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

The seasonal variability of hydrography, heat content, freshwater budget and sea ice is examined for the past century in Tvarminne, 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 (similar to 150 W m(-2)) and the highest heat gain in June (similar to 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 similar to 40W m(-2).