Trends in sea ice conditions in the Baltic Sea near the Estonian coast during the period 1949/1950–2003/2004 and their relationships to large-scale atmospheric circulation

In this paper, I study changes in the sea ice regime near the Estonian coast in 1949/1950–2003/2004. The trends in the dates of the first appearance of sea ice and final disappearance of sea ice, as well as in the number of days with sea ice, were analysed and related to the trends in air temperature and atmospheric circulation characteristics. The dependence of sea ice trends on circulation changes was estimated with the conditional Mann-Kendall test. The results revealed no similar trends in sea ice parameters in different areas of Estonia. I found a significant shift towards a later date in the first appearance of sea ice on the western Estonian coast of the inland sea, but not on the coasts of the Gulf of Finland and the Baltic Proper. This trend could not be explained by the trends in the circulation parameters. The date of the final disappearance of sea ice has shifted earlier by more than a month. The largest change has occurred at the westernmost stations and the smallest change was typical for the Gulf of Finland. This variable had a close correlation with the intensity of westerlies during the whole winter. The correlation coefficient between the date of the final disappearance of sea ice, NAO and AO indices and the frequency of the zonal circulation type W during the winter season (December–March) lay between –0.56 and –0.73. Concerning single months, the circulation in February plays a key role. The number of days with sea ice, as the main characteristic of the iciness, has decreased significantly in all the stations, except in Kunda and Narva-Jõesuu located on the southern coast of the Gulf of Finland. The most substantial decrease was observed at the westernmost stations located on the open coast of the Baltic Proper. This change was also induced by an increase in the intensity of winter westerlies.