Climatic turning points and regime shifts in the Baltic Sea region: the Baltic winter index (WIBIX) 1659–2002

The derived climate index is based on monthly values of the first principal component of (i) winter anomalies (January–March) of air pressure difference between Gibraltar and Reykjavik to describe the North Atlantic Oscillation, (ii) sea level anomalies of Landsort (Sweden) to characterise the filling level in the Baltic Proper, and (iii) maximum Baltic ice cover to include the influence of continentally dominated alignments of atmospheric centres of action (1897–2002). Resulting time coefficients were regressively completed by corresponding winter anomalies in air temperature over central England (1659–1896). The power spectrum of the obtained index (WIBIX 1659–2002) exhibits five significant quasi-cycles (2.2, 3, 6, 8, and 14 years), but also suggests periods of about 40 and 100 years. Severe (continental, WIBIX < 0) and mild (maritime, WIBIX > 0) winter types alternate and associated turning points characterise regime shifts affecting the ecosystem. Consequences for the Baltic fishery are discussed