The spatial distribution of solar radiation under a melting Arctic sea ice cover

The sea ice cover of the Chukchi and Beaufort Seas is currently undergoing a fundamental shift from multiyear ice to first‐year ice. Field observations of sea ice physical and optical properties were collected in this region during June–July 2010, revealing unexpectedly complex spatial distributions of solar radiation under the melt‐season ice cover. Based on our optical measurements of first‐year ice, we found the under‐ice light field in the upper ocean to be spatially heterogeneous and dependent on wavelength, ice thickness, and the areal and geometric distribution of melt ponded and bare ice surfaces. Much of the observed complexity in radiation fields arose because the transmission of light through ponded ice was generally an order of magnitude greater than through bare, unponded ice. Furthermore, while many sites exhibited a consistent, exponential decay in light transmission through both ponded and bare ice surfaces, light transmission under bare ice was also observed to increase with depth (reaching maximum values ∼5–10 m below the bottom of the ice). A simple geometric model shows these transmission peaks are a result of scattering in the ice and the interspersion of bare and ponded sea ice surfaces. These new observations of complex radiation fields beneath melt‐season first‐year sea ice have significant implications for biological production, biogeochemical processes, and the heat balance of sea ice and under‐ice ocean waters and should be carefully considered when modeling these sea ice‐related phenomena. Key Points The under‐ice light field is spatially heterogeneous and unexpectedly complex Subsurface transmission peaks are due to interspersion of bare and ponded ice The complexity of under‐ice radiation fields may impact biological processes