The polar vortex is a structure that mainly develops in the stratosphere at altitudes ranging from 12 to 50 km. The point of origin is situated above the troposphere, the layer in which the major meteorological processes occur. In the polar region, the atmosphere is thinner than it is in the equatorial region, and the stratosphere starts at an altitude of 8 km.
A similar circulatory system exists in the troposphere and can be partially linked with the stratosphere. We therefore make a distinction between the stratospheric and tropospheric vortex. Directly above the North Pole, when viewed from above, a circular, wavy movement of air masses can be observed flowing in a counterclockwise direction. The waves in this flow are known as Rossby waves in meteorology. A similar vortex exists above the South Pole, known as the Antarctic polar vortex.
The stratospheric polar vortex
The counterclockwise rotational movement (which happens clockwise at the South Pole) is caused by the Coriolis force, which is a consequence of the Earth’s rotation. The stratospheric polar vortex forms due to the strong westward flow that is generated in the mid-latitudes at higher altitudes. This flow, in turn, results from the differential warming of the northern and mid-latitudes and the resulting thermal imbalance. This imbalance is more pronounced during the cold season due to the more extreme cooling in the Arctic zones compared to the equatorial zones.
The stratospheric polar vortex becomes stronger and wider in the autumn and weakens in the spring. In the summer, it is completely absent. The polar vortex has a more uniform shape than the tropospheric polar vortex, but certain temporary events can disrupt it, causing it to break apart and disappear for a short period (lasting a few days to a few weeks). This can happen at any time, and not only when the polar vortex is at its weakest. The polar vortex can also be displaced due to the formation of a high-pressure area centred over the pole, which often occurs during the summer months. During the six months of summer, the polar vortex is replaced by the polar summer high, the circulation changes its direction, and the stratospheric winds blow in from the east.
The troposphere polar vortex
The tropospheric polar vortex is also a low-pressure area with its centre over the pole. It is located at an altitude of between 5,000 and 9,000 metres. The polar vortex typically extends to around the 40th or 50th latitude, making it more extensive than the stratospheric polar vortex. The tropospheric polar vortex is also constrained by the western flow of air, but it generally has a less compact structure than the stratospheric polar vortex.
On some days, the tropospheric polar vortex can have multiple centres, but typically the vortex will consist of just one or two centres. In the average boreal winter, there are two centres: one near Baffin Island (Canada) and the other over Northern Siberia.
The tropospheric polar vortex is weaker and less extensive in the summer, but unlike the stratospheric polar vortex, it does not disappear completely. As the polar vortex weakens, it tends to have stronger wave movements. This then leads to sudden changes in weather in the mid-latitudes. Although the tropopause separates the two polar vortexes, they can sometimes interact with each other and influence each other’s behaviour.
Governor of boreal cold
The tropospheric polar vortex interacts with subtropical high-pressure systems located in more southerly latitudes. The Earth’s rotation means that there is no direct exchange of air masses between the equator and the pole at higher altitudes. The jet streams in the mid-latitudes divert the air currents in an attempt to balance the energy imbalance between the equator and the poles. If the Earth did not rotate, a direct exchange between north and south could occur, and the dynamics of the Earth’s atmosphere would be much different.