Impact of meteorological and oceanographic conditions on the state of sea ice in Hornsund, Svalbard over 23 years

Abstract. Sea ice in fjords affects local hydrography, ecosystems, glacier stability and coastal erosion. The full archive of historical SAR imagery at ∼50 m resolution (Envisat ASAR, RADARSAT-2 and Sentinel-1) capturing the Hornsund fjord, Svalbard was used to create an unprecedented set of near-daily binary ice/open water maps over the fjord area for 23 seasons (2002–2025). We observe a general trend with the sea ice season shortening by 2.3 d yr−1, and a gradual decrease in average ice coverage, particularly in the main basin of Hornsund (−1.6 % yr−1). The interannual ice condition variability was strongly related to the autumn (October–December) and/or winter (January–March) air temperatures. The length of the sea ice season was shortened by 19.5 d for every 1 °C increase in mean autumn air temperature (R2=0.61, p<0.05). After freeze-up, landfast ice in fjords predominantly forms level ice and was here found to be preconditioned by air temperature remaining under the freezing point for over 90 d and drift ice presence in the fjord with an average coverage exceeding 20 % 40 d before the landfast ice onset and 26 % 1 d before the onset. The landfast ice season break-up period was characterised by a lack of drift ice, and positive air temperatures for over a month. The day of landfast ice freeze-up and break-up overlapped with an average sea ice thickness of 0.33 and 0.57 m as derived from thermodynamic terms, respectively, suggesting the importance of other processes.