Unseasonable warmth in December, a time traditionally associated with cold and snow, highlights a significant deviation from expected seasonal norms. When temperatures soar far above the historical average for a prolonged period, it requires specific meteorological conditions to occur. Understanding why the cold air remains at bay involves examining the immediate causes in the atmosphere and the larger, enduring global influences that set the stage.
Immediate Atmospheric Mechanics
The most direct reason for a warm December is the specific, temporary configuration of the upper atmosphere, particularly the path of the jet stream. This fast-moving river of air acts as the boundary between frigid arctic air masses to the north and warmer subtropical air to the south. When the jet stream takes a significant northern detour, it barricades the cold air near the pole, preventing it from plunging southward.
This northern shift is often accompanied by the formation of a persistent high-pressure ridge, sometimes referred to as a “blocking high.” A high-pressure system features sinking air, which warms as it descends and promotes dry, clear conditions. This dome of high pressure acts as a roadblock, stalling the normal progression of weather systems and locking the warm pattern in place for days or weeks. The resulting air flow on the western side of the ridge actively draws warm air masses from the south or southwest, contributing to the unusually high temperatures.
Large-Scale Natural Climate Cycles
While the jet stream’s position is the immediate cause of the warmth, its path can be influenced by large-scale, naturally occurring climate cycles that operate over months or years. One of the most significant of these global drivers is the El Niño Southern Oscillation (ENSO), which involves fluctuating sea surface temperatures in the equatorial Pacific Ocean. During its warm phase, known as El Niño, the ocean temperatures are warmer than average, which shifts atmospheric pressure patterns across the entire globe.
This shift creates a ripple effect, altering the position and strength of the jet stream over North America. An El Niño event typically causes the Pacific jet stream to be stronger and to extend further east. This configuration often results in warmer and drier winter conditions across the northern tier of the continent, including much of Canada and the northern United States. Therefore, an El Niño cycle can predispose the atmosphere to the jet stream pattern that produces unseasonable December warmth.
The Context of Global Warming
These warm December events occur against a backdrop of long-term, foundational change driven by the increase in global average temperatures. Human activities, primarily the burning of fossil fuels, have raised the planet’s baseline temperature compared to pre-industrial levels. This increase means that every weather event, including a warm spell in December, is happening in an environment that is already warmer than it was historically.
A small increase in the average temperature dramatically shifts the probability distribution of extreme events. What was once considered an unusually warm December day now occurs with greater frequency, and the intensity of record-breaking warmth is amplified. Scientists use “attribution” studies to determine the extent to which human-induced climate change has altered the probability or magnitude of a specific weather event. These studies show that global warming makes warm temperature extremes significantly more likely and more intense than they would have been otherwise.
The warming Arctic is a contributing factor to the jet stream’s behavior. The jet stream’s strength and stability are powered by the temperature difference between the pole and the mid-latitudes. As the Arctic warms faster than the rest of the planet, this temperature gradient weakens, causing the jet stream to slow down and become wavier. These exaggerated waves are more prone to creating the stagnant high-pressure blocks that lock in warm air. This combination of a higher baseline temperature and increased likelihood of warm-air-trapping patterns makes warm Decembers more common.