Warm
rivers play role in Arctic sea ice melt
These
images show sea surface temperatures of the Beaufort Sea where
Canada's Mackenzie River discharges into the Arctic Ocean, as
measured by the Moderate Resolution Imaging Spectroradiometer (MODIS)
instrument on NASA's Terra spacecraft.
5
March 2014
Date: March
5, 2014
Source: NASA/Jet
Propulsion Laboratory
Summary:
The
heat from warm river waters draining into the Arctic Ocean is
contributing to the melting of Arctic sea ice each summer, a new NASA
study finds.
A research team led by Son Nghiem of NASA's Jet Propulsion Laboratory in Pasadena, Calif., used satellite data to measure the surface temperature of the waters discharging from a Canadian river into the icy Beaufort Sea during the summer of 2012. They observed a sudden influx of warm river waters into the sea that rapidly warmed the surface layers of the ocean, enhancing the melting of sea ice. A paper describing the study is now published online in the journal Geophysical Research Letters
This Arctic process contrasts starkly with those that occur in Antarctica, a frozen continent without any large rivers. The sea ice cover in the Southern Ocean surrounding Antarctica has been relatively stable, while Arctic sea ice has been declining rapidly over the past decade
"River
discharge is a key factor contributing to the high sensitivity of
Arctic sea ice to climate change," said Nghiem. "We found
that rivers are effective conveyers of heat across immense watersheds
in the Northern Hemisphere. These watersheds undergo continental
warming in summertime, unleashing an enormous amount of energy into
the Arctic Ocean, and enhancing sea ice melt. You don't have this in
Antarctica."
The
team said the impacts of these warm river waters are increasing due
to three factors. First, the overall volume of water discharged from
rivers into the Arctic Ocean has increased. Second, rivers are
getting warmer as their watersheds (drainage basins) heat up. And
third, Arctic sea ice cover is becoming thinner and more fragmented,
making it more vulnerable to rapid melt. In addition, as river
heating contributes to earlier and greater loss of the Arctic's
reflective sea ice cover in summer, the amount of solar heat absorbed
into the ocean increases, causing even more sea ice to melt.
To
demonstrate the extensive intrusion of warm Arctic river waters onto
the Arctic sea surface, the team selected the Mackenzie River in
western Canada. They chose the summer of 2012 because that year holds
the record for the smallest total extent of sea ice measured across
the Arctic in the more than 30 years that satellites have been making
observations.
The
researchers used data from satellite microwave sensors to examine the
extent of sea ice in the study area from 1979 to 2012 and compared it
to reports of Mackenzie River discharge. "Within this period, we
found the record largest extent of open water in the Beaufort Sea
occurred in 1998, which corresponds to the year of record high
discharge from the river," noted co-author Ignatius Rigor of the
University of Washington in Seattle.
The
team analyzed data from the Moderate Resolution Imaging
Spectroradiometer (MODIS) instrument on NASA's Terra satellite to
examine sea ice patterns and sea surface temperatures in the Beaufort
Sea. They observed that on June 14, 2012, a stretch of landfast sea
ice (sea ice that is stuck to the coastline) formed a barrier that
held the river discharge close to its delta. After the river water
broke through the ice barrier, sometime between June 14 and July 5,
the team saw that the average surface temperature of the area of open
water increased by 11.7 degrees Fahrenheit (6.5 degrees Celsius).
"When
the Mackenzie River's water is held back behind the sea ice barrier,
it accumulates and gets warmer later in the summer," said
Nghiem. "So when it breaks through the barrier, it's like a
strong surge, unleashing warmer waters into the Arctic Ocean that are
very effective at melting sea ice. Without this ice barrier, the warm
river waters would trickle out little by little, and there would be
more time for the heat to dissipate to the atmosphere and to the
cooler, deeper ocean."
"If
you have an ice cube and drop a few water droplets on it, you're not
going to see rapid melt," said co-author Dorothy Hall of NASA's
Goddard Space Flight Center in Greenbelt, Md. "But if you pour a
pitcher of warm water on the ice cube, it will appear to get smaller
before your eyes. When warm river water surges onto sea ice, the ice
melts rapidly."
Nghiem's
team has linked this sea ice barrier, which forms recurrently and
persistently in this area, to the physical characteristics of the
shallow ocean continental shelf, and concludes the seafloor plays a
role in delaying river discharge by holding the barrier in place
along the shore of the Mackenzie delta.
The
team estimated the heating power carried by the discharge of the 72
rivers in North America, Europe and Asia that flow into the Arctic
Ocean. Based on published research of their average annual river
discharge, and assuming an average summer river water temperature of
around 41 degrees Fahrenheit (5 degrees Celsius), they calculated
that the rivers are carrying as much heat into the Arctic Ocean each
year as all of the electric energy used by the state of California in
50 years at today's consumption rate.
While
MODIS can accurately measure sea surface temperature where rivers
discharge warm waters into the Arctic Ocean, researchers currently
lack reliable field measurements of subsurface temperatures across
the mouths of river channels. Nghiem said more studies are needed to
establish water temperature readings in Arctic-draining rivers to
further understand their contribution to sea ice melt.
NASA
monitors Earth's vital signs from land, air and space with a fleet of
satellites and ambitious airborne and ground-based observation
campaigns. NASA develops new ways to observe and study Earth's
interconnected natural systems with long-term data records and
computer analysis tools to better understand how our planet is
changing. The agency shares this unique knowledge with the global
community and works with institutions in the United States and around
the world that contribute to understanding and protecting our home
planet.
For
more information about NASA's Earth science activities in 2014,
visit: http://www.nasa.gov/earthrightnow
. For information on the latest NASA Earth science findings, visit:
http://www.nasa.gov/earth
.
Story
Source:
The
above story is based on materials
provided by NASA/Jet
Propulsion Laboratory.
The original article was written by Maria-Jose Vinas. Note:
Materials may be edited for content and length.
Journal
Reference:
S.
V. Nghiem, D. K. Hall, I. G. Rigor, P. Li, G. Neumann. Effects
of Mackenzie River discharge and bathymetry on sea ice in the
Beaufort Sea.
Geophysical
Research Letters,
2014; DOI: 10.1002/2013GL058956
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