THE WESTERN BALTIC SEA-ICE SEASON IN TERMS OF A MASS-RELATED SEVERITYINDEX - 1879-1992 .1. TEMPORAL VARIABILITY AND ASSOCIATION WITH THE NORTH-ATLANTIC OSCILLATION

The variability of the severity of ice winters in the Western Baltic b etween 1879 and 1992 is statistically investigated using a time series of the accumulated areal ice volume (or V(ASIGMA) from the Baltic coa st of Schleswig-Holstein. Lowpass filtering of the original time serie s 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 poss ible of the variations in ice production by classifying V(ASIGMA) acco rding to ice winter severity types. The increased variability of ice p roduction since the 1920s is seen in the more frequent occurrence both of very strong and weak ice winters, while moderate and strong ice wi nters have decreased. The time series of the accumulated areal ice vol ume is negatively correlated with a temporally corresponding series of the NAO winter index, a measure of the strength of the zonal atmosphe ric circulation above the North Atlantic. Pearson's correlation coeffi cient, r(p) = -0.47. exceeds the 99.9 % confidence limit. In addition, a contingency table analysis revealed that this inverse correlation i s due to the preferential occurrence of (a) weak ice winters with stro ng westerlies (NAO winter index > 1 ) and (b) strong to very strong ic e winters with weak westerlies (NAO winter index < - 1).