The western Baltic sea ice season in terms of a mass-related severity index: 1879-1992 - Part I. Temporal variability and association with the north Atlantic oscillation

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 V_AΣ) 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 V_AΣ 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, r_p = -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).