Transport of photosynthetically active radiation in sea ice and ocean

A recently developed radiative transfer model is applied to study the transport of photosynthetically active radiation (PAR) in the whole coupled atmosphere, sea ice and ocean system. This model rigorously accounts for the multiple scattering and absorption by the atmospheric molecules, clouds, snow and sea water, as well as the brine pockets and air bubbles trapped in sea ice. Both the spectral distribution and the seasonal variation of PAR at various levels in the ice and ocean have been investigated for different conditions. Results show that clouds, snow and ice algae all have important effects on the PAR availability to the microbial community under ice. For example, 10 cm of new snow can reduce the total PAR entering the ocean under a 1.0 m of ice by a factor of 10. The algae in the ice also significantly alters the spectral distribution of PAR transmitted to the ocean. Compared with the effects of clouds, snow and ice algae, the effect of changes in the amount of ozone in the atmosphere, the main absorptive gas in the PAR spectrum, on the amount of PAR entering the ice and ocean is negligible. The total PAR transmitted to the ocean is also sensitive to the ice thickness, especially when the ice is thin and the solar elevation is high. Clouds not only reduce the total PAR entering the ocean, but also reduce the sensitivity of this PAR variation to the variation in ice thickness.