A record-early SSW in November 2025 and its implications for mid-latitude weather

Sudden Stratospheric Warming (SSW) is one of the most fascinating and, at the same time, one of the most important atmospheric phenomena occurring every winter in the polar stratosphere. Although it takes place high above us—specifically in the stratosphere, roughly 10 to 50 kilometres above Earth’s surface—it can exert a profound influence on weather conditions experienced at the ground. SSW events usually occur during the second half of the winter season or in early spring, but this season it is likely that such an event will occur already at the end of November, which is highly unusual.

What exactly is a Sudden Stratospheric Warming?

Focusing on the Northern Hemisphere, the wintertime circulation is typically dominated by the polar vortex, a region of extremely cold, rapidly rotating westerly winds centred over the Arctic in the stratosphere, approximately between 15 and 50 km altitude. This vortex acts as an “atmospheric barrier”, effectively confining cold Arctic air to high latitudes.

During an SSW, however, this stable configuration undergoes a dramatic disruption. Stratospheric temperatures can rise by 30 to 50 °C within just a few days, an exceptionally rapid and pronounced increase. This abrupt warming weakens the polar vortex and may even cause it to break down entirely. Once the vortex collapses, cold Arctic air is no longer “locked” at the pole, allowing it to spill into lower latitudes—which can have substantial impacts on weather patterns. SSW events usually occur during the second half of winter or in early spring. This season, however, is unusual in that an SSW event is likely to develop already at the end of November, making it one of the earliest occurrences on record. Figure 1 (see map) shows stratospheric wind speeds at the 10 hPa pressure level (approximately 33 km altitude) for November 30. This level is typically used to assess the state of the polar vortex. The letter L denotes a low-pressure system, representing the polar vortex, while the letter H marks a high-pressure system. This high-pressure system develops as a result of the intrusion of warmer air and disrupts the polar vortex.

Why do SSW events occur? And what is the role of planetary waves?

The primary trigger of SSW events is the activity of planetary (Rossby) waves, which originate in the troposphere—the lowest atmospheric layer where everyday weather occurs. These waves arise due to differential heating of Earth’s surface, the distribution of continents and oceans, and the presence of major mountain ranges. In particular, large mountain systems are key generators of strong planetary waves because they significantly disturb the atmospheric flow.

Planetary waves can propagate upward into the stratosphere when the background flow provides favourable conditions. If the waves are sufficiently strong, they act as a “brake” on the polar vortex, slowing its rotation and disrupting its symmetry. The process is analogous to disturbing a whirlpool in a bathtub—placing a hand into it disrupts its rotation, and the vortex weakens or collapses. In the atmosphere, strong planetary wave activity can similarly split or fully dismantle the polar vortex.

Thus, an SSW results from enhanced upward wave forcing, which transports energy from the troposphere into the stratosphere, displacing air masses, causing rapid warming, and weakening the polar vortex. This mechanism forms the crucial link between surface weather patterns and upper-atmospheric dynamics. Figure 2 shows the 7-day mean temperature anomalies at 10 hPa from ECMWF for the period from November 24 to December 1 (see forecast). A pronounced positive temperature anomaly is evident over the Arctic, indicating a disruption of the polar vortex.

How does an SSW affect the weather in mid-latitudes?

Although SSW events occur high above the surface, their consequences can be substantial. A weakened or collapsed polar vortex allows cold Arctic air to penetrate into the mid-latitudes—across Europe, North America, or Asia. This can lead to prolonged cold spells, increased frequency of snow events (even in regions where snow is normally infrequent), and enhanced winter severity. Conversely, the Arctic itself may experience anomalous warming, as cold air is displaced equatorward and replaced by warmer air masses.

These impacts do not appear immediately; it typically takes 1 to 3 weeks for the stratospheric signal to propagate downward into the troposphere—a process known as stratosphere–troposphere coupling. Moreover, not every SSW results in a severe winter outbreak; the outcome depends on the prevailing circulation patterns and a range of interacting atmospheric factors. Figure 3 shows the forecasted temperature anomalies for December 3, shortly after the SSW event (see map). Negative temperature anomalies are expected primarily over North America and Siberia, while temperatures in the Arctic are above normal. However, this represents only a preliminary estimate, and outbreaks of cold Arctic air may occur in different regions and at different times.

How often do SSW events occur?

In the Northern Hemisphere, SSW occur relatively regularly—about once or twice per winter. Their intensity varies, but the most significant events are termed major SSW, defined by a reversal of the zonal-mean westerly winds to easterlies at 60°N and 10 hPa. These major warmings, which tend to produce the strongest surface impacts, occur on average every two years. An SSW event is expected at the end of November, but as shown in Figure 4 (see forecast), it is not yet clear whether it will be a minor or major event. Figure 4 presents the ensemble forecast of potential zonal wind developments at 60°N, with the stronger blue line indicating the ensemble mean. For the end of November, this ensemble mean is in negative values, suggesting easterly winds and a potential major SSW. As the event approaches, the forecast is expected to become more precise.

Why is monitoring SSW events important?

Studying SSWs enables meteorologists to improve winter weather forecasting. While it is not possible to predict precisely which regions will experience cold outbreaks, the occurrence of an SSW provides an early indication that a substantial shift in weather patterns may unfold in the subsequent weeks. This makes SSW monitoring a crucial component of extended-range winter prediction.