The Paradox of Antarctic Sea Ice
02.09.2009 - Atmosphere & Space, Water & Oceans, Ice & Snow, Other, Antarctic
Even as average global temperature increases, the waters around most of Antarctica are gaining sea ice. While Arctic sea ice has been retreating at a rate of about 4% per decade since the 1970s, sea ice surrounding Antarctica has been expanding northward by about 1% (about 100,000 km2) per decade over the same period.
In an effort to answer the question of these changes, scientists from NASA's Goddard Space Flight Center and the University of Washington, Seattle, recently examined three factors that could possibly explain this phenomenon:
Ozone depletion: While a hole in the ozone layer increases the amount of ultraviolet radiation reaching the planet's surface, it cools the stratosphere. Since the hole in the ozone over Antarctica began developing, average temperatures have dropped between 2 and 6°C in the stratosphere, which alters the dynamics between the stratosphere and the lower layers of the atmosphere and strengthens the already strong winds in Antarctica.
Ozone depletion has also caused atmospheric pressure to drop in certain places, which has strengthened the polar vortex. The colder, stronger winds produced allow areas of open water to form near the coast known as polynyas, which promote sea ice production. At the same time warmer air from high pressure systems have been encroaching onto the Antarctic Peninsula, which is why this area has seen dramatic sea ice loss in recent years.
Changing ocean dynamics: Jinlun Zhang, an oceanographer at the University of Washington, has devised a computer model that shows warming temperatures have led to more stratified ocean layers in the Southern Ocean. Normally the Southern Ocean has two layers - a cold layer on top and a warmer one below - which mix due to convection, bringing warmer water to the surface and limiting sea ice growth. However warming temperatures have increased the amount of rain and snowfall, causing the upper layer of the ocean to become fresher and less dense, which hinders mixing with the warmer, denser layer below, so the surface water remains colder, allowing more sea ice to form.
While it has been shown that the bottom water between Antarctica and Australia has freshened since the 1990s, further research is needed to determine whether the entire Southern Ocean is freshening.
Waterlogged sea ice: Waterlogged ice is the third theory. In a process called snow-to-ice conversion, during which so much snowfall accumulates on the sea ice it weighs it down until it's nearly submerged. Once submerged, the snow turns into snow ice. Though insubstantial over a short period of time, on a large scale over time this phenomenon could explain the increase of sea ice. One study suggests as much as 38% of sea ice in the Amundsen Sea and eastern Ross Sea consists of snow-ice.
