Arctic sea ice has declined and become thinner and younger (more seasonal) during the last decade. One consequence of this is that the surface energy budget of the Arctic Ocean is changing. While the role of surface albedo has been studied intensively, it is still widely unknown how much light penetrates through sea ice into the upper ocean, affecting sea-ice mass balance, ecosystems, and geochemical processes. Here we present the first large-scale under-ice light measurements, operating spectral radiometers on a remotely operated vehicle (ROV) under Arctic sea ice in summer. This data set is used to produce an Arctic-wide map of light distribution under summer sea ice. Our results show that transmittance through first-year ice (FYI, 0.11) was almost three times larger than through multi-year ice (MYI, 0.04), and that this is mostly caused by the larger melt-pond coverage of FYI (42 vs. 23%). Also energy absorption was 50% larger in FYI than in MYI. Thus, a continuation of the observed sea-ice changes will increase the amount of light penetrating into the Arctic Ocean, enhancing sea-ice melt and affecting sea-ice and upper-ocean ecosystems.

Measurements of solar radiation over and under sea ice have been performed on various stations in the Arctic Ocean during the Polarstern cruise ARK-XXVI/3 (TransArc) between 11 August and 16 September 2011. All radiation measurements have been performed with Ramses spectral radiometers (Trios, Rastede, Germany). All data are given in full spectral resolution interpolated to 1.0 nm, integrated over the photosynthetically active radiation (PAR: 400-700 nm), and integrated over the entire wavelength range (broadband, total: 320 to 950 nm). Two sensors were mounted on a Remotely Operated Vehicle (ROV) and one radiometer was installed on the sea ice for surface reference measurements (solar irradiance). On the ROV, one irradiance sensor (cos-collector) for energy budget calculations and one radiance sensor (7° opening angle) to obtain high resolution spatial variability were installed. Along with the radiation measurements, sea-ice thickness, snow depth, freeboard, surface scattering layer depth, and other geometric parameters were recorded. All times are given in UTC.

Supplement to: Nicolaus, Marcel; Katlein, Christian; Maslanik, James A; Hendricks, Stefan (2012): Changes in Arctic sea ice result in increasing light transmittance and absorption. Geophysical Research Letters, 39(24), L24501