![]() ![]() The purpose of our paper is to document whether this can lead to skilful predictions of Baltic Sea MIE using output from the same forecast system. Recently, Scaife et al ( 2014) reported skilful forecasts of the winter mean NAO index and showed that this skill translates into skilful predictions of European winter climate. In particular, Tinz ( 1996) showed that a zonal index, closely related to the NAO, could explain 54% of interannual variability in the annual maximum sea ice extent (MIE) during the period 1899–1993. ![]() Indeed, the NAO was positive during the smallest sea ice extent year of 2007/8 and negative during the severe ice winter of 2010/11 (Maidens et al 2013). Many previous studies (Koslowski and Loewe 1994, Tinz 1996, Omstedt and Chen 2001, Jevrejeva et al 2003) have shown that interannual variability of the Baltic Sea seasonal ice cover is strongly influenced by atmospheric circulation variability associated with the North Atlantic Oscillation (NAO). The need to better prepare for such expense calls for skilful seasonal forecasts of Baltic Sea ice conditions. For example, ice-breaker costs during the severe ice winter of 2010/2011, when maximum ice extent was 309 000 km 2, reached 45 million euros, more than six times larger than during the mild winter of 2007/08 (Baltic Icebreaking Management 2008, 2011). Between these two extremes the ice cover has a strong interannual variability, which has a great influence on the economics of the region (Juga et al 2012). The smallest annual maximum of ice extent of 49 000 km 2 was observed in 2007/08 (Luomaranta et al 2014). During mild winters the ice cover is restricted to the Gulf of Bothnia, the Gulf of Finland and the Gulf of Riga. ![]() During the severest of winters ice can cover the whole Baltic Sea area of 422 000 km 2, although the last time these conditions were observed was in the winter of 1946/47, more than a half century ago. Ice season lasts for 3–6 months making shipping in the ice covered areas dependent on ice-breakers (Vihma and Haapala 2009). Each winter navigation in the Baltic Sea is restricted by ice cover. ![]()
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