Polar Vortex

Formation

Polar vortices are the result of two planetary phenomena: the heat difference between the equator and the poles, and the rotation of the Earth.

Because areas closer to the equator receive more sunlight than the poles, the air there is warmer. This is called differential heating. It creates an imbalance in atmospheric heat between the different latitudes. To even out this imbalance, warm air tends to move towards the poles, while cooler polar air moves towards the equator.

Once these air masses begin moving north and south, their direction becomes influenced by the Coriolis and centrifugal forces, which are generated by the Earth’s rotation. Because of these forces, any object in motion in the northern hemisphere will have its direction of movement curved to the right. In the southern hemisphere, it’s the opposite — movement is curved to the left.

These forces create east-west winds, also known as zonal winds. In the northern hemisphere, the warm air moving north tends to veer to the right and begin moving east. Meanwhile, cold, polar air moving southward will also veer to the right, causing it to move westward. Conversely, in the southern hemisphere, warm air moving south veers to the left and begins to move east, while cold air moving north ends up deviating westward. At high latitudes, the combined result of these atmospheric movements is a dominant wind towards the east — counter-clockwise around the North Pole and clockwise around the South Pole. These rotations form the polar vortices.

Did you know?
In each hemisphere, polar jet streams form. They are located at the southern boundary of the northern polar vortex, and the northern boundary of the southern polar vortex. These are tunnels of maximum wind speed which flow from west to east. They form by the same basic mechanism as polar vortices — warm air moving towards the poles, then being deviated eastward by the rotation of the Earth. The reason that they are areas of maximum wind speed is because they are located at the boundary between warm, subtropical air masses and cold, polar air masses. This boundary is also called a polar front. The boundary’s shape can vary considerably over time due to various factors.

Variability

Within polar vortices, different wind patterns can be found on smaller scales. These are mostly due to land masses and sea ice, which cause more friction with the air than oceans do. The behavior of polar vortices also varies seasonally. Since their formation depends on differential heating (see “Formation”), polar vortices are strongest in the winter, when the difference between temperatures at the equator and pole is highest. In the summer, they become weaker, and sometimes even reverse direction. Overall, the northern polar vortex exhibits more seasonal and regional variation than its southern counterpart. This is mainly because the Antarctic continent is much more symmetrical than the area surrounding the North Pole.

Effects

When the polar vortex is strong, winds are strong. Strong winds tend to carry air from the ocean’s surface towards land and vice-versa. In winter, the air above the ocean is warmer than on land. For this reason, a strong polar vortex tends to favour milder winters at mid-latitudes such as most of North America. A strong polar vortex also keeps polar air spinning around the pole, preventing it from travelling towards the equator. While it remains around the pole, though, it becomes colder and colder.

On the other hand, when the polar vortex weakens, there is a risk of cold weather further from the poles. Not only is there less air being blown from the ocean to land, but polar air may also change course and travel further away from the pole. This is because there is less of a rotation to constrain the polar air to polar regions. In Canada, this tends to happen in the latter stages of most winters. As spring approaches, the polar vortex and polar jet stream begin to weaken, and polar air begins to travel further south. The air is still very cold and many areas of the country experience exceptional cold for a few days to several weeks. As the polar vortex weakens, the jet stream and polar front also develop a meandering, wobbling shape. When this happens, a mass of polar air may even become completely detached from the polar vortex and travel south. This can bring bitter cold southwards all the way to the southern United States and Mexico.

Historical Examples

Since the northern polar vortex is less stable than its southern counterpart, it tends to have more noticeable effects. These occur during most winters, but exceptional, record-breaking low temperatures and snowfall were measured all over North America in February 1899, when the polar vortex reached all the way down to the southern United States. Several of those records still stand today, although some were surpassed in a similar event in January and February 2019. In Canada, more records were broken as recently as February 2021, as the temperature in several cities plunged to nearly -50°C.

Polar Vortices in a Changing Climate

Scientists have only been rigorously observing the atmosphere since the 1950s, which is a short time to gather historical data. Data from past decades have not been clear enough to identify if the polar vortices are changing overall. Establishing a trend is difficult because polar vortices exhibit natural variability determined by the complex Earth system. For example, they can be unusually strong for a decade, then weak for 5 years, then alternate in strength from one year to the next. This means that it may take a long time for scientists to single out what impact climate change has on them.