Bio-optical sensors would supplement satellite observations of the ocean's color by providing measurements of chlorophyll, light, and light scattering deep into the ocean interior throughout the year, in cloud - and ice - covered areas, or during the dark of polar winter. (eos.org)
The other key factor is that even if chemical conversion into reactive forms occurs during the cold, dark polar winter, the reactive chlorine must stick around until sunlight returns to the polar region for ozone destruction to take place. (realclimate.org)
I hope you agree that it is quite simple to establish that the deep ocean is cold because of the cold polar winter atmospheres. (skepticalscience.com)