Researchers from the University of Milano-Bicocca and Harvard University have identified a condition that anticipates anomalous temperatures in the polar stratosphere which cause the phenomena that allow freezing air to reach mid-latitudes such as ours. The mechanism analysed in the study by Mostafa Hamouda and Claudia Pasquero of the University of Milano-Bicocca together with Eli Tziperman of Harvard University also offers an explanation for the link between global warming and the increase in extreme cold events. The results of the study were published in “Nature Climate Change” in an article entitled “Decoupling of the Arctic Oscillation and North Atlantic Oscillation in a warmer climate” (https://www.nature.com/articles/s41558-020-00966-8).
Winter weather conditions at mid-latitudes are strongly influenced by the so-called polar vortex, an atmospheric circulation that traps cold Arctic air at high latitudes and prevents it from reaching other parts of the globe. On average, once every two years the polar vortex weakens and allows the cold air to reach the mid-latitudes, a phenomenon that can also affect Italy. We may remember, for example, the abundant snowfall in Rome in 2018 and February 2012 when the temperature in the Po Valley dropped below -20°C and the Venetian lagoon froze.
The weakening of the polar vortex is triggered by the rapid warming of the air at 30 kilometres altitude in the stratosphere and causes an anomaly in the winds. Within a couple of weeks, surface weather conditions start to feel the effects of anomalous winds, favouring the incursion of polar air into the mid-latitudes. This phenomenon is extremely topical: in fact, a particularly intense stratospheric warming in the Arctic (about 50°C) occurred at the turn of the New Year 2021, with possible weather instability consequences in Europe and/or North America in the following weeks, partly already manifested with the extreme wave of snow and cold in Spain.
It is already known that anomalous temperatures in the stratosphere are influenced by several climatic events, such as melting Arctic ice and intense tropical rainfall, however our current understanding does not permit accurate predictions of their occurrence. The study published in “Nature Climate Change” highlights a previously unrecognised condition that anticipates stratospheric anomalies. It relates to the surface temperature of the North Pacific Ocean: particularly warm water heats the cold air arriving from Siberia, causing it to rise and changing the stratospheric conditions.
The study shows that this mechanism becomes more important in a warmer climate. The North Pacific Ocean, due to ocean circulation, is warming much faster than the North Atlantic in response to the ongoing climate crisis. The significance of this discovery is that it may represent the link between global warming in recent decades and the increase in extreme cold events in boreal winter.