This paper focuses on relationships between the North Atlantic Oscillation (NAO) Index in selected months and periods and the ice conditions along the southern coast of the Baltic Sea. The data on ice conditions, including the number of days with ice (L) and the length of ice season (counted in days; S), cover 25 observation regions located between Arkona in the West and Kaliningrad in the East for winters of the 1950/51–1989/90 period. The NAO Index, understood to be the difference between normalized values of atmospheric pressure in Gibraltar and Reykjavik, was applied.
Correlation and regression analysis methods were applied to determine and study the relationships between NAO (independent variable) and the parameters of ice conditions L and S (dependent variables). The linear relationships recorded for sheltered basins (Szczecin Lagoon, Puck Bay Small, Vistula Lagoon) were stronger than those calculated for unsheltered ones (e.g., open Baltic Sea coast). The majority of relationships of ice conditions (L and S) to winter monthly NAO values, particularly for January (NAOII) and February (NAOII) in sheltered basins, are statistically significant, some of them even at the level of α = 0.01. The linear correlation coefficients, in most cases, were included within the range from −0.3 to −0.7. The strongest of them were calculated for January and February and encompassed the range from −0.5 to −0.7. The strength of these relationships tended to increase when particular months were connected into several-month periods of NAO Index, especially during the January–March and December–March periods. During these periods the correlation coefficients were usually in the −0.7 to −0.8 interval. In addition, the relationships of the NAO index with L were distinctly stronger than those with S. The strongest relationships, with linear correlation coefficients exceeding −0.80, were obtained for the number of days with ice (L) in protected areas, e.g., in Vistula Lagoon for a period of December–March (NAOXII–III) and in Puck Bay for January–March period (NAOI–III). These relationships are weaker in unsheltered areas, where the correlation coefficient does not exceed the value of −0.75. The strongest of them concern the estuarine regions of large rivers (Dziwnów, Świbno) and bays (Mie dzyzdroje), whereas the relationships between the NAO Index and L and S did not differ significantly from each other in unsheltered basins. However, for the NAO periods of December–March and January–March, they were stronger with S than with L.
The correlation coefficients show a decreasing tendency eastward along the southern Baltic Sea coast from Pomeranian Bay to Gdańsk Bay (northward of the Hel Peninsula). They become stronger in the western part of the Gulf of Gdańsk and in the mouth of the Vistula and again decrease eastward.
During some extremely severe or very mild winters, the extreme ice conditions do not always match the extreme NAO index values. During extremely severe winters, the ice cover could exist for a relatively long period, which is a result not only of the high inertia of thick ice covers but also of high cloudiness restricting the impact of insolation. However, during extremely mild winters, ice can disappear relatively quickly (thin covers, low cloudiness), comparable to the NAO index values.
