Sea Ice & Snowpack

The Arctic Ocean sea ice is in constant motion, rotating, and moving with currents.  When two ice floes (sheets of floating ice) converge, a pressure ridge is created with blocks of sea ice that extend vertically (a “sail”) that can be as high as 2-3 meters or more.  They typically extend in a semi-straight line along the points of convergence.  Sea ice can be annual (first-year) or perennial (multi-year) ice.  The annual ice melts in the summer, and re-forms as winter approaches, while perennial ice is ice that remains through the summer melt-season.  The amount of multi-year ice has been steadily decreasing (http://polarportal.dk/en/sea-ice-and-icebergs/understanding-the-arctic-sea-ice/).  At this point, most of the multi-year ice is now found along the western side of the Canadian Archipelago. 

Sea ice represents a refuge and hunting ground for polar bears, and a place for dens for seals and walrus, and a foraging area for Arctic fox and other mammals.  Arctic sea ice area extent is decreasing at a rate of a little over 1%/yr., on average.  As the sea ice retreats, this impacts the ability of these species to feed and thus survive.  As discussed in the Climate Change section of this blog, loss of sea ice represents a positive feedback in the Arctic climate system, as sea ice is highly reflective (it has a high albedo), while open water is relatively dark, and absorbs sunlight and thus heat, into the Arctic Ocean.  When ocean water freezes, most of the salt is expelled, most going downward into the water, while some of the salt is expelled to cracks and grain boundaries, and to the top of the ice.  As sea ice grows and ages, the top accumulates snow, which can wick up sea salt into that snow layer. That saline snowpack can represent reactive surfaces on which oxidants like ozone can convert the sea salt halogens (chloride, bromide, and iodide) into molecular halogens that evade from the surface into the atmosphere to initiate complex chemical cycles that destroy ozone and elemental mercury.  That process requires that the snowpack is somewhat acidified (see Pratt et al., DOI:10.1038/NGEO1779) relative to the pH of ocean water (~8.2).  That can happen as a result of deposition of acids such as nitric acid and sulfuric acid from the atmosphere. 

When the wind blows over the frozen snow-covered ocean at wind speeds of ~8m/s or greater, salty snow on that surface can be lofted into the atmosphere where it can desiccate, leaving sea salt particles behind.  Stress in moving ice floes can induce fractures in the ice leading to a long stretch of open water, that may be many kms long, and as much as a km wide.  If the open water is wide enough, breaking waves can eject droplets into the atmosphere that can become sea spray aerosol.  The halides in these sea salt aerosol particles can also react with oxidants in the atmosphere to release molecular halogens into the atmosphere.  All of these topics, about sea ice and snowpack and their study, and their impacts in a changing climate are discussed in detail in the videos found at arcticstories.net.