Experimental investigation of temperature-induced variations in the diffuse attenuation coefficient of young sea ice: depth and wavelength dependence

Abstract. The light attenuation properties of sea ice play a critical role in regulating the radiative energy budget and primary production in polar regions. Previous studies have shown that the diffuse attenuation coefficient of downwelling irradiance (K_d) of young, thin sea ice is sensitive to changes in ice temperature (T_ice). However, the depth- and wavelength-dependent behavior of K_d under changing T_ice conditions remains poorly understood. To address this gap, a comprehensive investigation was conducted using synchronized measurements of K_d  and T_ice from a cold laboratory experiment, a freezer experiment, and field observations in Liaodong Bay. The underlying mechanisms were further explored by analyzing the evolutions of sea ice microstructures and inherent optical properties, supported by Mie scattering theory and freezing equilibrium principles. Results revealed a distinct depth dependence: a negative correlation between K_d and T_ice was found in 95.96 % of surface-layer measurements, while a positive correlation occurred in 38.38 % of bottom-layer measurements. The overall correlation between K_d and T_ice was weaker in bottom layers compared to surface layers. This contrasting behavior is attributed to depth-dependent changes in the relative volume fractions of air and brine, which in turn affect scattering properties. No consistent trend in the K_d–T_ice relationship was found across wavelengths; however, increased variability was occasionally observed in the 443–555 nm range. These findings enhance our understanding of the thermal sensitivity of light attenuation in sea ice and provide valuable insights for improving radiative transfer parameterizations in climate models and marine ecosystem simulations.