As autumn begins, the Northern Hemisphere receives progressively less solar radiation, the temperatures in the polar stratosphere (around 15-50 km above the surface) become much lower than that in the mid-latitudes, creating a substantial north-south temperature difference and temperature gradient. Warmer mid-latitude air therefore flows toward the North Pole, but isdeflected to the rightdue to the Coriolis force caused by Earth's rotation, subsequently forming a vortex that rotates around the polar region from west to east, known as the “Polar Vortex” [1,2]. In general, the greater the temperature gradient, the stronger the vortex intensity.

The polar vortex is a cold-core low-pressure system that persists over the Arctic stratosphere during the boreal winter (Figure 1). As the polar vortex is a planetary scale weather system, its variation of intensity might influence the troposphere below and even the weather at the surface ^[4]. Therefore, scientists monitor the stratospheric zonal wind speeds and temperature using radiosondes and satellite data to assess the changes of the intensity of the polar vortex ^[5].

With the Northern Hemisphere receives less solar radiation, the temperatures in the Arctic reaches the annual minimum from December to January. The polar vortex peaks in strength with the strongest westerly winds in the stratosphere. When spring begins, the temperature in the Arctic gradually rise due to increase of solar radiation in the Northern Hemisphere, reducing the temperature gradient between the Arctic and the mid-latitudes, the westerly winds in the stratosphere will therefore weaken. When the zonal wind speed in the stratosphere drops to zero or shift to easterlies and last for more than 10 days, it indicates the polar vortex has weakened significant or breakdown completely, scientists refer to this as the “Final Warming” ^[7]. “Final Warming” typically occurs between March and May every year (Figure 2) where the stratospheric polar vortex completely breaks down and does not reform new structures. It will then re-establish in the next autumn (Figure 3).

From December to February, the polar vortex typically strong and stable, confining the extremely cold air to the polar regions. However, when there are sudden warmings over the Arctic stratosphere, the polar vortex might be weakened, causing it to become displaced or even splits ^[6]. This phenomenon is called “Sudden Stratospheric Warming” or SSW. It refers to a rapid rise in temperature of several tens of degrees in the polar stratosphere in a few days, persisting of about one to two weeks ^[6].

Research shows that SSW can influence the troposphere below, making the polar jet stream become wavier or even lead to blocking patterns, the airmasses originally confined over the Arctic may spills southward and bring very cold weather to the mid-latitudes ^[8]. Nevertheless, the understanding of how they affect both weather in the surface and the upper atmosphere is still incomplete ^[6]. Meanwhile, the influences of global warming andENSOon SSW and the polar vortex are also active research topics in academia ^[9][10].