Clouds are one of the biggest wildcards in predictions of how much and how fast the Arctic will continue to warm in the future. Depending on the time of the year and the changing environment in which they form and exist, clouds can both act to warm and cool the surface below them.
For decades, scientists have assumed that losses in
Arctic sea ice cover allow for the formation of more clouds near the ocean’s
surface. Now, new NASA research shows that by releasing heat and moisture
through a large hole in sea ice known as a polynya, the exposed ocean fuels the
formation of more clouds that trap heat in the atmosphere and hinder the
refreezing of new sea ice.
The findings come from a study over a section of northern Baffin Bay between Greenland and Canada
known as the North Water Polynya. The research is among the first to probe the
interactions between the polynya and clouds with active
sensors on satellites, which allowed
scientists to analyze clouds vertically at lower and higher levels in the
atmosphere.
The approach allowed scientists to more accurately
spot how cloud formation changed near the ocean’s surface over the polynya and
the surrounding sea ice, explained Emily Monroe, an atmospheric scientist at
NASA's Langley Research Center in Hampton, Virginia, who led the study.
“Instead of relying on model output and meteorological
reanalysis to test our hypothesis, we are able to pull near-instantaneous
satellite scan data from the area near the polynya,” Monroe said. “Since each
scan is collected over a time scale on the order of about 10 seconds, it is
more likely the polynya and nearby ice are experiencing the same large-scale
weather conditions, so we can more accurately tease apart what effect the
change from ice surface to water surface is having on the overlying clouds.”
A simplified
visualization showing cloud responses before, during, and after the opening of
a large hole surrounded by sea ice known as a polynya. The insulating effect of
sea ice is seen, as the opening of the polynya facilitates heat (red) and
moisture (yellow) exchanges. Heat emitted by clouds (purple) over the ice hole
helps keep the polynya open, and remains after new sea ice closes the ice hole.
Credits: NASA’s Goddard Space Flight Center Conceptual Image Lab/Jenny
McElligott
Sea ice acts like a lid on a pot of boiling water, explained Linette Boisvert, a sea ice scientist at
NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who was part of the
study. When the lid is removed, heat and steam escape into the air.
“We're getting more heat and moisture from the ocean going into the
atmosphere because the sea ice acts like a cap or a barrier between the
relatively warm ocean surface and the cold and dry atmosphere above,” Boisvert
said. “This warming and moistening of the atmosphere slows down the vertical
growth of the sea ice, meaning that it will not be as thick, so it's more
vulnerable to melt in the summer months.”
Like other polynyas in the Arctic and
Antarctic, the North Water Polynya forms when specific wind patterns blow in a
persistent direction and tear holes in the ice. These wind patterns only exist
in the winter months, and the holes open and close repeatedly, alternately
exposing and insulating the ocean.
The western edge of the North Water Polynya seen during an Operation IceBridge flight on April 3, 2019. The polynya, a large patch of exposed ocean within an area of substantial sea ice cover, opens four to five times during the colder months. The extent of the North Water Polynya varies from year to year, but can be large enough to cover the area of entire U.S. states such as Virginia. Credits: NASA/Jeremy Harbeck
The new insights come during a time when
Arctic sea ice appears to have hit its annual minimum extent after waning during the warmer months in 2021. They underscore how
sea ice influences a region that plays an integral role in regulating the pace
of global warming, sea level rise, and other effects of human-caused climate change.
Sea ice does not raise global sea levels directly.
Like ice cubes in a drink, melting sea ice does not directly increase the
volume of water in the ocean. Still, a shrinking Arctic sea ice extent can
expose relatively warm sea water to the region’s coastal ice sheets and
glaciers, causing more melting that contributes freshwater to the ocean and
does cause sea level rise.
The new research shows low clouds over the polynya
emitted more energy or heat than clouds in adjacent areas covered by sea ice.
Those low clouds contained more liquid water, too—nearly four times higher than
clouds over nearby sea ice. The increased cloud cover and heat under the clouds
persisted for about a week after each occasion the polynya refroze during the
time span of the study.
“Just because the sea ice reforms and the polynya
closes up, that doesn't mean that conditions go back to normal right away,”
Boisvert said. “Even though the moisture sources are essentially gone, this
effect of extra clouds and increased cloud radiative effect to the surface
remains for a time after [the polynya freezes].”
The findings also suggest the response of the clouds
to the polynya lengthened the time the hole remained open, said Patrick Taylor, a climate scientist at NASA Langley, who also was part of the study.
“They can create a thicker blanket and increase the
amount of heat emitted down to the surface,” Taylor said. “The emitted heat
helps keep the surface of the North Water Polynya a little warmer and helps
prolong the event itself.”
Large-scale meteorological processes often make
studies of Arctic warming difficult. However, repeated openings in the sea ice
in the same region create a natural laboratory to study the feedback between
clouds and the alternation between sea ice and polynyas.
“We can compare both sea ice and open water areas, and
the clouds over those two surface types in close enough proximity, so that we
don't have to worry about large changes in atmospheric conditions that have
confounded previous studies,” Taylor said. “If there's not a cloud response to
a polynya event where sea ice goes away over the course of a few days, you
wouldn't expect a response anywhere else. The opening of a polynya is a very
strong, distinct forcing.”
The team is planning to take their research to the
next level and test whether a similar cloud effect can be observed in other
areas where sea ice and open ocean meet.
Banner image: A section of the North Water Polynya and
adjacent sea ice seen during an Operation IceBridge flight on April 19, 2016.
Moisture evaporated from the ocean is seen condensing into small clouds. Credit: NASA/Jeremy Harbeck
By Roberto Molar Candanosa
NASA's Earth Science News Team
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