Understanding the impact of Stratospheric Polar Vortex (SPV)
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Arctic warming and severe winter weather - The link
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- The story: In the recent past, the Arctic region has warmed rapidly and studies have shown that the temperatures in the region have risen about twice as fast as global temperatures. This phenomenon is termed Arctic amplification and multiple factors including increased greenhouse gas emissions and atmospheric transport of heat to the region play important roles.
- New finding: A study has pointed that changes in the Arctic region induced by anthropogenic global warming have disrupted a wind system called Stratospheric Polar Vortex (SPV). This stretching of SPV can lead to extreme cold events in parts of Asia and North America.
- Indian and Brazilian researchers noted that Arctic warming was behind the heatwaves in India. The warming was found to have an impact on the frequency and intensity of heat waves.
- Strangely, studies also state that this Arctic warming has propelled the cold winter spells in North America.
- Understanding the SPV: The SPV consists of strong westerly winds from 10km to 50km above the surface. It peaks at around 50 m/s on average near 50km and is strongest in mid-winter, however, on occasion it can weaken, and sometimes the winds reverse direction entirely.
- These stretching events almost certainly occurred before greenhouse gas emissions began increasing, but the current emissions are leading to an increase in the occurrence of these types of events.
- When the polar vortex is nice and circular, that’s a sign all the cold air is bottled up over the Arctic…When it stretches like this, a piece of it goes into Asia and a piece of it goes towards eastern North America. So that’s what is seen now, say with the 2021 Texas cold wave.
- A reversal of the polar vortex is known to bring warm winter temperatures in Southern Asia.
- Impact on India: Can this disruption hit India? The polar vortex stretching events are associated with relatively cold temperatures in Central and East Asia but there are no strong signals in India (so far). These events might slightly favour relatively mild temperatures.
- Science can help: By identifying the precursor pattern to these stretching events, science can extend the warning lead time of cold extremes in Asia, Canada, and the United States. Preparing for only a decrease in severe winter weather can compound the human and economic cost when severe winter weather does occur.
- Knowledge centre:
- Sudden Stratospheric Warming - A SSW refers to a rapid rise in temperatures in the stratosphere, the layer of atmosphere lying 10-50km above our heads; see Vertical Temperature Structure. The polar stratosphere is dominated by the development of the Stratospheric Polar Vortex (SPV) each winter, and this is one of the dominant driving forces of mid-latitude weather during the cold season. Encircling the SPV is the Polar Night Jet, a jet stream which sits at about 1-10hPa in the stratosphere. The Polar Night Jet is the bigger brother of the tropospheric jet stream we hear TV weather presenters talking about (this one usually sits at about 200hPa). Both the Stratospheric Polar Vortex and Polar Night Jet form during the winter months when the pole is tilted away from the Sun, and when the ephemereal polar night results in significant cooling. The Polar Night Jet forms along the strong temperature gradient generated as a result, and helps to self-contain the vortex, resulting in further cooling.
- Rossby waves - The SPV is often impacted by events in the troposphere and vice-versa. Rossby waves in the troposhere (responsible for the 'typical' low and high pressure systems we observe) have a vertical component of propagation as well as a horizonal component. The Rossby Waves can only propagate vertically within an eastward-moving flow, which is usually the case in the mid-latitudes in winter. Their amplitudes can increase with height, and they can propagate vertically from the troposphere into the stratosphere - a process known as wave-mean flow interaction, exerting a drag on the polar vortex. The Polar Vortex can be remarkably resilient, and not all major SSWs result in a total breakdown of the vortex. Major SSW events can therefore be split into Vortex Split and Vortex Displacement events.
- From 50 km up to surface - How can something so delicate going on 50km above our heads impact the weather at the surface? The idea is that anomalies in the stratospheric circulation can propagate downwards through the stratosphere and into the troposhere over a number of weeks, rather like 'dripping paint' - which is often the analogy used to describe this process.
- EXAM QUESTIONS: (1) Explain the dynamics involved in the Stratospheric Polar Vortex (SPV) and warming/cooling effects on surface. (2) In what ways can global warming lead to unexpected events? Explain with an example.
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