Ioanna Merkouriadi

Critical Role of Snow on Sea Ice Growth in the Atlantic Sector of the Arctic Ocean

During the Norwegian young sea ICE (N-ICE2015) campaign in early 2015, a deep snow pack was observed, almost double the climatology for the region north of Svalbard. There were significant amounts of snow-ice in second-year ice (SYI), while much less in first-year ice (FYI). Here we use a 1-D snow/ice thermodynamic model, forced with reanalyses, to show that snow-ice contributes to thickness growth of SYI in absence of any bottom growth, due to the thick snow. Growth of FYI is tightly controlled by the timing of growth onset relative to precipitation events. A later growth-onset can be favorable for FYI growth due to less snow accumulation, which limits snow-ice formation. We surmise these findings are related to a phenomenon in the Atlantic sector of the Arctic, where frequent storm events bring heavy precipitation during autumn and winter, in a region with a thinning ice cover.

Influence of sea ice on the seasonal variability of hydrography and heat content in Tvärminne, Gulf of Finland

Abstract The seasonal variability of hydrography, heat content, freshwater budget and sea ice is examined for the past century in Tvärminne, Gulf of Finland. The aim was to evaluate the seasonal sea-ice impact on the physical properties of the coastal waters, which can also be subject to riverine influences. The Gulf of Finland is an optimal location for such a study, given the high interannual variability of the ice conditions. This is the first time annual cycles of physical properties have been analyzed based on such a long-term dataset in the Archipelago Sea. The maximum water temperature occurred in August at the sea surface, and at the bottom in September, with the 1 month lag due to vertical heat diffusion. The annual salinity cycle varied greatly in the surface layer. The variations were largely connected to sea-ice formation and break-up processes, and the ice cover, which prevented wind mixing of the surface waters. The largest heat loss occurred in November-December (∼150 W m-2) and the highest heat gain in June (∼180 W m-2) . Especially in April, latent heat released/absorbed in sea-ice formation/break-up contributed to a seasonal variation in heat budget of ∼40 W m -2.

Algal hot spots in a changing Arctic Ocean: Sea-ice ridges and the snow-ice interface

During the N-ICE2015 drift expedition north-west of Svalbard, we observed the establishment and development of algal communities in first-year ice (FYI) ridges and at the snow-ice interface. Despite some indications of being hot spots for biological activity, ridges are under-studied largely because they are complex structures that are difficult to sample. Snow infiltration communities can grow at the snow-ice interface when flooded. They have been commonly observed in the Antarctic, but rarely in the Arctic, where flooding is less common mainly due to a lower snow-to-ice thickness ratio. Combining biomass measurements and algal community analysis with under-ice irradiance and current measurements as well as light modeling, we comprehensively describe these two algal habitats in an Arctic pack ice environment. High biomass accumulation in ridges was facilitated by complex surfaces for algal deposition and attachment, increased light availability, and protection against strong under-ice currents. Notably, specific locations within the ridges were found to host distinct ice algal communities. The pennate diatoms Nitzschia frigida and Navicula species dominated the underside and inclined walls of submerged ice blocks, while the centric diatom Shionodiscus bioculatus dominated the top surfaces of the submerged ice blocks. Higher light levels than those in and below the sea ice, low mesozooplankton grazing, and physical concentration likely contributed to the high algal biomass at the snow-ice interface. These snow infiltration communities were dominated by Phaeocystis pouchetii and chain-forming pelagic diatoms (Fragilariopsis oceanica and Chaetoceros gelidus). Ridges are likely to form more frequently in a thinner and more dynamic ice pack, while the predicted increase in Arctic precipitation in some regions in combination with the thinning Arctic icescape might lead to larger areas of sea ice with negative freeboard and subsequent flooding during the melt season. Therefore, these two habitats are likely to become increasingly important in the new Arctic with implications for carbon export and transfer in the ice-associated ecosystem.