Polar sea ice is constantly changing. It contracts, expands, moves, breaks, reforms in response to changing seasons and rapid climate change. It is far from a homogeneous layer of frozen water on the ocean surface, but rather a dynamic mixture of water and ice, as well as tiny pockets of air and brine encased in the ice.
New research led by University of Utah mathematicians and climate scientists is generating new models to understand two critical processes in the sea ice system that have profound influences on global climate: the flow of heat through sea ice, which thermally links the ocean and atmosphere, and the Dynamics of the Marginal Ice Zone, or MIZ, a meandering region of the Arctic sea ice sheet that separates the dense ice from the open ocean.
In the past four decades since satellite images 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, professor of atmospheric sciences.
“It has also moved poleward while the size of the sea ice sheet has decreased,” said Strong, co-author of one of two studies published by University scientists in recent weeks. “Most of these changes occurred in the fall, around the time sea ice reaches its seasonal minimum.”
A story of two studios, one to the north and one to the south.
This study, which adapts a phase transition model typically used for alloys and binary solutions at laboratory scales to MIZ dynamics at 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 Antarctica, developed a model to understand the thermal conductivity of sea ice. The issue’s cover featured a photograph exposing regularly spaced brine channels in the bottom few centimeters of Antarctic sea ice.
The ice covering both polar regions has retreated dramatically in recent decades thanks to man-made global warming. Their disappearance is also creating a feedback loop in which more of the sun’s energy is absorbed by the open ocean, rather than being reflected back to space by the ice sheet.
Utah mathematics professors Elena Cherkaev and Ken Golden, a prominent sea ice researcher, are the authors of 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, delves into its microscale aspects.
Sea ice is not solid, but rather a sponge with small holes filled with saltwater or brine inclusions. When ocean water interacts with this ice, it can establish a flow that allows heat to move more quickly through the ice, just like when you stir a cup of coffee, according to Golden. Researchers in the Antarctic study used advanced mathematical tools to determine the extent to which this flow drives the movement of heat.
The thermal conductivity study also found that new ice, unlike ice that remains frozen year after year, allows for greater water flow, allowing for greater heat transfer. Current climate models could be underestimating the amount of heat moving through sea ice because they do not fully account for this water flow. By improving these models, scientists can better predict how quickly sea ice is melting and how this affects 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 is not a continuum. It’s a bunch of icebergs. It’s a composite material, like sea ice with little brine inclusions, but this is water with ice inclusions,” Golden said, describing the Arctic ice marginal zone. . “It’s basically the same physics and mathematics in a different context and environment, to find out what the effective large-scale thermal properties are given the geometry and information about the icebergs, which is analogous to giving detailed information about brine inclusions in the submillimeter scale.”
Golden likes to say that what happens in the Arctic doesn’t stay in the Arctic. Changes in the IMR are certainly occurring in other parts of the world in the form of altered weather patterns, so it is critical to understand what it is doing. The zone is defined as that part of the ocean surface where between 15% and 80% is covered by sea ice. When the ice cover is greater than 80%, it is considered ice and less than 15% is considered the outer fringe of the open ocean.
A disturbing image from space
“The MIZ is the region around the edge of the sea ice, where the ice is broken into smaller pieces by waves and melts,” Strong said. “Changes in the MIZ are important because they affect the way heat flows between the ocean and atmosphere, and the behavior of life in the Arctic, from microorganisms to polar bears to human navigation.”
With the advent of quality satellite data in the late 1970s, scientific interest in the MIZ has grown, as its changes are now easily documented. Strong was among those who discovered how to use images taken from space to measure the MIZ and document alarming changes.
“Over the last few decades, we have seen the MIZ expand by a dramatic 40%,” Strong said.
For years, scientists have examined sea ice as a so-called “soft layer.” As a metal alloy melts or solidifies from a liquid, it either passes through a porous or soft state where liquid and solid phases coexist. Frozen salt water is similar, resulting in a mass of pure ice with pockets of liquid brine, which is particularly porous or soft in the few centimeters of the bottom closest to the warmer ocean, with vertical channels called “chimneys” in the language of soft layers.
Strong’s team tested whether previously modeled soft-layer physics could be applied to the vast expanses of the MIZ. According to the study, the answer is yes, which could open a new look at a part of the Arctic that is constantly changing.
In summary, the study proposed a new way to think about the MIZ, as a large-scale phase transition region, similar to how ice melts into water. Traditionally, melting has been considered something that happens on a small scale, such as at the edges of ice floes. But when looking at the Arctic as a whole, the IMR can be seen as a broad transition zone between solid, dense ice and open water. This idea helps explain why the IMR is not just a sharp boundary, but rather a “soft” region where ice and water coexist.
“In climate science, we often use very complex models. This can lead to skillful predictions, but it can also make it difficult to understand what is physically happening in the system,” Strong said. “The goal here was to create the simplest possible model that can capture the changes we are seeing in the MIZ and then study that model to gain insight into how the system works and why it is changing.”
The objective of this study was to understand the seasonal cycle of the MIZ. The next step will be to apply this model to better understand what drives the MIZ trends observed in recent decades.