abundant. These changes in biomass of different copepod species had dramatic consequences on biomass, fisheries, and landings of key fish species: North Sea cod declined, cod in the CBS remained at low levels, and CBS sprat reached unprecedented high biomass levels resulting in high yields. The synchronous regime shifts in NS and CBS resulted in profound changes in both marine ecosystems. However, the reaction of fish populations to the bottom-up mechanisms caused by the same climatic shift was very different for the three fish stocks.

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The variability of the severity of ice winters in the Western Baltic between 1879 and 1992 is statistically investigated using a time series of the accumulated areal ice volume (or VAΣ) from the Baltic coast of Schleswig‐Holstein. Lowpass filtering of the original time series shows the level of ice production in the mid 1980s to have been the same as that almost 100 years ago. A vivid interpretation is made possible of the variations in ice production by classifying VAΣ according to ice winter severity types. The increased variability of ice production since the 1920s is seen in the more frequent occurrence both of very strong and weak ice winters, while moderate and strong ice winters have decreased. The time series of the accumulated areal ice volume is negatively correlated with a temporally corresponding series of the NAO winter index, a measure of the strength of the zonal atmospheric circulation above the North Atlantic. Pearson's correlation coefficient, rp = − 0.47, exceeds the 99.9% confidence limit. In addition, a contingency table analysis revealed that this inverse correlation is due to the preferential occurrence of (a) weak ice winters with strong westerlies (NAO winter index > 1) and (b) strong to very strong ice winters with weak westerlies (NAO winter index < – 1).

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