Interannual sea ice thickness variability in the Bay of Bothnia

Iina Ronkainen,Jari Haapala,Eero Rinne,Mikko Lensu,Christian Haas,Jonni Lehtiranta

doi:10.5194/tc-12-3459-2018

Abstract

Abstract. While variations of Baltic Sea ice extent and thickness have been extensively studied, there is little information about drift ice thickness, distribution, and its variability. In our study, we quantify the interannual variability of sea ice thickness in the Bay of Bothnia during the years 2003–2016. We use various different data sets: official ice charts, drilling data from the regular monitoring stations in the coastal fast ice zone, and helicopter and shipborne electromagnetic soundings. We analyze the different data sets and compare them to each other to characterize the interannual variability, to discuss the ratio of level and deformed ice, and to derive ice thickness distributions in the drift ice zone. In the fast ice zone the average ice thickness is 0.58±0.13 m. Deformed ice increases the variability of ice conditions in the drift ice zone, where the average ice thickness is 0.92±0.33 m. On average, the fraction of deformed ice is 50 % to 70 % of the total volume. In heavily ridged ice regions near the coast, mean ice thickness is approximately half a meter thicker than that of pure thermodynamically grown fast ice. Drift ice exhibits larger interannual variability than fast ice.

Highlights

The Baltic Sea belongs to the seasonal sea ice zone, extending from 54 to 66◦ N, and has a total area of 422 000 km2
According to the classification of the Finnish Ice Service, the Baltic ice season is mild when the ice extent is below 115 000 km2, severe when the extent is above 230 000 km2, and extremely severe when it is above 345 000 km2
The Bay of Bothnia is the northernmost basin of the Baltic Sea north of the sound of Quark at 63.5◦ N (Fig. 1)

Summary

Introduction

The Baltic Sea belongs to the seasonal sea ice zone, extending from 54 to 66◦ N, and has a total area of 422 000 km. The Bay of Bothnia is the northernmost basin of the Baltic Sea north of the sound of Quark at 63.5◦ N (Fig. 1). It is a semi-enclosed basin with a length of about 300 km, width of 100–150 km, and an area of approximately 36 000 km. During the last 100 years the Bay of Bothnia has always frozen completely over except in the extremely mild winters of 2014-2015 and most probably in 1929-1930 (Uotila et al, 2015). In the drift ice zone, the flux of the ice trough in the 25 km wide passage of Quark is small and has only a minor effect on the ice mass balance of the basin, which is mostly determined by the thermodynamic and dynamic processes within the basin itself; i.e., according to our understanding the exchange of ice with the Gulf of Bothnia in the south is negligible

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Discussion

Conclusion