Arctic ocean moorings shed light on winter sea ice loss
Date:
August 21, 2020
Source:
University of Alaska Fairbanks
Summary:
The eastern Arctic Ocean's winter ice grew less than half as much
as normal during the past decade, due to the growing influence of
heat from the ocean's interior, researchers have found.
FULL STORY ==========================================================================
The eastern Arctic Ocean's winter ice grew less than half as much as
normal during the past decade, due to the growing influence of heat from
the ocean's interior, researchers have found.
==========================================================================
The finding came from an international study led by the University
of Alaska Fairbanks and Finnish Meteorological Institute. The study,
published in the Journal of Climate, used data collected by ocean moorings
in the Eurasian Basin of the Arctic Ocean from 2003-2018.
The moorings measured the heat released from the ocean interior to the
upper ocean and sea ice during winter. In 2016-2018, the estimated heat
flux was about 10 watts per square meter, which is enough to prevent 80-90 centimeters (almost 3 feet) of sea ice from forming each year. Previous
heat flux measurements were about half of that much.
"In the past, when weighing the contribution of atmosphere and ocean to
melting sea ice in the Eurasian Basin, the atmosphere led," said Igor
Polyakov, an oceanographer at UAF's International Arctic Research Center
and FMI. "Now for the first time, ocean leads. That's a big change."
Typically, across much of the Arctic a thick layer of cold fresher water,
known as a halocline, isolates the heat associated with the intruding
Atlantic water from the sea surface and from sea ice.
This new study shows that an abnormal influx of salty warm water from
the Atlantic Ocean is weakening and thinning the halocline, allowing
more mixing.
According to the new study, warm water of Atlantic origin is now moving
much closer to the surface.
==========================================================================
"The normal position of the upper boundary of this water in this region
was about 150 meters. Now this water is at 80 meters," explained Polyakov.
A natural winter process increases this mixing. As sea water freezes,
the salt is expelled from ice into the water. This brine-enriched water
is heavier and sinks. In the absence of a strong halocline, the cold
salty water mixes much more efficiently with the shallower, warm Atlantic water. This heat is then transferred upward to the bottom of sea ice,
limiting the amount of ice that can form during winter.
"These new results show the growing and spreading influence of heat
associated with Atlantic water entering the Arctic Ocean," added Tom
Rippeth, a collaborator from Bangor University. "They also suggest a
new feedback mechanism is contributing to accelerating sea ice loss."
Polyakov and his team hypothesize that the ocean's ability to control
winter ice growth creates feedback that speeds overall sea ice loss in
the Arctic. In this feedback, both declining sea ice and the weakening halocline barrier cause the ocean's interior to release heat to the
surface, resulting in further sea ice loss. The mechanism augments the well-known ice-albedo feedback -- which occurs when the atmosphere
melts sea ice, causing open water, which in turn absorbs more heat,
melting more sea ice.
When these two feedback mechanisms combine, they accelerate sea ice
decline.
The ocean heat feedback limits sea ice growth in winter, while the
ice-albedo feedback more easily melts the thinner ice in summer.
==========================================================================
"As they start working together, the coupling between the atmosphere,
ice and ocean becomes very strong, much stronger than it was before,"
said Polyakov.
"Together they can maintain a very fast rate of ice melt in the Arctic." Polyakov and Rippeth collaborated on a second, associated study showing
how this new coupling between the ocean, ice and atmosphere is responsible
for stronger currents in the eastern Arctic Ocean.
According to that research, between 2004-2018 the currents in the upper
164 feet of the ocean doubled in strength. Loss of sea ice, making
surface waters more susceptible to the effects of wind, appears to be
one of the factors contributing to the increase.
The stronger currents create more turbulence, which increases the amount
of mixing, known as shear, that occurs between surface waters and the
deeper ocean. As described earlier, ocean mixing contributes to a feedback mechanism that further accelerates sea ice decline.
Accelerated currents have practical implications in the Arctic. Ship
captains need accurate maps of currents for navigation. Since currents
move sea ice, oil and gas extraction activities also need information
about currents.
========================================================================== Story Source: Materials provided by University_of_Alaska_Fairbanks. Note: Content may be edited for style and length.
========================================================================== Journal Reference:
1. Igor V. Polyakov, Tom P. Rippeth, Ilker Fer, Matthew B. Alkire,
Till M.
Baumann, Eddy C. Carmack, Randi Ingvaldsen, Vladimir V. Ivanov,
Markus Janout, Sigrid Lind, Laurie Padman, Andrey V. Pnyushkov,
Robert Rember.
Weakening of Cold Halocline Layer Exposes Sea Ice to Oceanic Heat
in the Eastern Arctic Ocean. Journal of Climate, 2020; 33 (18):
8107 DOI: 10.1175/JCLI-D-19-0976.1 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2020/08/200821155746.htm
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