Aurelia Butler

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Eddies are often seen as the weather of the ocean. Like large-scale circulations in the atmosphere, eddies swirl through the ocean as slow-moving sea cyclones, sweeping up nutrients and heat, and transporting them around the world.
In most oceans, eddies are observed at every depth and are stronger at the surface. But since the 1970s, researchers have observed a peculiar pattern in the Arctic: In the summer, Arctic eddies resemble their counterparts in other oceans, popping up throughout the water column. However, with the return of winter ice, Arctic waters go quiet, and eddies are nowhere to be found in the first 50 meters beneath the ice. Meanwhile, deeper layers continue to stir up eddies, unaffected by the abrupt change in shallower waters.
This seasonal turn in Arctic eddy activity has puzzled scientists for decades. Now an MIT team has an explanation. In a paper published today in the Journal of Physical Oceanography, the researchers show that the main ingredients for driving eddy behavior in the Arctic are ice friction and ocean stratification.
By modeling the physics of the ocean, they found that wintertime ice acts as a frictional brake, slowing surface waters and preventing them from speeding into turbulent eddies. This effect only goes so deep; between 50 and 300 meters deep, the researchers found, the ocean’s salty, denser layers act to insulate water from frictional effects, allowing eddies to swirl year-round.
The results highlight a new connection between eddy activity, Arctic ice, and ocean stratification, that can now be factored into climate models to produce more accurate predictions of Arctic evolution with climate change.
“As the Arctic warms up, this dissipation mechanism for eddies, i.e. the presence of ice, will go away, because the ice won’t be there in summer and will be more mobile in the winter,” says John Marshall, professor of oceanography at MIT. “So what we expect to see moving into the future is an Arctic that is much more vigorously unstable, and that has implications for the large-scale dynamics of the Arctic system.”
Marshall’s co-authors on the paper include lead author Gianluca Meneghello, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences, along with Camille Lique, Pal Erik Isachsen, Edward Doddridge, Jean-Michel Campin, Healther Regan, and Claude Talandier.
Beneath the surface
For their study, the researchers assembled data on Arctic ocean activity that were made available by the Woods Hole Oceanographic Institution. The data were collected between 2003 and 2018, from sensors measuring the velocity of the water at different depths throughout the water column.
The team averaged the data to produce a time series to produce a typical year of the Arctic Ocean’s velocities with depth. From these observations, a clear seasonal trend emerged: During the summer months with very little ice cover, they saw high velocities and more eddy activity at all depths of the ocean. In the winter, as ice grew and increased in thickness, shallow waters ground to a halt, and eddies disappeared, whereas deeper waters continued to show high-velocity activity.
“In most of the ocean, these eddies extend all the way to the surface,” Marshall says. “But in the Arctic winter, we find that eddies are kind of living beneath the surface, like submarines hanging out at depth, and they don’t get all the way up to the surface.”
To see what might be causing this curious seasonal change in eddy activity, the researchers carried out a “baroclinic instability analysis.” This model uses a set of equations describing the physics of the ocean, and determines how instabilities, such as weather systems in the atmosphere and eddies in the ocean, evolve under given conditions.

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An icy rub
The researchers plugged various conditions into the model, and for each condition they introduced small perturbations similar to ripples from surface winds or a passing boat, at various ocean depths. They then ran the model forward to see whether the perturbations would evolve into larger, faster eddies.
The researchers found that when they plugged in both the frictional effect of sea ice and the effect of stratification, as in the varying density layers of the Arctic waters, the model produced water velocities that matched what the researchers initially saw in actual observations. That is, they saw that without friction from ice, eddies formed freely at all ocean depths. With increasing friction and ice thickness, waters slowed and eddies disappeared in the ocean’s first 50 meters. Below this boundary, where the water’s density, i.e. its stratification, changes dramatically, eddies continued to swirl.
When they plugged in other initial conditions, such as a stratification that was less representative of the real Arctic ocean, the model’s results were a weaker match with observations.
“We’re the first to put forward a simple explanation for what we’re seeing, which is that subsurface eddies remain vigorous all year round, and surface eddies, as soon as ice is around, get rubbed out because of frictional effects,” Marshall explains.
Now that they have confirmed that ice friction and stratification have an effect on Arctic eddies, the researchers speculate that this relationship will have a large impact on shaping the Arctic in the next few decades. There have been other studies showing that summertime Arctic ice, already receding faster year by year, will completely disappear by the year 2050. With less ice, waters will be free to swirl up into eddies, at the surface and at depth. Increased eddy activity in the summer could bring in heat from other parts of the world, further warming the Arctic.
At the same time, the wintertime Arctic will be ice covered for the foreseeable future, notes Meneghello. Whether a warming Arctic will result in more ocean turbulence throughout the year or in a stronger variability over the seasons will depend on sea ice’s strength.
Regardless, “if we move into a world where there is no ice at all in the summer and weaker ice during winter, the eddy activity will increase,” Meneghello says. “That has important implications for things moving around in the water, like tracers and nutrients and heat, and feedback on the ice itself.”
This research is supported, in part, by the National Science Foundation. More

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Tree growth rings and ice cores illuminate the climatic conditions of times gone by. When combined with historical records and documents, climate data can also shed light on important events in human history — including the activities of nomadic groups such as the ancient Türks and Mongols.
“Climate data actually can tell us quite a lot about the history of nomadic empires,” said Nicola Di Cosmo, a professor at the Institute for Advanced Study and an expert on China and Inner Asian peoples and environmental history during a recent lecture in which he used a series of case studies to illustrate the links between climate history and nomadic empires. Such nomadic empires, Di Cosmo says, were “important historical protagonists” in shaping world events.
At the virtual event, Di Cosmo joined MIT associate professors David McGee, an expert on paleoclimate records, and Manduhai Buyandelger, a specialist in Mongolian anthropology.
“This is a really rich area of research,” said McGee. “Collaborations are the way forward — where you have experts who can read the complexity of the paleoclimate record and experts who can read the complexity of the human record.”
In the grassland steppes of Inner Asia, climate has a major impact on plant growth. Pastoral economies, which depend on grazing livestock like horse and sheep, are affected in turn. “The pastoral economy and the ecology of the steppe region is so sensitive — so vulnerable, also — to sudden climatic variability,” said Di Cosmo. A period of sudden drought or heavy snowfall can lead to major livestock die-offs.
A cold spell in the early seventh century hit the Eastern Türk empire, in the steppes of what is now Mongolia and China. Data from tree rings and ice cores show a brief period of unusually cool temperatures and heavy snowfall — the aftermath of a volcanic eruption. Volcanic eruptions push gases and small particles into the stratosphere, where they can deflect incoming solar rays and cause lower temperatures. In this case, historical documents indicate subsequent famine, increased taxation, political unrest, and a weakened military among the Eastern Türks — and the empire ultimately collapsed in 630 A.D.
But climate anomalies can also be beneficial. Recent tree ring data from 13th century Mongolia reveal that the normally warm, dry conditions were punctuated by a 15-year period that was unusually wet. This period coincides with the rise of Chinggis Khan’s Mongol empire. Di Cosmo and his colleagues came up with the idea that the empire’s success could partly be due to favorable climatic conditions during these wet years: with more plant growth, the fledgling Mongol empire could raise more horses, expand its army, and stage a flurry of intense military activity outside of Mongolia.
“A previous theory maintained that the Mongols were pushed out of Mongolia because of worsening climate conditions — in fact, because of drought,” said Di Cosmo. “We actually turned around that kind of thinking and conclude, based on new climate data, something quite different.”
Climate data can also shed light on particular, perplexing events. Why did the Mongols suddenly begin losing battles in 1242, after sweeping successfully across eastern Hungary only months before? Combining tree ring data with historical reports suggests the key lies in the weather. Dry, warm conditions in 1241 were favorable for the Mongol invasion of eastern Hungary — and an extremely cold, wet winter enabled them to cross the frozen Danube into western Hungary. But springtime thaws likely led to flooding and marshy conditions that made mounted warfare a liability.
“Di Cosmo demonstrates how new paleoclimate data helps to answer old or existing questions that have puzzled historians for a long time,” said Buyandelger.
Another puzzle is the Uyghur empire, which was “very different from other steppe empires,” said Di Cosmo. It grew less dependent on military power, perhaps due to limited resources, and developed a diversified economy that relied more on trade and agriculture rather than pastoralism or war. Di Cosmo’s work indicates that a 60-year drought around the start of the ninth century may have prompted this empire to develop in unusual ways.
Di Cosmo and his colleagues are currently focused on the 1257 eruption of the volcano Samalas, in Indonesia. This massive eruption — the largest release of volcanic gases in the past 2,000 years — triggered cool, wet conditions as far away as Europe. Di Cosmo’s team thinks these conditions might have hindered the Mongol invasion of Syria in 1260, and are using climate data to investigate.
One challenge of examining human history through a lens of paleoclimatology is that high-resolution climate data can be hard to obtain. But these nomadic empires coincided with clear data. “The tree ring records that Di Cosmo is working with are really the highest quality in terms of dating and spatially explicit information and calibration with modern data,” said McGee.
In systems where tree ring records are not available, there are alternatives. One project at MIT — led by Gabriela Serrato Marks PhD ‘20, a research specialist in McGee’s lab — is using stalagmites to help uncover the climatic context of past societal changes in Mexico. Ice cores and lake sediments provide more options for uncovering past climate conditions, and modeling can also be an important tool.
“One of the ways that paleoclimatologists are moving forward is by running climate models,” said McGee. “The models can help to fill in the gaps where we don’t have data and to understand things that the data can’t tell us, about wind patterns or about seasonality, for example.”
Meanwhile, present-day Mongolia is experiencing devastating droughts and winter weather as a result of global climate change. “Merging new climate data with the historical understanding of a place helps to put in perspective the catastrophic scale of climate change today,” said Buyandelger. “For instance, the heavy snowfalls in Mongolia in recent decades are the harshest in known history.”
In seeking solutions, we may be able to learn from past societies that adapted to poor climatic conditions. “Modern societies are much more complex than past societies, and so we have to factor in questions of industrialization and pollution and so forth,” said Di Cosmo. “But one of the things that history can do is help us understand better how societies in the past adapted or showed resilience.”
The virtual lecture was hosted by MIT Anthropology and co-sponsored by the Department of Earth, Atmospheric and Planetary Sciences and MIT History. More

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Extreme heat events — like those seen in California in 2020 — are expected to worsen over the century due to climate change and urban heat islands (UHIs). Cities will likely experience the brunt of those effects.
To help cities mitigate UHI and extreme heat, MIT Concrete Sustainability Hub postdoc Hessam AzariJafari is studying one of the most abundant urban surfaces: pavements. He has found that it’s possible to significantly lower urban air temperatures and greenhouse gas emissions by altering pavement surface reflectivity. However, as he explains below, the effects of reflective pavements can depend heavily on where they are implemented.
Q: What are reflective pavements and how do they impact climate change?
A: Reflective pavements are a paving strategy that can help solve the problem of urban heat islands. The so-called “cool pavement” strategy is currently practiced in a few cities, such as Los Angeles, by implementing reflective coatings and/or brighter-color materials in the pavement mixtures. These properties allow more sunlight to be reflected from a pavement’s surface, and less to be absorbed by its mass. As a result, reflective pavements can lower urban temperatures when the ambient temperature is lower than the pavement surface temperature. In Los Angeles, for instance, we found that reflective pavements would reduce the occurrence of heat waves by around 40 percent over 20 years.
In addition to altering air temperatures, pavements also influence climate change. By reflecting light into building envelopes, they can alter heating and cooling demands and their associated greenhouse gas (GHG) emissions in the surrounding neighborhoods. Moreover, by sending a larger amount of solar irradiation to the sky, they can alter the Earth’s energy balance. This process, known as a radiative forcing, creates a cooling effect that can help counteract climate change.
Q: How do the effects of reflective pavements vary by context?
A: The effectiveness of reflective pavement strategies in reducing climate change impact depends on several factors. One major factor is geographical context. The local climate condition, including real-time temperature, cloud factors, and relative humidity, plays an important role in the intensity of radiative forcing, as well as changes to building energy demand (BED) due to heating and cooling.
Within urban areas, the neighborhood morphology, such as building density and canyon aspect ratio [ratio of building heights to the adjacent pavement width], can considerably change the BED effect of reflective pavements. Building configuration characteristics, such as the ratio of the surface area to volume, the insulation system, and the heating and cooling technology also affect the intensity of BED change.
The efficiency and sustainability of the local grid play a part as well. For example, generating one kilowatt-hour of electricity in Phoenix emits 85 percent more greenhouse gas emissions than in Boston. That’s because a small proportion of the electricity generation is from low-GHG sources in Arizona. Therefore, increases in building energy demand in Phoenix can have a larger climate change impact.
Q: Many aspects of a pavement contribute to its life-cycle environmental footprint. Where does surface reflectivity fit into that total footprint?
A: Surface reflectivity is just one part of a pavement’s cumulative life-cycle emissions. Additional impacts include pavement construction and repairs, the extra fuel consumption of vehicles induced by pavement properties, and the end-of-life landfilling or recycling.
Just as with pavement reflectivity, these impacts can also vary by context. For example, in urban neighborhoods, the BED effect of pavements is more pronounced because there are hundreds of thousands of apartment units located in the city whose energy demands will be altered by the surface reflectivity of pavements. Since pavements in those dense urban areas also service a relatively low volume of traffic, the contribution of their reflectivity to their total life-cycle impact is more significant as well. However, on highways, which see significantly greater levels of traffic, the surface roughness and structural properties of a pavement contribute to a greater proportion of that pavement’s life-cycle emissions by influencing the fuel consumption of vehicles. Therefore, it is important to consider all elements of a life cycle when municipalities and transportation authorities decide on the environmentally preferred option. More