WHOI PO

The observed changes in physical properties of sea ice such as decreased thickness and increased melt pond cover severely impact the energy balance of Arctic sea ice. Increased light transmission leads to increased deposition of solar energy and thus plays a crucial role for sea-ice-melt as well as for the amount and timing of under-ice primary production. Recent developments in underwater technology provide new opportunities to undertake challenging research at the largely inaccessible underside of sea ice. We measured spectral under-ice radiance and irradiance onboard the new Nereid Under-Ice (Nereid-UI) and an Ocean Modules V8Si underwater robotic vehicle, during several cruises of R/V Polarstern to the Central Arctic Ocean in 2011,2012 and 2014. Nereid-UI is a next generation hybrid remotely operated vehicle (H-ROV) designed for both remotely-piloted and autonomous surveys underneath fixed and, moving sea ice. Here we present results from the first comprehensive scientific dive of Nereid-UI employing its interdisciplinary sensor suite. We combine under-ice optical measurements with three dimensional under-ice topography (multibeam sonar) and aerial images of the surface conditions. We investigate the influence of spatially varying ice-thickness and surface properties on the spatial variability of light transmittance on floe scale. Our results show, that light transmission through sea ice has increased threefold during the last decades. A combination of ROV measurements combined with modeling and lab experiments revealed a significant deviation from the widely used assumption of an isotropic under-ice light field. This deviation from isotropy is caused by micro-scale scattering processes determined by the sea-ice texture.