Scientists have discovered a new link between a pattern of fluctuations in jet stream air currents and crippling heatwaves that strike in multiple regions of the world at the same time.
Rossby waves are an atmospheric and oceanic phenomenon where giant meanders warp the flow of currents in a wave-like pattern. They don't only occur on Earth, but when they do, they're known to influence our planet's weather.
Now, the extent of that influence is becoming clearer. In a new study, researchers have found that when certain amplified wavelengths take shape in jet streams – fast-flowing air currents that course through the sky at high altitudes – the phenomenon is linked to the emergence of concurrent heatwaves that devastate food-producing regions.
"We found a 20-fold increase in the risk of simultaneous heatwaves in major crop-producing regions when these global-scale wind patterns are in place," says Earth systems researcher Kai Kornhuber from Columbia University.
Kornhuber and his team analysed climate data from 1979 to 2018 and found that when two particular wavelengths of circumglobal Rossby waves – called wave–5 and wave–7 – occurred in the Northern Hemisphere jet stream, they channel hot subtropical air into regions in North America, Europe, and Asia.
The hot air produces temperature spikes that can persist for weeks: extreme weather events that kill people, and threaten global food security through the widespread impact they have on crops.
"Two or more weeks per summer spent in the wave–5 or wave–7 regime are associated with 4 percent reductions in crop production when averaged across the affected regions, with regional decreases of up to 11 percent," the researchers write in their paper.
The fearful symmetry doesn't end there. After the crop losses, spikes in food prices often result, and while the researchers acknowledge that food pricing can be affected by a range of factors, the team nonetheless think that harvest impacts due to Rossby effects are a contributor.
According to the researchers, the wave patterns turned up five times in the study period (1983, 2003, 2006, 2012 and 2018), but while it's not yet clear how climate change might affect the phenomenon, the prospect of similarly great disturbances being unleashed in an already hotter world is not promising.
"Even if the frequency or the size of the [Rossby] waves doesn't change, the heat extremes linked to the patterns will become more severe, because the atmosphere as a whole is heating," says Kornhuber.
Nonetheless, the team acknowledge that further research is required to quantify how this discovery might be impacted by future climate projections. Until that's carried out, we won't know what Rossby waves could do to the world of tomorrow; what they're already doing is something to be concerned about.
"Until now, this was an under-explored vulnerability in the food system," Kornhuber says.
"During these events there actually is a global structure in the otherwise quite chaotic circulation. The bell can ring in multiple regions at once."
The findings are reported in Nature Climate Change.