Unpacking polar sea ice
Utah mathematics and climate researchers build new models for understanding sea ice, which is not as solid as you might think.
image:
An upside-down sea ice slab showcasing brine channels that facilitate the drainage of liquid brine and support convection along the interface.
view moreCredit: Ken Golden, University of Utah
Polar sea ice is ever-changing. It shrinks, expands, moves, breaks apart, reforms in response to changing seasons, and rapid climate change. It is far from a homogenous layer of frozen water on the ocean’s surface, but rather a dynamic mix of water and ice, as well as minute pockets of air and brine encased in the ice.
New research led by University of Utah mathematicians and climate scientists is generating fresh models for understanding two critical processes in the sea ice system that have profound influences on global climate: the flux of heat through sea ice, thermally linking the ocean and atmosphere, and the dynamics of the marginal ice zone, or MIZ, a serpentine region of the Arctic sea ice cover that separates dense pack ice from open ocean.
In the last four decades since satellite imagery became widely available, the width of the MIZ has grown by 40% and its northern edge has migrated 1,600 kilometers northward, according to Court Strong, a professor of atmospheric sciences.
“It has also shifted toward the pole while the size of the sea ice pack has declined,” said Strong, a co-author on one of two studies published by U scientists in recent weeks. “Most of these changes have happened in the fall, around the time when sea ice reaches its seasonal minimum.”
A tale of two studies, one north and one south
This study, which adapts a phase transition model normally used for alloys and binary solutions on laboratory scales to MIZ dynamics on the scale of the Arctic Ocean, appears in Scientific Reports. A second study, published in the Proceedings of the Royal Society A and based on field research in the Antarctic, developed a model for understanding the thermal conductivity of sea ice. The issue cover featured a photo exposing regularly spaced brine channels in the bottom few centimeters of Antarctic sea ice.
Ice covering both polar regions has sharply receded in recent decades thanks to human-driven global warming. Its disappearance is also driving a feed-back loop where more of the sun energy’s is absorbed by the open ocean, rather than getting reflected back to space by ice cover.
Utah mathematics professors Elena Cherkaev and Ken Golden, a leading sea ice researcher, are authors on both studies. The Arctic study led by Strong examines the macrostructures of sea ice, while the Antarctic study, led by former Utah postdoctoral researcher Noa Kraitzman, gets into its micro-scale aspects.
Sea ice is not solid, but rather is more like a sponge with tiny holes filled with salty water, or brine inclusions. When the ocean water below interacts with this ice, it can set up a flow that allows heat to move more quickly through the ice, just as when you stir a cup of coffee, according to Golden. Researchers in the Antarctic study used advanced mathematical tools to figure out how much this flow boosts heat movement.
The thermal conductivity study also found that new ice, as opposed to the ice that remains frozen year after year, allows more water flow, thereby enabling greater heat transfer. Current climate models could be underestimating the amount of heat moving through the sea ice because they don’t fully account for this water flow. By improving these models, scientists can better predict how fast sea ice melts and how this affects the global climate.
While the aspects of ice investigated in the two studies are quite different, the mathematical principles for modeling them are the same, according to Golden.
“The ice not a continuum. It’s a bunch of floes. It’s a composite material, just like the sea ice with the tiny brine inclusions, but this is water with ice inclusions,” said Golden, describing the Arctic’s marginal ice zone. “It’s basically the same physics and math in a different context and setting, to figure out what are the effective thermal properties on the big scale given the geometry and information about the floes, which is analogous to giving detailed information about the brine inclusions at the sub-millimeter scale.”
Golden is fond of saying what happens in the Arctic does not stay in the Arctic. Changes in the MIZ are certainly playing out elsewhere in the world in the form of disrupted climate patterns, so it is critical to understand what it’s doing. The zone is defined as that part of the ocean surface where 15% to 80% is covered by sea ice. Where the ice cover is greater than 80% it is considered pack ice and less than 15% it’s considered to be the outer fringes of open ocean.
A troubling picture from space
“The MIZ is the region around the edge of the sea ice, where the ice gets broken into smaller chunks by waves and melting,” Strong said. “Changes in the MIZ are important because they affect how heat flows between the ocean and atmosphere, and the behavior of life in the Arctic, from microorganisms to polar bears, and navigating humans.”
With the advent of quality satellite data beginning in the late 1970s, scientific interest in the MIZ has grown, since now its changes are easily documented. Strong was among those who figured out how to use imagery shot from space to measure the MIZ and document alarming changes.
“Over the past several decades, we’ve seen the MIZ widen by a dramatic 40%,” Strong said.
For years, scientists have scrutinized sea ice as a so-called “mushy layer.” As a metal alloy melts or solidifies from liquid, either way it passes through a porous or mushy state where the liquid and solid phases coexist. Freezing salt water is similar, resulting in a pure ice host with liquid brine pockets, which is particularly porous or mushy in the bottom few centimeters nearest the warmer ocean, with vertical channels called “chimneys” in mushy layer language.
Strong’s team tested whether previously modeled mushy layer physics could be applied to the vast reaches of the MIZ. According to the study, the answer is yes, potentially opening a fresh look at a part of the Arctic that is in constant flux.
In short, the study proposed a new way of thinking about the MIZ, as a large-scale phase transition region, similar to how ice melts into water. Traditionally, melting has been viewed as something that happens on a small scale, like at the edges of ice floes. But when the Arctic is viewed in its entirety, the MIZ can be seen as a broad transition zone between solid, dense pack ice and open water. This idea helps explain why the MIZ is not just a sharp boundary, but rather a “mushy” region where both ice and water coexist.
“In climate science, we often use very complex models. This can lead to skillful prediction, but can also make it difficult to understand what’s happening physically in the system,” Strong said. “The goal here was to make the simplest possible model that can capture the changes we’re seeing in the MIZ, and then to study that model to gain insight into how the system works and why it’s changing.”
The focus in this study was to understand the MIZ’s seasonal cycle. The next step will be applying this model to better understand what drives MIZ trends observed over the past few decades.
The study “Homogenization for convection-enhanced thermal transport in sea ice” appeared Aug. 28 in the journal Proceedings of the Royal Society A. Co-authors include Rebecca Hardenbrook of Dartmouth University and Huy Dinh, N. Benjamin Murphy, Elena Cherkaev and Jingyi Zhu of the U’s Department of Mathematics. The Arctic study titled, “Multiscale mushy layer model for Arctic marginal ice zone dynamics,” appeared Sept. 3 in Scientific Reports. Funding for this research came from the National Science Foundation and the U.S. Office of Naval Research.
Journal
Scientific Reports
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
Multiscale mushy layer model for Arctic marginal ice zone dynamics
New MBARI research reveals the dynamic processes that sculpt the Arctic seafloor
An international team of researchers used MBARI’s advanced underwater technology to document how melting permafrost and new ice formation contribute to the dramatic underwater landscape in a remote area of the Arctic
Monterey Bay Aquarium Research Institute
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An international team of researchers led by MBARI Senior Scientist Charlie Paull has used MBARI’s advanced underwater technology to document the dynamic processes that sculpt the seafloor in a remote region of the Arctic Ocean. The team has discovered large underwater ice formations in the Canadian Beaufort Sea. This discovery reveals an unanticipated mechanism for the ongoing formation of submarine permafrost ice. Image: Dave Caress © 2022 MBARI
view moreCredit: Dave Caress © 2022 MBARI
MBARI researchers, working alongside a team of international collaborators, have discovered large underwater ice formations at the edge of the Canadian Beaufort Sea, located in a remote region of the Arctic. This discovery reveals an unanticipated mechanism for the ongoing formation of submarine permafrost ice.
In a previous MBARI study, researchers observed enormous craters on the seafloor in this area, attributed to the thawing of ancient permafrost submerged underwater. While exploring the flanks of these craters on a subsequent expedition, MBARI researchers and collaborators from the Korea Polar Research Institute (KOPRI), the Korea Institute of Geoscience and Mineral Resources, the Geological Survey of Canada, and the U.S. Naval Research Laboratory observed exposed layers of submarine permafrost ice.
The recently discovered layers of ice are not the same as the ancient permafrost formed during the last ice age, but rather were created under present-day conditions. This ice is produced when deeper layers of ancient submarine permafrost melt, creating brackish groundwater that rises and refreezes as it approaches the seafloor, where the ambient temperature is approximately -1.4 degrees Celsius (29.5 degrees Fahrenheit).
The complex morphology of the seafloor in this region of the Arctic tells a story that involves both the melting of ancient permafrost that was submerged beneath the sea long ago and the disfiguration of the modern seafloor that occurs when released water refreezes.
After the last ice age, sea levels rose and covered the ancient permafrost on the Arctic shelf. The base of this body of ancient permafrost is slowly warming and thawing because of heat flowing out of the Earth—much older, slower climatic shifts are contributing to the melting of this Arctic submarine permafrost, not human-driven climate change. When this water migrates up to the colder seafloor, it freezes. Freezing ice pushes up ridges and mounds. Seawater seeps into the blistered seafloor surface, melting the ice layers and leaving massive sinkholes behind. The dynamic interplay between large changes in salinity and small changes in temperature near the seafloor drives this process.
The research team has published these new findings in the Journal of Geophysical Research: Earth Surface.
“Our work shows that permafrost ice is both actively forming and decomposing near the seafloor over widespread areas, creating a dynamic underwater landscape with massive sinkholes and large mounds of ice covered in sediment,” said Charlie Paull, a geologist at MBARI and the lead author of the study. “These dramatic and ongoing seafloor changes have huge implications for policymakers who are making decisions about underwater infrastructure in the Arctic.”
Since 2003, MBARI has been part of an international collaboration to study the seafloor at the edge of the Canadian Arctic shelf. This remote area that only recently became accessible to scientists as warmer temperatures caused sea ice to retreat.
A mapping survey by Canadian researchers in 2010 first uncovered the region’s distinctively rugged seafloor terrain. In 2013, MBARI researchers and their collaborators conducted the first high-resolution mapping surveys in this region. Using an MBARI autonomous underwater vehicle (AUV), the research team documented the seafloor terrain in detail.
Five mapping surveys—two conducted from Canadian research ships and three with MBARI’s advanced underwater technology—in this area over a 12-year period revealed 65 newly-formed craters on the seafloor. The largest crater was the size of a city block of six-story buildings.
In 2022, the team returned to the Arctic aboard KOPRI’s icebreaker research vessel Araon. They first used MBARI’s two seafloor mapping AUVs to identify recently formed craters. Then, they conducted visual surveys within those specific craters with MBARI’s MiniROV. This portable remotely operated vehicle developed by MBARI engineers can be configured for a variety of science missions. Equipped with cameras and sampling equipment, it has been integral to studying the Arctic seafloor. While exploring the seafloor with the MiniROV, researchers observed ice formations inside two recently formed large seafloor craters.
Isotopic analysis of these formations and samples of the surrounding seafloor sediments confirmed that the ice came from brackish groundwater, created partly by the melting ancient permafrost rising up through the seafloor. The ascending groundwaters refreeze near the seafloor, forming widespread sub-bottom ice layers that blister the seafloor, producing ice-cored mounds.
Minor temperature and salinity variations cause shifts between freezing of ascending brackish groundwater and melting of near-seafloor ice layers. These ongoing processes work in tandem to create a dramatic submarine landscape composed of numerous depressions and ice-filled mounds of varying ages.
“These findings upend our assumptions about underwater permafrost,” said Paull. “We previously believed all underwater permafrost was leftover from the last ice age, but we’ve learned that submarine permafrost ice is also actively forming and decomposing on the modern seafloor.”
The process that creates these sub-seafloor ice formations has not been considered before and may occur where bottom-water temperatures are below zero degrees Celsius.
“This discovery means that the techniques we’ve previously used to locate submarine permafrost don’t work for the types of near-seafloor ice that we recently discovered exist in the Arctic. We now need to revisit where permafrost may exist under the Arctic Shelf,” said Paull.
This work was funded by the David and Lucile Packard Foundation, the Korean Ministry of Ocean and Fisheries (KIMST grant No. 20210632), the Geological Survey of Canada, and the U.S. Naval Research Laboratory.
About MBARI
MBARI (Monterey Bay Aquarium Research Institute) is a non-profit oceanographic research center founded in 1987 by the late Silicon Valley innovator and philanthropist David Packard. Our mission is to advance marine science and technology to understand a changing ocean. Visit mbari.org to learn more.
Journal
Journal of Geophysical Research Earth Surface
Article Title
Massive Ice Outcrops and Thermokarst Along the Arctic Shelf Edge: By-Products of Ongoing Groundwater Freezing and Thawing in the Sub-Surface
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