Unseasonably warm November weather is a noticeable departure from expected historical averages for late autumn. This phenomenon is defined by a temperature anomaly, which measures how much the current temperature deviates from a long-term average, usually calculated over 30 years or more. When November feels warmer than usual, it indicates a significant positive temperature anomaly. This warmth results from an interplay between immediate atmospheric mechanics and broader, long-term climate influences.
Atmospheric Mechanics Behind the Anomaly
The immediate cause of a warm November is typically the behavior of the polar jet stream high in the atmosphere. This fast-moving river of air separates cold polar air from warmer mid-latitude air. When the jet stream develops deep meanders, it creates large north-south bulges called Rossby waves.
A pronounced northward bulge, known as a ridge, allows warm air to surge far poleward, diverting the normal flow of colder Arctic air. This configuration locks a warm pattern in place for days or weeks. Within this ridge, a high-pressure system, or anticyclone, contributes to surface warmth.
This dome of high pressure causes air to slowly sink toward the ground, and as the air compresses, it naturally warms up. The sinking air also clears out clouds, resulting in clear, sunny skies. These conditions allow solar radiation to efficiently heat the ground, creating the potential for record-breaking late-season temperatures.
Influence of Large-Scale Climate Cycles
The jet stream’s position and intensity are often influenced by natural, recurring patterns that operate over months or years. The El Niño-Southern Oscillation (ENSO) is one such pattern, characterized by fluctuations in Pacific Ocean sea surface temperatures. El Niño, the warm phase of ENSO, influences global weather by shifting the jet stream’s typical path.
During an El Niño event, the jet stream tends to shift farther south across the United States, leading to milder winter temperatures across the northern US. This remote connection, known as a teleconnection, means distant ocean conditions can steer continental atmospheric circulation. A strong El Niño increases confidence in seasonal forecasts predicting warmer than average late autumn and winter conditions.
Another important influence, particularly for North America and Europe, is the North Atlantic Oscillation (NAO). The NAO is the difference in atmospheric pressure between the Icelandic Low and the Azores High. A strong positive NAO phase is associated with increased westerly winds across the Atlantic, often bringing above-normal temperatures to northern Europe and southeastern North America. The NAO is a major driver of late autumn and winter weather variability in this region.
Context of Long-Term Warming Trends
To understand why extreme November heat is becoming more common, it is necessary to differentiate between weather and climate. Weather is the day-to-day atmospheric state, while climate is the average of weather conditions measured over decades. The immediate atmospheric mechanics, like the high-pressure ridge, represent the weather event, but the long-term rise in global temperatures provides the climate context.
The planet’s average temperature has been persistently rising due to the buildup of greenhouse gases in the atmosphere, which raises the baseline temperature from which all weather events begin. Scientists describe this as “shifting the curve,” meaning the entire distribution of daily temperatures is moving toward warmer values. A hot November day caused by a natural weather pattern now starts from a significantly warmer starting point than it would have fifty years ago.
This shifted baseline dramatically increases the probability of breaking temperature records, even late in the season. The underlying climate trend makes these positive temperature anomalies more frequent and more intense. Research has shown that human-caused climate change has increased the frequency and duration of heatwaves since the 1950s.
Every additional increment of global warming causes a discernible increase in the intensity and frequency of these temperature extremes. The atmospheric mechanics that bring warm air north are now operating in a warmer world. This results in more impactful and longer-lasting late-season heat events. Attribution science confirms that many extreme weather events are now made worse by this long-term, human-induced warming trend.