Seasonal Evolution of Light Transmission Distributions Through Arctic Sea Ice

Light transmission through sea ice is a critical process for energy partitioning at the polar atmosphere-ice-ocean boundary. Transmission of sunlight strongly impacts sea ice melting by absorption, as well as heat deposition, and primary productivity in the upper ocean. While earlier observations relied on a limited number of point observations, the recent years have seen an increase in spatially distributed light measurements underneath sea ice using remotely operated vehicles covering a wide range of ice conditions. These measurements allow us to reconstruct the seasonal evolution of the spatial variability in light transmission. Here we present measurements of sea ice light transmittance distributions from 6 years of Arctic under-ice remotely operated vehicle operations. The data set covers the entire melt period of Central Arctic sea ice. Data are combined into a pseudo time series describing the seasonal evolution of the spatial variability of sea ice optical properties from spring to autumn freezeup. In spring, snowmelt increases light transmission continuously, until a secondary mode originating from translucent melt ponds appears in the histograms of light transmittance. This secondary mode persists long into autumn, before snowfall reduces overall light levels again. Comparison to several autonomous time series measurements from single locations confirms the detected general patterns of the seasonal evolution of light transmittance variability. This also includes characteristic spectral features caused by biological processes at the ice underside. The results allow for the evaluation of three different light transmittance parameterizations, implying that light transmission in current ice-ocean models may not be accurately represented on large scales throughout all seasons while ice thickness alone is a poor predictor of light transmittance. Plain Language Summary The interaction of sunlight with sea ice is an important key to the understanding of Arctic climate. The amount of sunlight penetrating through the ice is strongly dependent on ice and snow thickness. Optical properties of ice and snow change throughout the season. Also, sea ice properties, such as ice thickness, the cover by snow, or melt ponds are drastically changing within a few meters. To investigate the effect of these differences, we use an underwater robot to acquire high-resolution light measurements under Arctic sea ice. This enables us to describe the variations of optical properties of sea ice during the seasonal cycle. On the basis of a compilation of data from multiple expeditions, we discuss how current parameterizations in climate models need to be adapted to correctly describe the amount of light present under sea ice.