Optical properties of melting landfast sea ice and underlying seawater in Santala Bay, Gulf of Finland

In March and early April 2000, spectral irradiance measurements were conducted on 10 occasions above and beneath the landfast sea ice in Santala Bay near the entrance to the Gulf of Finland. The measurements included the spectral albedo and transmittance of the sea ice and the downwelling and upwelling spectral irradiance at different depths in the water column. Spectral albedos integrated over 400–700 nm photosynthetically active radiation (PAR) were in the range of 0.16–0.58, with most common values between 0.33 and 0.42. Maximum transmittances for the ice are found close to 570 nm, decreasing toward shorter and longer wavelengths. Correspondingly, diffuse attenuation coefficients for both seawater and sea ice agree well with earlier measurements in the Baltic Sea and have a PAR attenuation of 0.4–0.7 and 3.1–4.7 m −1 , respectively. During the measurement period the ice reached its maximum thickness of 28 cm and thereafter started to decay. The sea ice was snow free; however, the formation of high‐scattering melt/freeze layers above the freeboard largely increased the surface reflectance. Discharged meltwater was retained beneath the sea ice, forming a low‐salinity layer which only disappeared as the ice ablated in April. Compared with many parts of the Arctic, the sea ice and seawater in Santala Bay contain higher amounts of dissolved and particulate matter, which are indicated by high absorption at wavelengths below 700 nm and thus potentially increase the melt rate of the sea ice.