Table of Content

• 1 Overview of the ice winter 2020/21
• 2 The ice winter at the German North Sea and Baltic Sea coasts
• 2.1 Weather conditions at the German coastal areas
• 2.2 Ice conditions at the German North Sea and Baltic Sea coasts
• 2.3 Navigational conditions at the German North Sea and Baltic Sea coasts
• 2.4 Ice winter severity
• 3 Ice conditions in the western and southern Baltic Sea, Kattegat and Skagerrak
• 4 Ice conditions in the northern Baltic Sea
• 5 The ice winter 2020/21 in long-term comparison
• 6 Reporting of the BSH
• 7 Bibliography
• Appendix
• A. Ice conditions at the German coasts
• B. Baltic Sea ice codes German stations

1 Overview of the ice winter 2020/21

The ice season 2020/21 at the German North Sea and Baltic Sea coast was a weak ice winter, that was dominated be an approximately two week long frost period in ther first half of February. During this period, severe to very severe frost caused ice formation along the entire German coast. Unusually warm weather in the second half of February lead to a relatively rapid melting of the sea ice. The ice season of the entire Baltic Sea was also a weak ice winter. Ice formation started late in the season and the first significant ice outside the archipelagoes of the Bay of Bothnia formed in the second half of December. At the time of the maximum ice extent the Bay of Bothnia, the Quark, coastal areas of the Sea of Bothnia, large parts of the Gulf of Finland and the Gulf of Riga, coastal areas of Germany, southern Sweden, Danmark and Norwegian fjords in the Skagerrak were covered by ice. The last ice in the Bay of Bothnia melted at the end of May.

2 The ice winter at the German North Sea and Baltic Sea coast

2.1 Weather conditions at the German coastal areas

The winter 2020/21 was dominated by a cold period in the first half of February that caused up to very severe frost at some places. Daily mean temperatures of some selected weather stations along the German coast are shown in Figure 1. Warmer and colder periods alternate in December and January but daily mean temperatures stay mostly above 0 °C. Mid of January, a short period of sub-zero daily mean temperatures occured at some stations along the German Baltic Sea coast. At the end of January a significant drop in air temperatures occured when cold polar air masses were brought to northern Germany. In the first half of February, daily mean air temperatures were partly clearly below the freezing point. The weather changed quickly mid February when warm air masses from the southwest significantly increased the daily mean air temperatures to locally more than 10 °C. The daily maximum temperatures were often the highest in a February at all times since the start of the records. Temperatures at the beginning of March were slightly colder but no significant frost occured again.

Despite the relative cold first half of February, monthly mean temperatures of the selected weather stations were above the long term mean value of the reference period from 1981-2010 for all winter months (sea Table 1). November and December were 1.6 to 2.7 °C warmer than the reference period while January, February and March were only up to 1 °C warmer. The winter 2020/21 was the tenth too warm winter in a row [1].

Compared to the last two winters, this winter was much colder. This is demonstrated in Figure 2 by the number of ice days, the days with a daily maximum temperature below the freezing point, and the cold sum, the sum of negative daily mean temperatures between November and March. The winter 2017/18 however was slightly colder than this winter; only at the North sea, there were some more ice days this winter. The past winters along the coast can all be classified as mild winters (cold sum < 100), only for Greifswald a cold sum slightly above 100 was obtained in the winter 2017/18.

The water temperatures along the German coast reflect the course of the air temperatures and the lowest temperatures are reached in the first three weeks of February. At the North Sea coast (see Figure 3) the temperatures drop continuously during the winter and reach their minimum values mid February. However, only at the measuring station in Büsum sub freezing temperatures are recorded. Along the outer coasts of the Baltic Sea, water temperatures also drop contiunously in the course of the winter and reach their minimun temperatures in mid February. Only the station in Sassnitz recorded temperatures below 0 °C for a short period. Water temperatures at the Bodden waters of the Darß and around Rügen and in the Szczecin Lagoon show the largest drop in November while the temperatures in December and January remain between 0 bis 5 °C (see Figure 5). Mid January the first temperatures below 0 °C are registered at some stations. Beginning of February most stations but Karlshagen show temperatures below the freezing point. With the warmer temperatures after mid February, water temperatures increase in all sea areas. During the beginning of march, temperatures remain quite static before they increase further.

2.2 Ice conditions at the German North Sea and Baltic Sea coasts

The ice winter 2020/21 along the German North Sea and Baltic Sea coasts followed mostly the weather pattern. A first period of sea ice formation was observed mid January in sheltered areas of the Baltic Sea. The cold temperatures in the first half of February caused ice formation along the entire North and Baltic Sea coasts.
The first ice along the German Baltic Sea coast formed during the first short frost period in mid January. Ice formation was first reported on 16.01.2021 by the ice observation stations in Schleswig at the Schlei, Neuendorf/Hiddensee and Rankwitz/Peene. In the following days ice formed in coastal areas in the Bay of Greifswald, along the river Peene and in the Szczecin Lagoon. This first short period of ice ended around the 20.01.2021 when warmer air masses were brought in by a low pressure system moving from Great Britain towards Norway. With falling temperatures, ice formation resumed in the end of January/beginning of February and increased noticeable from the 10.02.2021 onwards. The maximum ice extent was observed on the 16.02.2021. The ice situation at this point is shown in Figure 5.

The Szczecin Lagoon, the Darss-Zingst Bodden Chain, the Bodden waters around Rügen were entirely and the Greifswald Bodden mostly covered by ice. In the western Bay of Pomerania was open drift ice in a few places. More westwards, the Salzhaff and Wismar Bay up to the island Walfisch were covered by ice. In most of the inner fjords of Schleswig-Holstein new ice had formed and the Schlei was mostly covered with ice. At the outer coasts and at open sea ice was reported only very occasionally for Mecklenburg-Vorpommern and not at all for Schleswig-Holstein. With the start of the warmer weather in mid February, the thin ice vanished quickly. Only in the sheltered areas of the Bodden waters, the Greifswald Bodden and in the Szczecin Lagoon, where ice thicknesses above 20cm were reached locally, the ice remained for a longer period. The last areas were ice free on 26.02.2021. An overview of the progress of the ice winter is given by the daily areal ice volume sum of 13 ice climatological stations (Koserow, Arkona, Landtiefrinne, Vierendehlrinne, Warnemünde/Seegebiet, Rostock-Warnemünde, Walfisch-Timmendorf, Travemünde-Lübeck, Marienleuchte/Seegebiet, Westermarkelsdorf/Seegebiet, Eckernförde (Bucht und Hafen), Schleimünde-Schleswig) shown in Figure 7. The two periods of ice formation and the break in between are represented in the graph. It is also visible that the ice amount increases significantly after the 10th of February. With the warmer temperatures the daily ice volume first decreases rapidly when the thinner ice melts. The thicker and more persistant ice causes a stagnation in ice melt before the ice finally melted away.

The first ice at the North Sea coast was reported by the ice observing station at Emden Binnenhafen on the 30th January 2021. In sheltered areas, ice formed in the following days. Noticeable ice formation at the North and East Frisian Wadden Seas and the river Elbe took place after February 10th. In the Wadden Seas there was mostly very open to open drift ice with close ice at some places. Ice thicknesses were mostly about 5-15 cm. In some harbours and locally at sea, ice thicknesses up to about 40 cm were reported which mostly were caused by ice floes freezing together due to constant tidal motions of the water. The ice melted quickly with the warmer temperatures from mid February onwards. The last ice was reported on February 19th. The course of the ice winter is also shown in the daily areal ice volume sum of 13 ice climatological stations along the North Sea coast (Borkum/Westerems, Emden (Emsgebiet und Außenhafen), Norderney/Seegatt, Wangerooge/Watten, Hohe Weg/Leuchtturm, Brake (Weser), Helgoland, Stadersand/Elbegebiet, Brunsbüttel, Hamburg/Landungsbrücken, Husum (Au und Hafen), Amrum/Schmaltief, Tönnig (Hafen)) in Figure 7. Ice formation quickly increased from February 10th onwards. With warmer temperatures and the effect of the tides, the ice vanished quickly at the climatological stations.
All in all, 97 of 123 ice observation stations along the North Sea and Baltic Sea coasts reported ice for at least one day. At the Baltic Sea, the maximum number of reported ice days was 30 at the Schlei. In some places of the Bodden waters, Peenestrom and the Szczecin Lagoon ice was reported on 23-26 days. The maximum number of days with ice at the North Sea coast were reported for Amrum (Hafen Wittdün) with 19 and Lister Tief with 15.

2.3 Navigational conditions at the North Sea and Baltic Sea

At the North Sea coast, there were locally obstructions for shipping due to the ice situation. Ferry and freight traffic between Harlesiel and Wangerooge had to be suspended for some days due to the ice situation in the harbour of Harlesiel. At the German Baltic Coast only daytime navigation was allowed from February 11th in the northern approach to Stralsund (including Bodden waters west), eastern approach to Stralsund (from the sea mark „Landtief B“ to the ports of the Bight of Greifswald and to the harbour Stralsund), fairway „Osttief“ and northern Peenestrom, southern Peenestrom, Achterwasser and the Kleines Haff. From February 15th traffic in the Strelasund, eastern approach to Stralsund (from the sea mark „Landtief B“ to the ports of the Bight of Greifswald and to the harbour Stralsund) and fairway „Osttief“ and northern Peenestrom was restricted to vessels with minimum 1000 kW or vessels suitable for navigation in ice. Both restrictions were lifted on February 26th.

2.4 Ice winter severity

The ice winter severity for the German North Sea and Baltic Sea coast is defined by the areal ice volume sum of 13 ice climatological stations for each coastal area (see 2.2). The accumulated ice volume sum of the winter 2020/21 is shown in Figure 8. At the Baltic Sea coast, an areal ice volume sum of 0.37 m and at the North Sea coast of 0.26 m was calculated. Therefore, the ice winter at both German coastal areas is a weak ice winter. Moderate ice winters start at an areal ice volume sum of 0.5 m at the Baltic Sea and 0.35 m at the North Sea. It is the ninth weak winter in a row at both coasts as shown in a time series of areal ice volume sum in Figure 9 and 10. The black lines show the boundaries for weak, moderate, severe, very severe and extremely severe ice winters. The ice volume sum also reflects that the winter 2020/21 was stronger than the two previous winter but slightly weaker then the winter 2017/18. The time series also shows that since the start of the new millenium, there was only one winter, the winter 2009/2010 at the Baltic Coast, that was a strong ice winter at both coastal areas. Overall a decreasing trend in the ice winter severity can be observed. Alltogether since 1878/79 (142 years), 50 winters were equal or weaker in terms of the ice volume sum at the Baltic Sea coast compared to this winter. At the North Sea coast, 53 winters were equal or weaker compared to this winter since 1896/97 (125 years).

3 Ice conditions in the western and southern Baltic Sea, Kattegat and Skagerrak

Noticeable ice formation startet in fjords and archipelagoes of the Norwegian and Swedish coasts in Skagerrak and Kattegat in the beginning of February. In sheltered areas sea ice thicknesses of up to 15 cm were reached but in some Norwegian fjords the fast ice got up to 30 cm thick. At the end of February the area was mostly ice free. In Danish waters first sea ice formation was observed mid of January in some sheltered places, that quickly melted away. Since the beginning of February increased ice formation took place in fjords and sheltered areas. The maximum ice thickness reached up to 15 cm in some areas. Navigation was not obstructed and most of the ice was gone by the 25th of February. In the southern Baltic Sea, first ice formation started mid of January in the Szczecin Lagoon, Vistula Lagoon and Curonian Lagoon. In the Szczecin Lagoon the ice persisted only for a few days before ice formation started again with the beginning of February. Szczecin Lagoon was covered by fast ice from the 10th of February onwards that reached a maximum ice thickness of about 20 cm. The area was ice free from the beginning of March. In the Bay of Pomerania some partly open drift ice was observed from 15th - 17th of February with a thickness of up to 10 cm. Vistula Lagoon and Curonian Lagoon were continuously covered by ice and most of the ice vanished by mid March. The maximum ice thickness was about 15-25 cm.

4 Ice conditions in the northern Baltic Sea

In the Bay of Bothnia, the first ice formed in the inner archipelagoes by the end of November. It took however until the end of December that noticable ice formed outside the sheltered coastal areas. By the end of January the entire area of the Quark was covered with close or new ice, the southwest part of the Bay of Bothnia was, however, still mostly ice free at sea. In the Sea of Bothnia ice started to form in the northern part beginning of January and by mid January new ice had formed along the entire eastern coast and a bit later also along the western coast. Noticeable ice formation in the eastern Gulf of Finland started around the 10th of December. Only slowly the ice edge advaced westwards along the Finnish coast. In sheltered areas of the Gulf of Riga ice formation started in the northeastern part around the 10th of January. The maximum ice extent was reached on February 15th. The timeliest ice chart issued by the BSH on February 12th is shown in Figure 11.

At this point the entire Bay of Bothnia and the Quark were covered by ice. Along the coasts of the Sea of Bothnia, new ice or thin level ice had formed; in the archipelagoes of the eastern coast and in sheletered bays of the west coast, there was also fast ice. The Gulf of Finland was covered by very close ice in the eastern part and close ice in the west from about Tallinn to the Åland Islands. The Gulf of Riga was allmost entirely covered by new ice, with very close or fast ice in Väinameri and the northeastern part. Along the southeast coast of Sweden ice had formed in sheltered areas down to Karlskrona. The maximum thickness of the fast ice during the winter in the Bay of Bothnia was about 60-70 cm. The drift ice in the Bay of Bothnia was at maximum 50 cm thick. In the eastern Gulf of Finland the thickness of the fast ice reached about 45 cm, and the drift ice was up to 30 cm thick. Most of the ice along the east coast of Sweden vanished by mid March except in some sheltered bays and Angermanälven. In Väinameri and the northeastern Gulf of Riga some drift ice remained until the end of March and the last ice in the Bay of Pärnu vanished by the 10th of April. In the Gulf of Finland, the drift ice was pushed more and more eastwards during March but fast ice remained in the northern archipelagoes. From the 20th of April the Gulf of Finland was mostly ice free. In the Archipelago Sea and the eastern Sea of Bothnia the ice vanished in the first half of April. In the Bay of Bothnia, the drift ice was pushed eastwards by the end of March and at sea, the western Bay of Bothnia remained mostly ice free except for some sporadic ice floes. The last remaining ice in the Bay of Bothnia vanished by May 20th.
In the Bay of Bothnia restrictions for navigation regarding ice class and ship size/deadweight were in place from 27.12.2021. Later there were also restrictions in the Sea of Bothnia, the Gulf of Finland and the Gulf of Riga. Several icebreakers were deployed for navigational assistance. Navigational restrictions for the Saimaa Canal were in place from 02.01.2021 and from 08.01.2021 to 23.03.2021 the Saimaa Canal and Lake Saimaa were closed for traffic. The traffic separation scheme in the Quark was out of use between January 19th and April 9th. Transit traffic west of Holmöarna was prohibited from February 1st to March 29th.

5 Ice winter severity of the Baltic Sea

The ice winter severity for the entire Baltic Sea is determined by the maximum ice extent. This was reached on February 15th 2021 and was 127000 km² according to the Swedish and Finnish Ice Service (SMHI and FMI, respectively). From the ice charts of the BSH the maximum ice extent was 120373 km² . The differences in the ice extent stem from different land masks, differences in satellite data interpretation as well as different methods of determining the maximum ice extent, but the generally follow the same trend. This is shown by a time series of the maximum ice extent from the Finnish Ice Service and the BSH in Figure 12. The boundaries of the different ice winter severities (black lines in Figure 12) are determined with the method from Nusser [3]. The ice winter 2020/21 was a weak ice winter according to this classification. According to the Swedish Ice Service this winter is classified as a normal ice winter (115000 km² to 230000 km²) on a three fold scala of mild, normal and strong. After the record low ice extent in 2020 the extent in this winter was therefore relatively normal.
For the entire area covered by the ice charts issued from the BSH (Baltic Sea with the Swedish lakes Mälaren and Vänern, Skagerrak and North Sea up to the IJsselmeer) the maximum ice extent was reached on February 14th 2021 with 131424 km² .

6 Reporting of the BSH

The BSH informed about ice conditions and expected ice development in the entire Baltic Sea and German coastal waters by the following reports and ice charts:

106 Ice reports (Amtsblatt, official report issued Monday - Friday)
19 German Ice Reports (Information about ice conditions in German fairways)
19 Baltic Sea reports (detailed description of the ice situation along the German Baltic Sea coast)
14 North Sea reports (detailed description of the ice situation along the German Baltic Sea coast)
21 Weekly reports (Review and outlook of the ice situation)
21 Ice charts of the entire Baltic Sea (once per week)
12 Ice charts of the German coastal areas and Skagerrak (up to daily)
11 Ice charts of the German Baltic Sea coast (up to daily)

7 Bibliography

[1] Deutscher Wetterdienst, 2021: Deutschlandwetter im Winter 2020/21 [Pressemeldung], https://www.dwd.de/DE/presse/pressemitteilungen/DE/2021/20210226_deutschlandwetter_winter2020_2021_news.html
[2] Koslowski, G., 1989: Die flächenbezogene Eisvolumensumme, eine neue Maßzahl für die Bewertung des Eiswinters an der Ostseeküste Schleswig-Holsteins und ihr Zusammenhang mit dem Charakter des meteorologischen Winters. Dt. hydrogr. Z. 42, 61-80
[3] Nusser, F., 1948: Die Eisverhältnisse des Winters 1947/48 an den deutschen Küsten. Dt. hydrogr. Z. 1, 149–156

Appendix

A. Ice conditions at the German coasts

B. Baltic Sea ice codes German stations

Table of Content

• 1 Overview of the ice winter 2021/22
• 2 The ice winter at the German North Sea and Baltic Sea coasts
• 2.1 Weather conditions at the German coastal areas
• 2.2 Ice conditions at the German North Sea and Baltic Sea coasts
• 2.3 Navigational conditions at the German North Sea and Baltic Sea coasts
• 2.4 Ice winter severity
• 3 Ice conditions in the western and southern Baltic Sea, Kattegat and Skagerrak
• 4 Ice conditions in the northern Baltic Sea
• 5 The ice winter 2021/22 in long-term comparison
• 6 Reporting of the BSH
• Bibliography
• Appendix
• A. Ice conditions at the German coasts
• B. Baltic Sea ice codes German stations

1 Overview of the ice winter 2021/22

The ice winter 2021/2022 at the German North Sea and Baltic Sea coast was a weak ice winter. Noticeable ice formation occurred only in sheltered places along the Baltic Sea coast and the Szczecin Lagoon for a short-period at the end of December. Warm temperatures at the turn of the year quickly ended the ice occurrence. The ice winter for the entire Baltic Sea was a weak ice winter with respect to the maximum sea ice extent. The length of the ice season in the northern part of the Bay of Bothnia from the first ice formation in October to the melting of the last ice beginning of June was comparatively long. In other regions of the Baltic Sea the ice season was mostly shorter than on average.

2 The ice winter at the German North Sea and Baltic Sea coast

2.1 Weather conditions at the German coastal areas

The winter 2021/22 in Germany was the eleventh winter in a row that was too warm [1]. This is underlined by the comparison of the monthly mean air temperatures with the long-term mean values of the reference period 1981-2010 for four weather stations along the German coast in Table 1. All monthly mean values were above the values of the reference period. The largest deviation occurred for January and February was the with 2.8 °C to 3.8 °C. For December and March the deviations were around 0.4 °C to 1.3 °C.

The weather situation during the winter is shown in more detail by the daily mean air temperatures of these four weather stations in Figure 1. In December warmer and colder periods were alternating and the first daily mean air temperatures below freezing occurred around 10th December 2021 at the eastern stations. However, temperatures increased significantly after this short cold period. Cold airmasses of Polar origin reached Germany around the 20th December 2021 [2]. At all four stations, daily mean temperatures below 0 °C occurred some days later (see areas highlighted in blue in Figure 1). Particularly cold temperatures were measured at the stations in Schleswig and Greifswald. There was mostly moderate frost with severe frost at places and permanent frost occurred locally. Around the turn of the year temperatures rose quickly and two-digit daily mean air temperatures were reached at places. Daily mean air temperatures remained, with only a few exemptions, above the freezing point for the rest of the winter. Only at the beginning of March, there were a few days with night frost and generally colder temperatures.

The number of ice days, days with maximum temperature below 0 °C, and the cold sum, the absolute value of the sum of negative daily mean temperatures, for a number of weather stations are shown in Figure 2. The number of ice days in the winter 2021/22 is mostly lower than the number in previous winters; only Greifswald and Greifswalder Oie had more ice days than 2018/19. The very warm winter of 2019/20 had no ice days at all. The cold sum is a measure for the strength of the winter. The last four winters are all classified as a mild winter with a cold sum below 100 for the months November to March.

The water temperatures along the coast are shown in the Figures 3-5. In November and December, the water temperatures decreased constantly. The air temperature variabilities in December are also reflected in the water temperatures and most pronounced in the Bodden waters and the Szczecin Lagoon. Most of the stations reach their minimum temperature with the cold weather period at the end of December. Along the North Sea coast and the outer coastal areas, the water temperatures stayed well above 0°C for the entire winter and remained constant throughout January and February before water temperatures started rising end of March. In the Bodden waters around Rügen and the Szczecin Lagoon water temperatures below 0°C were measured during the cold period at the end of December (see areas highlighted in blue in Figure 6). In those waters temperatures started to raise slowly mid/end January and then significantly in the second half of March. The water temperatures show that ice formation was only possible in sheltered places and for a very short period. At the outer coast and the North Sea coast temperatures were too high for ice formation.

2.2 Ice conditions at the German North Sea and Baltic Sea coasts

The ice winter 2021/22 in the German coastal waters was a very short one. With the cold weather period end of December and the accordingly low air temperatures ice formation started in the Schlei at 21st December 2021 and in the harbour of Wismar on 22nd December 2021. At first the ice could not persist and around 25th/26th December 2021 ice formation started in sheltered places along the coast of Mecklenburg-Vorpommern, the Schlei and the Flensburg Fjord. The ice situation along the eastern coast at 27th December 2021 is nicely shown in an optical image of Sentinel-2 (Figure 6) and a radar image of Sentinel-1 (Figure 7). In the optical image, ice formation is visible by white structures with a dark background especially in the Szczecin Lagoon. In the Darß-Zingst Bodden Chain ice formation is hardly noticable in the image without zooming in. In the radar image, the ice formation at the Szczecin Lagoon, the Darß-Zingst Bodden Chain and the Salzhaff is clearly visible by the darker tone that is caused by dampening of surface waves due to ice formation.



At the North Sea coast, ice was only observed at the harbours of Büsum, Husum and Emden after the Christmas holidays. With significantly warmer temperatures above the freezing point and partly rainy weather over the turn of the year the ice vanished quickly. Only in the Szczecin Lagoon ice persisted at places until 4th January 2022. Due to the relative cold air temperatures and persistent frost for a few days, the ice reached up to 10 cm thickness in sheltered areas. The cold temperatures with night frost at the beginning of March caused some new ice formation in the Flensburg Fjord, the Schlei and the harbour of Wismar. The ice did mostly not survive the day.

The course of the ice winter is also reflected by the daily areal ice volume sum of 13 ice climatological stations along the North Sea and Baltic Sea coast, respectively; shown in Figure 8. The two short periods of ice formation at the end of December and beginning of March in the Baltic Sea and at the end of December in the North Sea are clearly visible. The ice climatological stations are Borkum/Westerems, Emden (Ems and outer harbour), Norderney/Seegat, Wangerooge/wadden, Hohe Weg/lighthouse, Brake (Weser), Helgoland, Stadersand/Elbe, Brunsbüttel, Hamburg/Landungsbrücken, Husum (harbour), Amrum/Schmaltief und Tönning (harbour) for the North Sea and Koserow, Arkona, Landtiefrinne, Vierendehlrinne, Warnemünde/sea area, Rostock-Warnemünde, Walfisch-Timmendorf, Travemünde-Lübeck, Marienleuchte/sea area, Westermarkelsdorf/sea area, Eckernförde (harbour), Schleimünde-Schleswig und Flensburg-Holnis for the Baltic Sea. The areal ice volume sum is a characteristic number to determine the ice winter strength along the German coasts, which is calculated from the ice concentration, the ice thickness and the duration of the ice occurrence [3]. Husum (harbour) was the only climatological station at the North Sea that reported ice during this winter. In the Baltic Sea, Flensburg-Holnis and Schleimünde-Schleswig were the only stations to report ice. The choice of the climatological stations diminishes the difference between the two short cold weather periods because the stations in the Szczecin Lagoon and the inner Bodden waters are not considered in the calculation of the areal ice volume sum.

Overall 20 of 123 ice stations along the North Sea and Baltic Sea coast reported ice during the winter 2021/22. The most days with ice were reported at the station Flensburg-Holnis with 11 days and at the Schlei with 10 days. At the North Sea coast Emden (Binnenhafen) reported 4 days with ice. An overview of all German ice observation stations that reported ice this winter is given in the appendices A und B.

2.3 Navigational conditions at the North Sea and Baltic Sea

Navigation in German coastal waters was not noticable impacted by the ice conditions in the winter 2021/22. Small craft and leisure boat traffic was locally hampered by the ice conditions.

2.4 Ice winter severity

The ice winter severity at the German North Sea and Baltic Sea coasts is determined by the accumulated areal ice volume sum of the 13 respective ice climatological ice stations (see 2.2). The accumulated areal ice volume sum of the winter 2021/22 is shown in Figure 9 for the North Sea and Baltic Sea. At the North Sea an areal ice volume of 0.004 m and at the Baltic Sea of 0.026 m is obtained. Therefore, these were the tenth weak ice winters in a row at the North Sea and Baltic Sea, as shown in Figures 9 and 10. The black lines in the figures denote the boundaries for weak, moderate, severe, very severe and extremely severe ice winters. At the Baltic sea coast, 20 winters of the time series from 1878/79 (142 years) were milder or equal than the winter 2021/22. At the North Sea, 18 winters were milder or equal since 1896/97 (125 years). The time series shows that with exception of the winter 2009/10 at the Baltic Sea there has not been a severe ice winter at the German coastal waters since the start of the new millennium.

3 Ice conditions in the western and southern Baltic Sea, Kattegat and Skagerrak

In the Skagerrak, ice has formed locally in sheltered bays along the Norwegian coast and sporadically along the Swedish coast from December into March. At a few very sheltered places, thicker fast ice occurred along the Norwegian coast. Along the Danish islands and coast, new ice formed occassionally during the winter but never survived a longer period. In the southern Baltic Sea, ice formed in the Szczecin Lagoon around 26th December 2021. The last ice melted shortly after New Year. Along the outer coasts of the southern and southeastern Baltic Sea and the southern Swedish coast no noteworthy ice has formed during the winter. Navigation was mostly unobstructed. Only in some smaller Norwegian fjords navigation was impeded or temporarily closed due to the ice conditions.

4 Ice conditions in the northern Baltic Sea

The ice season in the northern Baltic Sea was comparatively long. The first ice formed in the northern Bay of Bothnia end of October and the last ice disappeared there by the beginning of June. Further south, ice formation started earlier than usual at places but the ice season was mostly shorter than expected. Comparatively little ice formed at sea this winter. In the coastal regions however, the ice remained more or less continuously throughout the winter season. The first ice of the season formed in the northern Bay of Bothnia by the end of October. But it was not before the end of November that ice formed outside sheltered coastal areas. At that time, ice formation started also further south along the coasts of the southern Bay of Bothnia and the northern Sea of Bothnia, sheltered areas in Norra Kvarken as well as in Lake Mälaren. New ice formation in the eastern Gulf of Finland and the northern Gulf of Riga started early December. In the course of December ice formation was mostly limited to the coastal areas. The ice continuously persisted in those areas and the first fast ice formed in the northern Bay of Bothnia, Norra Kvarken and the eastern Gulf of Finland. With the beginning of January ice formation extended further out at sea in the Bay of Bothnia and the eastern Gulf of Finland. The ice could not persist at sea in the north, because higher winds destroyed the new ice or the ice drifted away. In the Gulf of Finland, the ice extended to approximately the island Seskar. At the end of January/beginning of February cold temperatures facilitated ice formation and ice growth and the maximum ice extent was reached on 3rd/4th February 2022. The ice situation at that time is shown by the BSH ice chart of 3rd February 2022 in Figure 12. At that time, the entire Bay of Bothnia and Norra Kvarken were covered by 5–20 cm thick ice, but at places the drift ice was up to 50 cm thick. Along the coast, there was up to 70 cm thick fast ice. In the Sea of Bothnia, there was fast ice along the coast and new ice further out in the east. The sea however was ice free. In the Gulf of Finland, there was fast ice along the northern and eastern coast and at sea, there was 5–30 cm thick drift ice up to the lighthouse Rodser. In the northeastern Gulf of Riga, there was thin ice in Väinameri and drift ice from Saaremaa to Pärnu. Along the Swedish coast of the northern Baltic, there was new ice at places. Further south there were only remnants of ice in the Curonian Lagoon.

The ice situation did not significantly change during the course of February and beginning of March. However, the edge of the drift ice in the Bay of Bothnia and eastern Gulf of Finland retreated slightly to the north and east, respectively. South of Norra Kvarken with the exception of the eastern Gulf of Finland, sea ice was mostly restricted to coastal areas. In the southern parts, thawing started around mid-March and the ice slowly started to melt in the Gulf of Riga and the southern Swedish coast. In the Bay of Bothnia, the drift ice moved northwards during March and reached a thickness of up to 70 cm by the end of the month. The fast ice in the area was up to 85 cm thick. In the eastern Gulf of Finland, the up to 30 cm thick drift ice at sea extended approximately to the island Moščnyj before it drifted further east beginning of April. The fast ice in the eastern Gulf of Finland was up to 45 cm thick. The last ice in the Gulf of Riga vanished in the Bay of Pärnu mid of April. While the harbour of St. Petersburg was ice free 20th April 2022, the ice in the Gulf of Finland, the Archipelago Sea and the Sea of Bothnia vanished end of April/beginning of May. At that time, there was still a larger area of thick drift ice from about Simpgrund/Falkensgrund to the Finnish coast. Off the fast ice in the north and west, there was mainly open water with some single floes. During May, the ice slowly melted in the Bay of Bothnia. In the west, the ice vanished much quicker than in the east, because westerly winds held the drifting ice at the eastern coast. Mid of May the western coast was mostly ice free, while there was still very close drift ice along the Finnish coast from Raahe to Kalajoki. The last ice melted by 3rd June 2022.
Restrictions of navigation regarding ice class and ship size/deadweight in the northern Bay of Bothnia were in place from 4th December 2021 to the 25th May 2022. At times, there were also restrictions in the Sea of Bothnia, the Gulf of Finland and the Gulf of Riga. Several icebreakers were deployed for navigational assistance. The Saimaa Canal was closed from 30th January 2022 to 30th April 2022. The traffic separation scheme in Norra Kvarken was out of use from 15th January to 2nd May 2022.

5 Ice winter severity of the Baltic Sea

The ice winter severity of the entire Baltic Sea is determined by the maximum ice extent. The maximum ice extent was reached on 4th February 2022 according to the Finnish and Swedish ice services and was about 93000 km². The maximum ice extent in the ice charts of the BSH was reached on 3rd February with 87410 km². The differences in the ice extent stem from different land masks, differences in satellite data interpretation as well as different methods of determining the maximum ice extent, but the generally follow the same trend. This is shown by a time series of maximum sea ice extent from the Finnish Ice Service and the BSH in Figure 13. The boundaries of the different ice winter severities from extremely weak to extremely strong winters (black lines in Figure 13) are determined with the method from Nusser [4]. The winter 2021/2022 was a weak ice winter according to this classification, but was just above the border to an extremely weak winter. In a threepart classification from mild to strong of the Swedish and Finnish ice services this winter was a mild ice winter (< 115000 km²). The maximum ice extent is slightly smaller than the last winter.

6 Reporting of the BSH

The BSH informed about ice conditions and expected ice development in the entire Baltic Sea and German coastal waters by the following reports and ice charts:

1. Ice reports (Amtsblatt, official report issued Monday - Friday)
2. German Ice Reports (Information about ice conditions in German fairways)
3. Baltic Sea reports (detailed description of the ice situation along the German Baltic Sea coast)
4. North Sea reports (detailed description of the ice situation along the German Baltic Sea coast)
5. Weekly reports (Review and outlook of the ice situation)
6. Ice charts of the German coastal areas and Skagerrak (up to daily)
7. Ice charts of the German coastal areas and Skagerrak (up to daily)
8. Ice charts of the German Baltic Sea coast (up to daily)

Bibliography

1. Deutscher Wetterdienst (2022, 28. Februar), Deutschlandwetter im Winter 2021/22 [Pressemitteilung], https://www.dwd.de/DE/presse/pressemitteilungen/DE/2022/20220228_deutschlandwetter_winter2022_news.html
2. Haeseler, S., Der Wetterlotse, Ausgabe 6/2021, Deutscher Wetterdienst
3. Koslowski, G., 1989: Die flächenbezogene Eisvolumensumme, eine neue Maßzahl für die Bewertung des Eiswinters an der Ostseeküste Schleswig-Holsteins und ihr Zusammenhang mit dem Charakter des meteorologischen Winters. Dt. hydrogr. Z. 42, 61-80
4. Nusser, F., 1948: Die Eisverhältnisse des Winters 1947/48 an den deutschen Küsten. Dt. hydrogr. Z. 1, 149–156

Appendix

A. Ice conditions at the German coasts

B. Baltic Sea ice codes German stations