## 4 Synoptic Situation and Sea Ice Conditions  
The analysis of the synoptic situation, including the passages of fronts and sea ice conditions was based on the daily FMI operational weather analysis and ice charts. Until recently, the Bothnian Bay has been entirely frozen every winter. However, the ice thickness, the maximum annual ice extent, and the length of the ice season have shown decreasing trends in recent decades [95]. Winters 2014/2015 (Uotila2015) and 2015/2016 were the first for which we can be certain that parts of the Bothnian Bay remained ice-free. The maximum ice extent is typically reached in March. In the shallow waters close to the coast, land-fast ice prevails and can grow up to a thickness of 0.8 m. Even in mild winters, the level ice thickness reaches 0.3 m to 0.5 m. The land-fast ice is typically free of leads, and the compact sea ice field with snow pack on top effectively insulates the atmosphere from the relatively warm sea. The sea ice season 2016/2017 was mild in the Baltic Sea. Its length in the Bothnian Bay was, however, close to the average of 1965-1986 (reference period used in FMI ice service). The ice growth started during the first half of November 2016 and was fast during a cold period in early January, leading to an overall ice extent of 44,000 km\({}^{2}\) in Bothnian Bay. Shortly thereafter, temperatures increased and for the rest of the month, mild southwesterns prevailed, preventing new ice formation and packing the ice densely towards the coast within Bothnian Bay. By the end of January, the Baltic Sea ice extent had reduced to only 28,000 km\({}^{2}\). Figure 4: Data availability and corresponding altitude (only for profiling systems) for the different observation systems during the campaign period. Green indicates the availability of good quality data where applicable; orange corresponds to available data of limited quality; and the white gaps indicate poor quality or missing data. In the beginning of February, a large high pressure system strengthened over Finland, causing fair weather and occasional extremely cold temperatures. Especially from 6 to 9 February, there were very cold temperatures in most of the country. The ice extent increased then rapidly, and a maximum ice extent in the Baltic Sea of 88,000 km\({}^{2}\) was observed on 12 February. At this time, Bothnian Bay was almost completely ice-covered by 10 cm to 25 cm thick drift ice, and the thickness of the land-fast ice was between 5 cm to 55 cm, as shown in Figure 5 (left panel). In the middle of February, a westerly to northwesterly flow pattern strengthened over the region, causing dry and warm Fohn wind from the Scandinavian Mountains. Over Bothnian Bay, the ice field was packed against the Northeastern coast, and a large ice-free area in the center of the Bay formed (Figure 5, right panel). Almost all ships to Oulu, Kemi, and Tornio had to be assisted by ice breakers. In the end of February, ice extent of the Baltic Sea was 77,000 km\({}^{2}\). On Hailuoto, the 2-m air temperature was 2 \({}^{\circ}\)C higher and the 10-m wind speed was 0.5 m s\({}^{-1}\) lower than the climatological mean values for February during 1981-2010. In the first week of the ISOBAR campaign Hailuoto-I, from 11 to 18 February, the synoptic-scale conditions were characterized by a high-pressure center, first located over Southern Scandinavia and then moving over Central and Eastern Europe. Low pressure systems were passing over the North Atlantic, Norwegian Sea, and Barents Sea from southwest to northeast, resulting in variable winds, occasionally approaching 20 m s\({}^{-1}\) in Hailuoto (Figure 6). Depending on the air-mass origin, wind speed, and cloud cover, the 2-m air temperature in Hailuoto varied between \(-17\,^{\circ}\)C to 4 \({}^{\circ}\)C (Figure 6). By 19 February, the high pressure center had moved north of the Azores, and a small low pressure system passed over Europe during 19 to 24 February. A passage of a warm front resulted in snow fall (8 mm water equivalent) on 23 February.