While in most of Europe there was an intense wave of summer heat, in recent days several countries of South America – where it is now winter – have recorded an ‘wave of exceptional coldwith –1.9 ° C in Buenos Aires (the lowest temperature since 1991) and numerous weather stations that have recorded Record temperatures In Argentina (-16 ° C in the Cuyo region in Patagonia, -9 ° C in the regions of Chubut and Neuquén), in Chile (–9.3 ° C in Chillán, –8.1 ° C in Puerto Montt, –7.9 ° C in Temuco) and in Uruguay (5.8 ° C maximum in Montevideo, the lowest since 1967). We have seen snow in the city and places where this phenomenon is absolutely unusual, such as the Miramar beach in Argentina and the Atacama desertin Chile, considered among the most arid places on the planet, which on June 26 was whitewashed for the first time for 10 years. The polar cold would have caused the death of 15 people In Argentina and Uruguay.
Data in hand, Argentina and Chile have found themselves in recent days until 12 ° C below the climatological average calculated compared to the period 1979-2000. This wave of anomalous cold is the result of the incursion of a intense anticyclone (i.e. a high pressure area in rotation) arrived directly from the Antarctic Region and which has been hanging up to Paraguay and Uruguay. But how does an Antarctic anticyclone do to South America?
To understand it we have to talk about the polar vortexthat is, a sort of permanent cyclone above the terrestrial poles fueled by very low temperatures. Both polar vortices are distinguished in two components: one at high altitude (in the stratosphere) with high -speed currents of almost circular shape, and a lower component (in the troposphere) with a lower latitude: this component is called jetor jet stream.
As can be seen from the figure above, the jet currents have one less regular trajectory Compared to the tropospheric component: this is true above all in the boreal hemisphere, where currents meet orographic obstacles and other geomorphological characteristics that deviate its direction. Antarctic jet currents, on the other hand, tend to be much more stable because they blow over the Antarctic ocean, where there is nothing almost to “disturb them”.
The role of jet currents is of “Confinte” the very cold polar airpreventing her from reaching lower latitudes. It may happen, however, that the polar vortex weakens, typically due to a quick stratospheric heating (Sudden Stratospheric WarmingSSW). In the Antarctic area, SSW are sometimes linked to the absorption of UV sun radiation by the Ozone layer in the high atmosphere. In case of high altitude heating, the polar vortex can weaken or even “break”, becoming discontinuous. Here then a door opens in the “wall” that borders the polar cold air. This air then moves to lower latitudes, bringing Older of anomalous cold Like the one that occurred in recent days in South America.
The same thing happens of course also in the northern hemisphere: the instability of the polar vortex has as its effect an increase in the vertical component of the motion of the jet streamwhich therefore can also lead to European latitudes cold air from the Arctic, causing Sudden cold even out of seasonespecially towards the end of the boreal winter when the polar vortex begins to lose part of its strength.
