The Effect of Light Attenuation in Water Column on Sea Ice Simulations Jinlun Zhang PSC/APL/UW Polar Science Center Jinlun Zhang Synthesis of Primary Productivity Observations Workshop 1/2008

Light attenuation/penetration in the water column affects biology. Does and to what degree light attenuation/penetration in the ocean affect the simulation of sea ice?

Light attenuation is affected by sea water, plankton algae, and other suspended or dissolved substances: R(z ) = R o *exp(−Kz) K = k 1 + k 2 *Chl + k 3 *X Not much info about K for the Arctic Ocean. Numerical experiments using varying attenuation coefficients K to examine their effects on sea ice calculations:

Three Cases Case 1: K = ∞; allows no light penetration at the ocean surface; all the net surface solar radiation used to calculate ice growth/decay. Case 2: K = 0.12 (1/m); allows certain light penetration at the ocean surface; this part of solar energy is stored in the ocean, not used to calculate ice growth/decay. Case 3: K = 0.06 (1/m); allows more light penetration at the ocean surface and less solar energy for calculating ice growth/decay.

Red: satellite data; blue: K = ∞; green: K = 0.12; yellow: K = 0.06 Observed and simulated arctic sea ice extent

Observed ice edge and simulated ice thickness, case 1: K = ∞

Ice thickness difference: case 2 minus case 1

Conclusions Allowing light penetration/attenuation in the ocean does affect sea ice simulations. This particular model overestimates ice extent in winter, but allowing light penetration in the ocean tends to improve the calculation of ice edge because of summer storage of solar energy in the ocean. Allowing no light penetration into the ocean tends to underestimate summer ice extent because all the solar energy at the surface is used to melt ice. Proper light attenuation in the ocean is likely to improve both summer and winter ice extent. Spatially and temporarily varying K is needed for model improvement.