POLAR VORTEX
The polar vortex is a large area of low pressure and cold air that surrounds both of the Earth’s poles. It typically exists near the poles and weakens during summer, while strengthening during winter.
- Expansion of the Polar Vortex: During winter in the northern hemisphere, the polar vortex can expand, sending cold air southward. This leads to Arctic outbreaks of cold air in regions such as the United States and Europe.
- Impact: Although the polar vortex itself is not a surface-level phenomenon, it exists tens of thousands of feet up in the atmosphere. When the vortex expands, it can bring extremely cold air southward, leading to severe winter weather in affected areas.
- Threat to Humans: The primary danger to humans is the intensity of the cold temperatures brought by the expanded polar vortex, which sends Arctic air into regions that are typically much warmer.
Characteristics of the Polar Vortex:
- Single Vortex: The polar vortex can form a single vortex when the Arctic Vortex becomes elongated in shape and has two cyclone centers.
- Duration: The Arctic polar vortex usually breaks up between mid-March to mid-May, marking the transition from the end of winter to the beginning of spring. This event influences farming activities, ecosystems, and weather cycles.
- Affects Climate: The breakup of the polar vortex impacts sea ice, ozone levels, air temperature, and cloud formation.
- Warming Periods: An early breakup of the polar vortex leads to a warming period that lasts from late February to mid-March. A late breakup of the polar vortex can result in two warming periods: one in January and one in March.
- Polar Vortex Breakoff: Sometimes, the polar vortex breaks off before the end of the last warming period. If the breakoff is large enough, the cold air can move into regions such as Canada and the Midwestern United States.
Natural Cycles Affect the Polar Vortex:
- See-Saw Pattern: The configuration of the polar vortex is connected to the phase of the Arctic Oscillation, which follows a see-saw pattern of rising and falling atmospheric pressure over the Arctic and mid-latitudes.
- Positive Phase of Arctic Oscillation: When the Arctic Oscillation enters a positive phase, the polar vortex tends to remain closer to the pole, which helps keep cold Arctic air contained.
- Negative Phase of Arctic Oscillation: When the Arctic Oscillation switches to a negative phase, the polar vortex expands southward, allowing cold air to move farther toward the middle latitudes. The vortex also becomes more wavy, meaning the ridges and troughs of the atmospheric pressure system become more pronounced.
Polar Cyclone:
- Area of Low Pressure: Polar cyclones, also referred to as arctic cyclones or polar vortices, are large areas of low pressure that form near the poles. They should not be confused with tropical cyclones.
- Counterclockwise Spiral: In the Northern Hemisphere, polar cyclones rotate in a counterclockwise spiral, much like tropical cyclones, due to the Coriolis effect.
- Reason for Rotation: The rotation of polar cyclones is driven by the Coriolis effect, similar to tropical cyclones. Polar cyclones also form in regions like the Eurasian Arctic, Greenland, and northern Canada. About 15 polar cyclones form during winter in these regions.
- Occurring Intensity: Polar cyclones can form year-round, but winter polar cyclones are stronger than those in the summer.
Disruption of the Polar Vortex:
- Rossby Wave: Rossby Waves can disrupt the polar vortex by affecting its circulation.
- Originate Due to Temperature Difference: Disruptions in the polar vortex originate during winter due to sharp temperature differentials between the poles and the equator. The polar vortex often expands, sending cold air southward along with the jet stream.
- Break of Arctic Air in the US: In the United States, the polar vortex commonly breaks during winter, causing outbreaks of Arctic air. For example, cities like Chicago and Dakota have experienced extreme cold temperatures as low as -30°C during such events.
Strong Polar Vortex:
- Common State: The strong polar vortex is the more common state of the vortex, creating strong low pressure in the Arctic region.
- Mechanism: Air flows from the mid-latitudes to the Arctic due to the pressure difference, keeping the cold air confined to the polar region.
- Where: This state results in milder winters across the Eastern US, Europe, and East Asia when the polar vortex is strong.
- Direction: During a strong polar vortex, airflow is fast and moves from west to east. This situation corresponds to the positive phase of the Arctic Oscillation (AO), also known as the North Atlantic Oscillation (NAO).
Weak Polar Vortex:
- Mechanism: The polar vortex occasionally weakens due to disruptions caused by wave energy propagating from the lower atmosphere to higher levels.
- Effects: When this occurs, the stratosphere warms, triggering sudden stratospheric warming (SSW) events that lead to rapid temperature changes in the stratosphere, miles above Earth’s surface.
- Shift in Location: When the vortex weakens, it may shift southward or split into two or more “sister vortices”. This shift can bring extreme winter weather and delayed warming to regions such as the eastern US and Europe.
- Where: The split can cause delayed or severe winter conditions in eastern US and northern/western Europe.
Factors Affecting the Strength of the Polar Vortex:
- Seasonal: The strength of the polar vortex is highest during winter and lowest during summer, with seasonal variations significantly affecting its intensity.
- Volcanic Eruptions: Volcanic eruptions can strengthen the polar vortex. After a volcanic eruption, the polar vortex may remain stronger than usual for up to two years due to the lasting effects of the eruption on the atmosphere.
- Variants: The Antarctic polar vortex is typically more persistent than the Arctic variant, making it more resistant to disruption.
- Climate Phenomena: La Niña and other climate phenomena can significantly strengthen the polar vortex, leading to more severe winter conditions.
Effects of the Polar Vortex:
- Affect the Ozone Layer: The Antarctic polar vortex has caused severe ozone depletion. The nitric acid in stratospheric clouds reacts with chlorofluorocarbons (CFCs) to form chlorine, which accelerates the destruction of the ozone layer.
- Global Warming: The Arctic region has warmed over twice the global average, reducing the temperature differential between the North Pole and regions such as North America. This energy imbalance disrupts the jet stream and polar vortex, causing the vortex to split.
- For example, two “child” vortices visited North America, contributing to record temperatures.
- Warmer Arctic and Severe Winter: Sudden stratospheric warming can lead to a warm Arctic but can also result in severe winter weather across other regions, such as the United States and Europe, as the vortex weakens and cold air escapes from the poles.