Planetary and Gravity Waves as Drivers of Sudden Stratospheric Warmings (PEGASUS)

Sudden stratospheric warmings (SSWs) are some of the most dramatic events in the entire atmosphere. Over just a few days, the high-altitude jet stream stops and sometimes even reverses, and polar stratospheric temperatures can shoot up as much as 50 degrees Celsius. Their effects propagate upwards, where they modulate the near-Earth space environment of the ionosphere, and downwards, where they can cause extreme winter weather in densely-populated regions such as Europe and North America.

SSWs occur on average twice every three winters, but may occur several times in one winter and then not at all for several years afterwards. Forecasting them more than a few days in advance is extremely challenging. Their effects are also very difficult to predict - while (for example) the 'Beast From The East' of February 2018 in Europe and the 'Polar Vortex Winter' of January 2014 in the eastern United States were directly attributable to SSWs which happened a few days earlier, many SSWs have occurred with almost no effect on surface weather.

In PEGASUS, we will use new satellite measurement techniques and advanced computer models to better understand the physics of how SSWs develop, and of how and why they affect both surface weather and space weather. We will (i) test a recent theory that changes our understanding of how and why SSWs happen, (ii) investigate the details of how, when and where SSWs affect surface weather and (iii) measure the effects of SSWs on the global upper atmosphere, with implications for GPS and radio communications.

(i) Traditionally, we thought that SSWs were triggered by extremely large and unusually intense 'planetary waves' travelling through the atmosphere. These large waves seriously disrupt the jet stream, making it collapse and triggering an SSW. However, recent work has shown that this conceptual model does not properly explain the observed SSW record. Instead, a new theory challenges this model at a fundamental level. This new theory is that smaller-scale 'gravity waves' over the weeks before the SSW nudge the jet stream into a less robust state, weak enough that normal winter weather can be enough to trigger the start of an SSW. The precise distribution of these gravity waves, in space, time and intensity, may also affect how severe the surface effects of the SSW are. There is thus an important need to test this new theory. PEGASUS will do so. We will use advanced new satellite methods of measuring both the large planetary waves and the much smaller gravity waves to study the development of every SSW in the last sixteen years. We will also study idealised mathematical models (i.e. models which strip away unnecessary details) to understand the underlying physics and mathematics of how SSWs evolve and develop. This will provide a robust and critical test of the new theory.

(ii) This combination of observational and theoretical insight will let us test and assess how well forty leading climate models reproduce SSWs. We will use this information to select the best such models, tested against both observations and theory from (i). We will then study these selected models in close detail to understand what features of SSWs cause them to affect the surface and the upper atmosphere, with the aim of better predicting both SSW development and surface effects in future. In particular, we will closely study the differences between the surface effects of two different types of SSW, known as 'splits' and 'displacements' based on how they affect the jet stream.

(iii) Finally, we will quantify how SSWs affect global GPS signals and radio communications, allowing us to understand not just the surface weather effects of SSWs but also their space-weather effects. This will use a chain of five state-of-the-art radars spanning from pole-to-pole, and global measurements of upper-atmospheric composition from satellite measurements.

SSWs are major stratospheric weather events, which have knock-on effects throughout the atmosphere above and below them, from the Earth's surface to the edge of space.

At the surface, SSWs are an important driver of unusually extreme winter weather at surface level, especially the densely-populated regions of Europe and North America. Such weather has a major and direct impact on a vast range of industries, from logistics and transport to hospitality, oil and gas supply and public service. It also has broad implications for human health and mortality both during and after the events. PEGASUS will improve our scientific knowledge of how and why SSWs occur, facilitating advances in forecasting both when they happen and, when they do occur, how they will affect the surface.

Furthermore, a range of recent studies (e.g Sigmond et al, Nat. Geosci. 2013; Tripathi et al, Environ. Res. Lett 2015) have suggested that our limited ability to predict SSWs is a key roadblock on the path to producing meaningful weather forecasts at sub-seasonal and seasonal timescales, i.e. timescales of roughly a month or longer. Our work will directly address this block on the pathway to this highly challenging goal, with broad implications on long-term planning in industries such as farming and civil engineering.