A3 Atmospheric Blockages and Anomalous Circulation

Topic

Atmospheric blocking is a persistent large-scale circulation pattern in which a blocking anticyclone settles into an anomalous position at mid- or high latitudes and remains there for a period ranging from five to thirty days or more. While the blocking anticyclone holds this position, it disrupts the normal flow of the jet stream—the wind current that typically guides weather systems from west to east—deflecting them around the blockage rather than allowing them to advance. This prolonged diversion of weather systems is why atmospheric blocking is responsible for a disproportionate share of temperature extremes, such as the heatwaves striking Europe and the cold snaps hitting North America, as well as the flooding caused by persistent rainfall from systems that stall or become stationary under its influence.

What sustains the blocking anticyclone in this anomalous position is the absorption of mid-latitude Rossby waves into the blocking zone: instead of propagating their energy eastward as they would under normal conditions, these Rossby waves become trapped and accumulate within the blocking zone. This energy accumulation is precisely what maintains the disturbance, reinforcing the very blocking anticyclone that prevented the waves from escaping, thereby closing the feedback loop that makes atmospheric blocking persistent. Indeed, this persistence is the criterion used to operationally identify atmospheric blocking: it is considered active when the pressure gradient in the area remains reversed for at least five consecutive days.

There is also a fundamental explanation for the persistence of the blocking anticyclone: ​​mid-latitude atmospheric flow allows for two distinct equilibrium states between which it can switch. The first is the normal zonal flow, in which weather systems move unimpeded from west to east; The second is the blocked state, in which a blocking anticyclone establishes itself and is sustained by the previously described accumulation of Rossby waves. The system shifts from one state to the other when a sufficiently large-amplitude disturbance pushes it out of the zonal state; once it falls into the blocked state, the accumulation of Rossby waves keeps it there until another disturbance of sufficient amplitude returns it to the zonal flow.

When Rossby waves do not remain trapped in the mid-latitude troposphere but instead amplify and propagate vertically into the stratosphere, they can produce an even more extreme form of anomalous circulation: the breakdown of the polar vortex. Within the stratosphere, the polar vortex is the circulation pattern that, under normal conditions, confines cold Arctic air within the polar circle; when the vertical propagation of amplified Rossby waves strikes it with sufficient force, the polar vortex weakens or splits, allowing the cold Arctic air to escape toward mid-latitudes. This escape of cold Arctic air produces historic cold waves capable of reaching regions as far from the polar circle as the southern United States or Central Europe.

Both atmospheric blocking in the troposphere and the breakdown of the polar vortex in the stratosphere are, at their core, the same type of anomalous circulation sustained by Rossby waves that fail to propagate normally: when these waves accumulate in the troposphere, they create a persistent blocking anticyclone that diverts weather systems and causes temperature extremes and flooding; when they amplify and rise into the stratosphere, they disrupt the polar vortex and release cold Arctic air, resulting in historic cold spells. In both cases, atmospheric blocking and its variants demonstrate how a single large-scale circulation disturbance—depending on the atmospheric layer where it persists—can manifest as extreme heat, prolonged flooding, or unseasonable polar cold.

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