BIOLOGICALLY-ACTIVE INSOLATION OVER ANTARCTIC WATERS - EFFECT OF A HIGHLY REFLECTING COASTLINE

Near an Antarctic coastline or sea ice edge, multiple reflection of ph otons between the high-albedo surface and a cloud will increase the do wnwelling surface insolation not only over the high-albedo surface its elf but also out over the adjacent open water. This insolation enhance ment is examined with a Monte Carlo radiative transfer model. The inso lation enhancement extends to a typical distance of 4 km out to sea, w ith the most important effects being within 2 km of the coastline. The strength of the multiple reflection effect depends primarily on cloud base height and cloud optical depth and only slightly on cloud geomet rical thickness. The insolation enhancement is also a function of wave length, being larger for ultraviolet wavelengths than for the visible. This is due to a slightly greater contribution from Rayleigh scatteri ng at the shorter wavelengths, although at ultraviolet wavelengths whe re ozone absorption is strong, tropospheric ozone absorption can offse t the Rayleigh scattering contribution at larger cloud optical depths. On the basis of the limited range of the multiple reflection effect ( 2-4 km out to sea) the insolation enhancement due to the high-albedo c oastline is unlikely to be a major influence on the primary productivi ty of all Antarctic waters; however, it may influence phytoplankton bl ooms near the coast and photobiological experiments carried out at coa stal research stations. Also, the insolation enhancement may have sign ificance in sea ice leads and polynyas.