The global climate is significantly influenced by ocean currents. The most significant of these is the global conveyor belt, also called the thermohaline circulation, which connects the major oceans. It is driven by global density differences of water masses, which are determined by temperature and salinity. The impetus for the Earth-wide circulation essentially occurs where cold, salty water sinks to depth due to its higher density, especially in the North Atlantic. In addition to these important deep water formation zones, however, ice that forms below the ocean surface in Antarctica also plays a crucial role, as a Japanese research team has now discovered. The study was published in the journal Science Advances.
The Southern Ocean around Antarctica connects the Atlantic, Pacific, and Indian Oceans, making it a central component in global circulation. Large amounts of sea ice form near the Antarctic continent, and salt is not trapped in the ice, leaving behind salty and cold, i.e. dense, water that sinks to the ocean floor. This so-called Antarctic Bottom Water (AABW) is the coldest and densest water mass in the thermohaline circulation, and it spreads from there over most of the global deep sea.
As thermohaline circulation affects global climate, Kay Ohshima of Hokkaido University and his team wanted to understand the mechanism of AABW formation and how global warming affects it. “We have found surprising new results about the shape of sea ice growth in a key AABW production area near Cape Darnley in Antarctica that may have far-reaching implications for other areas,” Kay says.
Satellite monitoring and ocean-anchored sensors provided data that showed the importance of so-called “frazil ice” in the formation of dense, cold water. This needle-shaped ice forms below the ocean surface when strong cold winds continue to cool and mix the water, causing its temperature to drop below freezing. Depending on wind strength and local conditions, this cooling can occur to depths of 80 meters or more.
Frazil ice is formed mainly where katabatic winds from the Antarctic continent meet open water, for example in coastal polynyas or polynyas in the pack ice.
“It’s important to learn that such an important process is occurring underwater, revealing an aspect of the circulation system that has been at least partially lost from view,” Kay said.
Even in nutrient cycles, frazil ice could play a role. The research team suspects that the ice crystals on the seafloor take up sediment, which they release as they melt, fertilizing plankton and affecting the overall biological productivity of the waters around Antarctica.
“Our next step is to incorporate these new processes into understanding Southern Ocean biogeochemistry and carbon circulation, which will require extensive new fieldwork and research,” Kay concludes.
Julia Hager, PolarJournal
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