Sea ice is a key climate factor and it restricts considerably the winter navigation in severe seasons on the Baltic Sea. So determining ice conditions severity and describing ice cover behavior at severe seasons are necessary. The ice seasons severity degree is studied at the years 1982 to 2016. A new integrative characteristic named the weighted ice days number of the season is introduced to determine the ice season severity. The ice concentration data on the Baltic Sea published in the European Copernicus Programme are used to calculate the maximal ice extent and the weighted ice days number of the seasons. Both the ice season severity characteristics are used to classify the winters with respect of severity. The ice seasons 1981/82, 1984/85, 1985/86, 1986/87, 1995/96 and 2002/03 are classified as severe by the weighted ice days number. Only three seasons of this list are severe by both the criteria. We interpret this coincidence as the evidence of enough-during extensive ice cover in these three seasons. In the winter 2010/11 ice cover extended widely for some time, but did not last longer. At 2002/03 and a few other ice seasons the Baltic Sea was ice-covered in moderate extent, but the ice cover stayed long time. For 11 winters (32 % of the period) the relational weighted ice days number differs considerably (> 10 %) from the relational maximal ice extent. These winters yield one third of the studied ice seasons. Statistically every 6th winter is severe by the weighted ice days number whereas only statistically every 8th winter is severe by the maximal ice extent on the Baltic. Hence there are more intrinsically severe seasons than the maximal ice extent gives. The maximal ice extent fails to account with the ice cover durability. The weighted ice days number enables to describe the ice cover behavior more representatively. Using the weighted ice days number adds the temporal dimension to the ice season severity study.

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The distributions of ice season characteristics in the Baltic Sea during 1982-2016 were studied. A shift in the ice season regime was observed in 2006-2007. Ice season characteristics before and after the shift were determined in the northern and southern regions of the Baltic Sea, and the contributions of these areas to the shift regime were evaluated. To study changes in ice conditions, satellite data on the daily ice concentration over the Baltic Sea provided by the Copernicus Marine Environment Monitoring Service were used. The ice cover extent and number of ice days were calculated. The maximal ice extent event of the ice season shifted from the beginning of March to the end of January in 2007. The average ice concentration over the Baltic Sea was 18% and 10% before and after the shift, respectively. On average, 32 and 19 ice days occurred before and after the shift, respectively. The average ice concentration in the northern Baltic was 54% before and 34% after the shift, and the concentration in the southern Baltic was 8% before and 7% after the shift. The determined shift of the ice season characteristics indicated that extensive ice cover does not last long during the after-shift seasons. Sažetak Proučavale su se karakteristike rasporeda ledene sezone na Baltičkome moru od godine 1982. do 2016. Promjena u režimu ledene sezone uočena je 2006. i 2007. godine. Karakteristike ledene sezone prije izmjene i nakon nje određene su u sjevernim i južnim regijama Baltičkoga mora te su evaluirani doprinosi ovih prostora na promjenu režima. Da bismo proučili promjene u uvjetima leda, satelitski podatci o koncentraciji leda nad Batičkim morem, korišteni su podatci koje je priskrbila Copernicus nadzorna služba za pomorski okoliš. Izračunata je debljina sniježnoga pokrivača i broj ledenih dana. Maksimalan razmjer ledenoga događaja, ledene sezone, pomakao se od početka ožujka do kraja siječnja 2007. Prosječna koncentracija nad Baltičkim morem bila je 18 %, a 10 % prije promjene i nakon nje, redoslijedom navođenja. Prosječno 32 i 19 ledenih dana dogodilo se prije promjene i nakon nje prema redoslijedu navođenja. Prosječna koncentracija leda na sjevernome Baltiku bila je 8 % prije, a 7 % nakon promjene, a koncentracija na južnome Baltiku bila je 8 % prije, a 7 % nakon promjene. Određena promjena karakteristika ledene sezone pokazuje da sniježni pokrivač ne traje dugo tijekom sezona nakon promjene. KEY WORDSBaltic Sea climate ice season ice concentration climatic regime regime shift ice days KLJUČNE RIJEČI klima Baltičkoga mora ledena sezona koncentracija leda klimatski režim promjena režima ledeni dani

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