Jet stream waves linked to multiple crop failures

Two or more weeks per summer under the influence of these waves are associated with 11 percent drop in crop production

In 2016, Canadians were horrified by the ferocity of Alberta’s Fort McMurray wildfire. It was suspected to be human caused, but also in play were an unusually hot, dry air mass over the northern part of the province, record-setting temperatures, relative low humidity and high wind gusts.

It led to the largest wildfire evacuation in Alberta’s history (more than 88,000 people), burned more than 1.5 million acres before being brought under control and destroyed 2,400 homes and buildings.

However, less known was the fact that a particular circulation pattern in the jet stream was spanning the entire Northern Hemisphere. It is known among atmospheric scientists as non-dimensional zonal wave number 4, and the strongest wing of this airborne pattern was parked over Western Canada.

Now, scientists have shown how specific wave patterns in the jet stream strongly increase the chance of co-occurring heat waves — and the potential for wildfires — in major food-producing regions of North America, Western Europe and Asia. They can lead to the threat of reduced crop production, risk of harvest failures and possible complications that can come from social unrest.

“In our study, we focus on the Rossby waves found in the atmospheric circulation,” said Kai Kornhuber with the University of Oxford’s physics department in the United Kingdom and Columbia University’s Earth Institute in the United States.

“Those are known to be tightly connected to extreme weather events. Those large-scale meanders are driven by disturbances and instabilities in the mid-latitude jet stream.”

According to the scientists’ report published in the journal Nature Climate Change, in the mid-latitudes the circumglobal Rossby waves are associated with a strongly meandering jet stream and might cause simultaneous heat waves and floods across the Northern Hemisphere.

Rossby waves, also known as planetary waves, are a type of wave that naturally occur in rotating fluids and are seen in atmospheres and oceans owing to the planet’s rotation. They are giant meanders in high-altitude winds and have a major influence on weather.

According to the National Oceanic and Atmospheric Administration, these waves form primarily from Earth’s geography. They help transfer heat from the tropics toward the poles and cold air from the Arctic to temperate regions. They are identified by specific numbers, and each number indicates the number of up and down swings observed around the planet.

“We found that the conditions for the Alberta wildfires in 2016 were likely favoured by a circulation pattern in the jet stream that was expanding beyond the Fort McMurray region and was actually spanning around the whole hemisphere,” said Kornhuber.

“The wave number 4 pattern refers to the number of up and down swings observed across the hemisphere. Wave numbers 5 and 7 seem to re-occur over the exact same positions. Thus, they tend to affect the same regions again and again.”

These wave patterns trigger simultaneous heat extremes in specific regions. Wave 5 affects North America, Eastern Europe and eastern Asia while wave 7 affects west-central North America, Western Europe and western Asia.

When these atmospheric waves dominate the jet stream circulation, vulnerable regions may see increases by a factor of up to 20 for severe heat extremes. The report said that two or more weeks per summer spent under the influence of wave 5 or wave 7 are associated with four percent reductions in crop production when averaged out, with regional decreases up to 11 percent.

“Until now this was an underexplored vulnerability in the food system,” said Kornhuber.

“We have found that during these events there actually is a global structure in the otherwise quite chaotic circulation. The bell can ring in multiple regions at once and the impacts of those specific interconnections were not quantified previously.”

Coincidentally, the affected regions are important global food production areas. Multiple harvest failures would potentially lead to food supply crises, threats to food security and other far-reaching social upheavals.

“Extreme weather events apply stress on societies as they often implicate a scarcity of resources or the loss of infrastructure,” said Kornhuber.

“Though this seems to be intuitively clear, it is often hard to quantify such effect. It has been shown that extreme weather can lead to social unrest in countries that are already fractionalized. When harvests are hit in one or several countries, export bans can lead to price spikes, which especially affect importing countries. High oil and bread prices can be a spark for social unrest as seen in recent history.”

The research is particularly valuable for producers and food security experts. Under normal conditions, low harvests in one area are balanced out by good harvests elsewhere, but the influences of these waves can change things.

“These waves can cause reduced harvests in several important breadbaskets simultaneously, creating risks for global food production,” co-author Dim Coumou, who works with the Institute for Environmental Studies at VU Amsterdam, said in the news release.

Added co-author Radley Horton of the Lamont-Doherty Earth Observatory at Columbia University: “If climate models are unable to reproduce these wave patterns, risk managers such as reinsurers and food security experts may face a blind spot when assessing how simultaneous heat waves and their impacts could change in a warming climate.”

Kornhuber said that co-occurring heat waves will become more severe in the coming decades if greenhouse gases are not mitigated.

“This can lead to food price spikes and have impacts on food availability even in remote regions not directly affected by heat waves.”

The scientists said that a thorough understanding of what is driving this jet stream behaviour may lead to improvements in seasonal predictions for agricultural production at a global level and provide assessments for potential harvest failures across the world’s food-producing regions.

“In our paper we identify the specific circulation patterns,” said Kornhuber.

“Establishing reliable predictions or early warning systems will remain for further research.”

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