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A new study on the freezing and melting cycles of the Arctic sea ice has shed more light into the exchanges of heat between the atmosphere and the ocean. This is crucial to study the impact of climate change on the Arctic Ocean, as past reports show us temperatures in the Arctic are rising four times as fast as in the rest of the world.
The surface and bottom of the Arctic sea ice often do not melt and freeze simultaneously, a new study has found. Researchers from China, Europe and the United States published their findings in the not-for-profit scientific journal The Cryosphere.
Until recently, most studies monitored sea ice melting and freezing only from the surface. Scientists have now measured the ice’s surface and bottom to paint a complete picture of the freeze-thaw cycle of the Arctic sea ice.
The Arctic sea ice is seasonal. It forms in the winter when seawater freezes into massive floating ice blocks and thaws during the summer. When sea ice is present, it insulates the ocean against heat loss.
The sea ice reflects sunlight into space, regulating ocean and air temperature and maintaining habitats above and below the water, according to the US’s National Aeronautics and Space Administration (NASA).
Understanding the timings of the thawing of sea ice and freeze onsets, as well as the lengths of the melt and freeze seasons is crucial. These play a vital role in the “heat budget” of the atmosphere-ice-ocean system.
A heat budget is a balance between incoming heat absorbed by the earth and outgoing heat escaping it.
The researchers analysed data from 2001 to 2018 to explore how melting and freezing vary at the ice surface and bottom. They also were keen to understand the mechanism behind them.
At the Beaufort Gyre — a major ocean current in the Arctic Ocean — the average ice melt at the bottom began on May 23, approximately 17 days earlier than the surface melt onset, the study found.
The Gyre impacts climate by setting temperature and salt levels in the Arctic, Isabela Le Bras from the Woods Hole Oceanographic Institution said in an interview with the Institute.
“The longer melt season under the ice is likely playing a key role in the enhanced thinning of the ice that is keeping our summer extents low,” Julienne Stroeve, a senior scientist at the National Snow and Ice Data Center, told Down To Earth. Stroeve was not involved in the study.
The Arctic sea ice extent or cover has been on a decline, especially in late summer, according to NASA. These new findings “can improve our understanding of how much ice is melted each summer,” the expert added.
Further, the average ice melt at the bottom began at the central Arctic Ocean on June 17. This is comparable with the surface melt.
The differences in the onset between Beaufort Gyre and the central Arctic Ocean could be because of the two regions’ seasonal variations in oceanic heat available for melting.
For example, earlier warming of the surface ocean in Beaufort Gyre could be responsible for the earlier onset of melting under the ice relative to the surface. As for the ice growth or freezing, the bottom lagged behind the surface by three months across the Arctic Ocean, the findings stated.
The freeze-delay of the ice at the basal region (bottom) is relative to the surface, lead author Long Lin from the Polar Research Institute of China said in a statement. It can be attributed to the regulation of the sea ice’s heat capacity and the heat released from the ocean-mixed layer, which is tens of metres deep and the subsurface layer.
However, the researchers also observed a trend towards earlier ice growth at the bottom. This could be due to thinner ice thickness and snow cover, according to Lin. Still, he added that a longer freezing season due to thinning of ice could not compensate for the ice lost in summer.
This study sheds more light on the processes in the Arctic Ocean, Stroeve said, adding that it is a fascinating study in terms of the differences in the timing of melt onset and freeze-up from below compared to what she has measured from the top.