I study the stratosphere, the layer of atmosphere that extends above the troposphere from about 10-50 km. Friends and colleagues of mine often joke (I hope…) that “nobody cares about the stratosphere” *, primarily because it contains no real ‘weather’ – such as what happens in the troposphere. With little to no water vapour, it can’t be seen on visible satellite imagery – unlike the huge and beautiful weather systems in the troposphere. To visualise the stratosphere, we rely primarily on computer-generated graphics – and it’s not like you can walk outside and experience it, either. So, why do we care? What follows is a relatively simple (I hope!) explanation.
Weather forecasts, particularly on TV, often explain that our weather is “all down to the position of the jet stream” (the band of fast flowing air high in the troposphere that forms on the boundary between warmer and cooler airmasses). Now, that’s almost always true in the UK, but it’s particularly potent in winter – when the temperature contrasts either side of the jet become enhanced thanks to the Polar Night. One of the main driving factors behind the speed and position of the jet stream (particularly the Atlantic jet stream) in winter is… the stratosphere!
Rather like the jet streams we know and love/loathe in the troposphere that guide the development and evolution of weather systems, in the stratosphere there exists another jet stream – the Polar Night Jet (Figure 1). This encircles the Stratospheric Polar Vortex (SPV). Both of these form as the pole tilts away from the Sun in winter, leading to intense cooling. The strong temperature gradient then forms a jet stream and cyclonic vortex, which isolates the air within the vortex, and it cools further…etc. The Polar Vortex is a normal phenomenon which forms each winter – nothing sensational like some headlines suggest.
Figure 1: GFS zonal wind analysis from February 4th 2018. Reds indicate westerly winds. A strong Polar Night Jet exists in the stratosphere, associated with a strong tropospheric jet.
Through a process known as stratosphere-troposphere coupling, the stratosphere and the troposphere beneath can ‘talk’ via the influence of planetary/Rossby waves. These very large waves in the mid-latitude westerly flow can propagate vertically from the troposphere into the stratosphere and influence the circulation there – a process known as wave-mean flow interaction. Sometimes, this is strong enough to strongly disrupt the SPV, and when that happens, the isolated reservoir of cold air is broken down and the temperature sky-rockets… by as much as 50C in only a few days. This is known as a Sudden Stratospheric Warming (SSW). Very strong SSWs – called major SSWs – occur in approximately 6 winters per decade, and result in a reversal of the Polar Night Jet to easterlies. The Polar Vortex is either displaced, split up, or destroyed (2018’s SSW is shown in Figure 2).
Figure 2: The February 2018 Major SSW, as told through daily analyses from the GFS of 10 hPa wind (filled) and geopotential height (contours). This is classified as a ‘split’ SSW, for obvious reasons.
This has implications for our weather, because anomalies in the strength and position of the SPV and the Polar Night Jet can propagate downwards and influence the tropospheric jet stream. A stronger than normal SPV is associated with a strengthened tropospheric jet stream – and for us in the UK, that means Atlantic westerlies and generally mild winter weather. In contrast, following a major SSW, the easterlies propagate downwards (Figures 3 and 4) – resulting in a reduction in strength of the Atlantic westerlies. Sometimes, there can be a complete reversal of circulation – this happened in March 2018 with the infamous ‘Beast from the East’, bringing cold and snowy weather.
Figure 3: As in Figure 1, but for February 17th, following the major SSW. Note the weaker tropospheric jet and surface easterlies as the ‘Beast from the East’ developed in response.
Thus, being able to predict the state of the Stratospheric Polar Vortex is a source of skill for wintertime forecasts. Moreover, because there tends to be some lag between the events in the stratosphere and their maximum impact at the surface (~2 weeks), stratospheric predictability can provide increased predictability on the sub-seasonal timeframe (~15-30 days). Additionally, anomalies associated with a major SSW tend to persist in the lower stratosphere for even longer – which again, is a source of skill.
Figure 4: Anomalies in geopotential height for January-March 2018. Note how anomalies associated with the major SSW (red blob in the centre) propagate downwards like ‘dripping paint’.
And that is why we care about the stratosphere!
Further reading:
Kidston et al., 2015: Stratospheric influence on tropospheric jet streams, storm tracks and surface weather. Nature Geoscience, 8, 433-440.
*A tongue-in-cheek quote from Reading Meteorology’s weekly ‘Weather and Climate Discussion’ a few years ago that stuck with me was “the stratosphere – nothing of interest lies therein”. I plan to use that in my thesis…
