Baltic Sea ice seasons in the twentieth century

Abstract

We examine the evolution of ice seasons in the Baltic Sea during the 20th century based on a set of 37 time series from the coastal observation stations. The statistical question of combining data from sites with different ice probabilities is solved by using fractiles of the distributions. These 100 yr long time series, including date of freezing, ice break-up, number of days with ice, and maximum annual ice thickness, provide evidence of a general trend toward easier ice conditions; the largest change is in the length of ice season, which is decreasing by 14 to 44 d per century. The trends of a reduction of about 8 to 20 d per century to earliest ice break-up are in a good agreement with a warming trend in winter air temperature over Europe. A statistically significant decreasing trend in probability of ice occurrence in the southern part of the Baltic Sea was detected; however, there is no change in probability of ice occurrence in the northern part. © Inter-Research 2004.