Stratospheric warming and its role on the planet's climate

Stratospheric heating can cause intense heating at the level of the stratosphere right at the pole, whereby the intense high-altitude winds reverse the rotational direction

by Lorenzo Ciotti
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Stratospheric warming and its role on the planet's climate

Stratospheric heating can cause intense heating at the level of the stratosphere right at the pole, whereby the intense high-altitude winds reverse the rotational direction, splitting the polar vortex into two or more secondary lobes that descend in latitude, replacing it with a high pressure area in the same location, with above-average ground temperatures.

In the winter season, this phenomenon causes rapid descents of icy air towards the European continent, towards Asia, or towards Canada and the United States which can cause intense and abundant snowfalls, even persistent for several days, in the areas around the low pressure that generally forms at the southern end reached by the polar cold front.

This configuration was at the origin of the most intense frost waves that hit the whole of Europe in 1929, 1963 and 1985. At the end of the strong warming, the vortex currents change direction: they no longer rotate from west to east but from east to west.

When the stratwarming is of a greater type, the heating is often also transmitted to the troposphere, together with the circulatory inversion of the winds. However, this situation is temporary and reversible and the polar vortex can recompose itself after 15/20 days.

At the end of February 2018, stratospheric heating took place with complete crushing of the polar vortex of which a lobe flowed into Siberia and from here, in the form of burian, it spread throughout Europe. While temperatures on the continent suddenly dropped by tens of degrees, in the Arctic circle, due to the hot anticyclone that expelled the polar vortex, temperatures were even above 0 °C.

After about ten days the vortex slowly resumed its formation above the pole. The depression area conditions on a large scale, with its variable displacements in time and space, the meteorological situation of the northern hemisphere, thanks to the remote interaction with the subtropical anticyclones; the counterclockwise rotation of the Earth feeds the mid-latitude jet stream, diverting the consequent exchanges of air masses of different temperatures which otherwise would only take place along the north-south meridian.

A strong polar vortex causes a tense zonal flow in mid-latitudes. Extension and depth of the depression area of the polar vortex are variable over time, being able to deepen and expand towards the south, especially in winter, thus profoundly influencing the meteorological conditions of the northern part of the European, Asian or American continent, depending on the area expansion to the south.

In the southern hemisphere cyclogenesis would be simpler as the Antarctic vortex should have a more regular circumpolar shape and less subject to variations.