[Reading Science] Why Do Cold Waves Linger So Long?

The Science of Winter: Persistence Matters More Than Intensity

This winter's cold spell has been unusually prolonged. Instead of temperatures dropping sharply and then quickly recovering, there have been repeated stretches of several days with persistently low temperatures. In Seoul, for example, over the past two weeks, there have been multiple days when the morning low hovered around minus 10 degrees Celsius, and daytime temperatures often remained below freezing as well. The wind chill barely improved, and cold wave advisories were repeatedly issued and lifted. The sensation among citizens is simple: it is not just "cold," but "constantly cold."

On the 25th, when the cold wave continued, ice floes were floating near Hangang Hangjudaegyo Bridge. Photo by Yonhap News

This perception is not an illusion. Recently, meteorologists and climate scientists have described this winter's cold spell not as a "severe cold wave," but rather as a "stagnant cold wave." What has changed compared to the past is not the record low temperatures themselves, but the fact that the cold air remains trapped over a region for a longer period, unable to move on with the atmospheric flow. Why has this winter's cold spell lasted so long?

Cold waves no longer just pass by

In the past, winter cold snaps followed a relatively clear pattern: cold air would descend from the north, causing temperatures to plummet, and then, after a few days, conditions would recover to normal levels. Cold waves were short, intense, and moved quickly. Weather forecasts were also based on this pattern.

However, recent winters have been different. Once cold air settles over the Korean Peninsula, it does not easily retreat. Temperatures remain low, and the chill does not dissipate even during the day. This is not simply a drop in temperature, but rather a sign that atmospheric circulation itself is slowing down. The cold is no longer a fleeting event, but a persistent state lingering over a specific region.

Kim Seung-bae, Director of the Korea Natural Disaster Association and former spokesperson for the Korea Meteorological Administration, describes this winter's cold spell as "highly volatile and difficult to end once it starts." He explains, "While winters in the past followed relatively regular cycles, recent winters have seen unpredictable alternations between warmth and cold, making forecasting much more difficult." He particularly points out that, unlike before, cold waves now do not subside quickly once they begin.

When the jet stream slows down, the cold gets trapped

The key concept for understanding winter cold waves is the jet stream. The jet stream is a fast-moving band of wind that flows from west to east between the Arctic and the mid-latitudes, acting as a barrier that keeps Arctic cold air trapped to the north. In a typical winter, this wind band flows relatively straight, holding the cold air in place.

The problem is that, over the past few decades, this flow has weakened. As Arctic warming has progressed more rapidly than the global average, the temperature difference between the Arctic and the mid-latitudes has decreased. This temperature difference is the energy source that drives the jet stream. As the difference narrows, the force pushing the jet stream weakens, causing it to lose speed and meander instead of maintaining a straight path.

As the jet stream meanders, cold air moves southward along its troughs, while warm air moves northward along its ridges. Once these meanders form, they do not resolve easily. With the jet stream moving more slowly, cold air remains over the Korean Peninsula for extended periods. As a result, cold waves become not short-lived events, but persistent states.

The gap created by Arctic Oscillation: Room for cold waves to linger

Another factor is the Arctic Oscillation (AO) index. The Arctic Oscillation is an index that represents the pressure difference between the Arctic and the mid-latitudes, indicating how strongly the Arctic cold air is being contained. When this index is negative, it creates conditions that make it easier for cold Arctic air to move southward into the mid-latitudes.

Lee Myung-in, professor at the School of Urban and Environmental Engineering at Ulsan National Institute of Science and Technology (UNIST), explains, "Winter temperatures in Korea are highly correlated with the Arctic Oscillation index," and adds, "This winter, the Arctic Oscillation has repeatedly shown negative phases, creating conditions favorable for cold air to move southward."

However, some experts argue that it is difficult to explain the "persistence" of cold waves with the Arctic Oscillation index alone. Woo Jin-gyu, forecaster at the Korea Meteorological Administration, emphasizes that the prolonged cold wave is the result of a complex atmospheric structure rather than a single factor. While a negative Arctic Oscillation phase increases the likelihood of a cold wave, for the cold to persist, several "opportunistic conditions" must coincide, such as the degree of jet stream meandering, the development of upper-level high pressure, and the formation of so-called blocking patterns in specific regions.

In fact, when a strong upper-level high develops near the Bering Sea, it can cause atmospheric flows to stagnate, and if this effect extends to East Asia, it can create a structure in which cold air remains trapped over the Korean Peninsula. When these conditions coincide, cold waves are characterized more by their persistence than their intensity.

This winter’s atmosphere as seen in NOAA indicators... The unstable trend of the AO index

This analysis is supported by actual observational data. According to the Arctic Oscillation (AO) index graph provided by the National Oceanic and Atmospheric Administration (NOAA) of the United States, since last autumn, the AO index has consistently remained below zero, repeating negative values. This indicates that the force containing the Arctic cold air has been weak for an extended period.

In particular, since January of this year, there have been periods when the AO index dropped sharply, followed by repeated cycles of recovery and decline, showing an unstable trend. When the AO index is negative, the likelihood of Arctic cold air expanding into the mid-latitudes increases, and this winter, such conditions have been formed not just temporarily but continuously.

NOAA’s ensemble forecast (GEFS) also suggests that the AO index is likely to remain weak and volatile, rather than recovering strongly in the short term. This indicates that after a cold wave passes, it may not subside immediately, leaving open the possibility for cold air to re-enter.

On the morning of the 9th, when the first cold wave advisory of this winter was issued across Seoul, citizens were walking in thick clothing at Gwanghwamun Square in Seoul. Photo by Yonhap News

A cold winter does not contradict the climate crisis

This brings up a common question: "If we are in an era of global warming, why is it so cold?" Scientists explain that this is not a contradiction, but rather a result of warming. Global warming is a change in averages, while cold waves are a matter of variability.

Although the global average temperature is clearly rising, the balance of energy in the atmosphere and oceans is being disrupted in the process, leading to more frequent and prolonged extreme weather events. In particular, rapid warming in the Arctic destabilizes winters in the mid-latitudes. Professor Lee Myung-in describes this as the "Arctic amplification" phenomenon, stating, "A warmer Arctic is actually less able to contain the cold air."

The prolonged cold wave also affects society as a whole, leading to increased energy burdens due to higher heating demand, damage to crops, disruptions in transportation and industry, and health problems for vulnerable groups. Director Kim Seung-bae emphasizes, "Now, society must prepare for the persistence of cold waves rather than just their intensity," and adds, "New standards are needed across energy, health, and transportation systems." This winter’s cold spell is now less about how low temperatures drop, and more about how long they linger.