A 70-degree Fahrenheit day in January can feel surprising for regions accustomed to winter’s chill. This unusual warmth results from immediate weather phenomena, broader global climate patterns, and long-term climate change. Understanding these interacting factors helps explain why such unseasonably warm temperatures occur.
Understanding Immediate Weather Patterns
Unseasonably warm winter days are often due to specific, short-term atmospheric conditions. A primary factor involves the position and behavior of the jet stream, a fast-moving current of air high in the atmosphere that separates cold polar air from warmer tropical air. When the jet stream shifts northward from its typical winter position, it allows warmer air masses from lower latitudes to extend further north into regions that would normally be cold. This northward shift prevents cold Arctic air from plunging southward, allowing warmer air to extend into otherwise chilly areas.
Another contributing element is the presence of strong high-pressure systems. These systems are characterized by sinking air that compresses and warms as it descends, leading to clear skies. In the Northern Hemisphere, high-pressure systems rotate clockwise, and if positioned appropriately, they can draw warm air from the south into a region. This process, known as warm air advection, involves the horizontal transport of warmer air into a cooler area, causing temperatures to rise. When these conditions align, a local area can experience significantly elevated temperatures.
How Global Climate Patterns Influence Local Weather
Beyond immediate weather, larger climate patterns can set the stage for warm anomalies. The El Niño-Southern Oscillation (ENSO) is a significant example, with its El Niño phase often influencing winter weather across North America. During an El Niño, unusually warm ocean waters in the equatorial Pacific affect global atmospheric circulation, causing the Pacific jet stream to shift southward and extend eastward over the southern United States. This shift often leads to warmer and drier conditions in the northern U.S. and Canada during winter.
Similarly, the Arctic Oscillation (AO) describes a seesaw in atmospheric pressure between the Arctic and mid-latitudes, impacting the polar jet stream. In its positive phase, lower-than-average pressure over the Arctic and higher pressure over the mid-latitudes cause a strong, consistent west-to-east jet stream, keeping frigid Arctic air contained near the pole. This positive AO phase often results in milder winters across mid-latitude regions like North America, Europe, and Asia. Conversely, a negative AO phase, with higher pressure in the Arctic, can weaken the jet stream, allowing cold air to spill into middle latitudes.
The Impact of Long-Term Climate Change
Underlying these immediate and cyclical weather patterns is the pervasive influence of long-term climate change, which increases the probability and intensity of unseasonably warm events. Rising global average temperatures, driven by human activities, shift the baseline, making a rare warm anomaly more common. Winter, in particular, is the fastest-warming season across much of the United States, with average winter temperatures rising since 1970.
This warming trend means that cold winter days are warming faster than hot summer days, reducing the temperature difference between the Arctic and mid-latitudes. A warmer atmosphere holds more moisture, which can lead to increased precipitation. Climate change increases the odds that natural variability will result in extreme warm events. The overarching warming trend makes such occurrences more frequent and potentially more intense.