Understanding ‘planetary waves’ helps forecast dangerous heatwaves
The European heatwave of 2003 is believed to have been responsible for over 70 000 deaths. In 2010 a heatwave in Russia killed around 50 000 people and had a major impact on the wheat harvest resulting in economic losses of around USD 15 billion. Many extreme events, including the European and Russian heatwaves, are associated with particular atmospheric circulation patterns – specifically the propagation of a type of atmospheric wave known as a Rossby wave along atmospheric waveguides. Rossby waves, also known as planetary waves, are a type of large-scale wave in the atmospheric winds or ocean currents, with typical wavelengths measuring 100s to 1 000s of km. These waves occur naturally owing to the rotation of the planet. Waveguides are formed by particular configurations of the atmospheric winds, or the ‘jet streams’, the fast-flowing air moving from west to east in the upper atmosphere approximately 8-12 km above the Earth’s surface. Waveguides help determine the direction Rossby waves travel in, and thus what regions of the world they impact.
Seasonal predictability examined
The EU-funded PROTECT project set out to investigate the connection between Rossby waves and extreme weather, as well as the predictability of such circulation patterns on sub-seasonal to seasonal (S2S) timescales to improve the prediction of the probability of extreme events several months in advance. The research was undertaken with the support of the Marie Skłodowska-Curie Actions programme (MSCA). The MSCA research fellow Rachel White studied atmospheric waveguides that trap Rossby waves within a certain latitude range of the Earth. “Trapping Rossby waves can allow them to be larger in amplitude and to remain in place for extended periods of time, both of which contribute to the occurrence of extreme temperature events. Although PROTECT focused on heatwaves, Rossby waves have also been shown in the scientific literature to be connected to extreme cold events and extreme rainfall,” White explains.
Connection confirmed
A key result of the project was the development of a data set containing daily maps of atmospheric waveguide occurrence. According to White: “Before this project, waveguides were typically studied in average conditions, as an average either over time or over longitude. However, PROTECT developed an objective algorithm to detect waveguides in data for winds observed over the past 35 years, allowing the waveguide data set to be produced.” Analysis of the dataset showed that waveguides are indeed associated with an increased occurrence of Rossby waves, especially slowly propagating Rossby waves that contribute to extended extreme temperatures such as heatwaves. “The research helped advance our understanding of the links between large-scale atmospheric circulation patterns and extreme weather events,” adds White. The research initiated through the PROTECT project is ongoing, exploring the predictability of these waveguides in operational S2S forecast systems, which is an exciting route for potentially improving the ability to predict extreme events. “Results showing connections between waveguides and the types of Rossby waves that typically lead to extreme events suggest that improvements to the S2S prediction skill of heatwaves may indeed be possible using the approach laid out by the PROTECT project,” White concludes.
Keywords
PROTECT, Rossby waves, waveguides, heatwave, extreme weather, circulation patterns, seasonal, forecast, planetary waves
