The northern hemisphere has experienced a spate of extreme weather in recent times. In 2012 there were destructive heat waves in the US and southern Europe, accompanied by floods in China. This followed a heat wave in the US in 2011 and one in Russia in 2010, coinciding with the unprecedented Pakistan flood – and the list doesn’t stop there.
Now we believe we have detected a common physical cause hidden behind all these individual events: each time one of these extremes struck, a strong wave train had developed in the atmosphere, circling the globe in mid-latitudes. These so-called planetary waves are well-known and a normal part of atmospheric flow. What is not normal is that the usually moving waves ground to a halt and were greatly amplified during the extreme events.
Looking into the physics behind this, we found it is due to a resonance phenomenon. Under special conditions, the atmosphere can start to resonate like a bell. The wind patterns form a regular wave train, with six, seven or eight peaks and troughs going once around the globe (see graph). This is what we propose in a study published this week together with our colleagues of the Potsdam Institute for Climate Impact Research (PIK).
Normally, an important part of the global air motion in the mid-latitudes of the Earth takes the form of waves wandering around the planet, oscillating irregularly between the tropical and polar regions. So when they swing northward, these waves suck warm air from the tropics to Europe, Russia, or the US; and when they swing southward, they do the same thing with cold air from the Arctic. This is a well-known feature of our planet’s atmospheric circulation system.
However, during several recent extreme weather events these planetary waves almost froze in their tracks for weeks. So instead of bringing cool air after having brought warm air before, the heat just stays. And stays. And stays. In fact, we detected a strong amplification of the usually weak, slowly moving component of these waves.
Time is critical here: two or three days of 30°C are no problem, but 20 or more days lead to extreme heat stress. Since many ecosystems and cities are not adapted to this, prolonged hot periods can result in a high death toll, forest fires, and devastating harvest losses.
What does climate change have to to with it?
Climate change caused by greenhouse-gas emissions from fossil-fuel burning does not bring a uniform global warming. In the Arctic, the warming is amplified by the loss of snow and ice. This in turn reduces the temperature difference between the Arctic and, for example, Europe. Yet temperature differences are a main driver of air flow, thereby influencing the planetary waves. Additionally, continents generally warm and cool more readily than the oceans.
These two factors are crucial for the mechanism now detected. They result in a changing pattern of the mid-latitude air flow, so that for extended periods the slow waves get trapped. The irregular surface temperature patterns disturb the global air flow. This analysis is based on equations that our team of scientists developed, mathematically describing the wave motions in the extra-tropical atmosphere. The conclusions drawn from the equations were tested using standard daily weather data from the US National Centers for Environmental Prediction (NCEP).
During recent periods in which several major weather extremes occurred, the trapping and strong amplification of particular waves – like “wave seven” (which has seven troughs and crests spanning the globe) – was observed. The data show an increase in the occurrence of these specific atmospheric patterns.
This analysis helps to explain the increasing number of unprecedented weather extremes. It complements previous research that already showed that climate change strongly increases the number of heat records around the world, but which could not explain why previous records were broken by such stunning margins. The findings should significantly advance the understanding of weather extremes and their relation to man-made climate change.
The new data show that the emergence of extraordinary weather is not just a linear response to the mean warming trend, and the proposed mechanism could explain that.
Still, things are not at all simple. The suggested physical process increases the probability of weather extremes, but additional factors certainly play a role as well, including natural variability. Also, the 32-year period studied in the project provides a good indication of the mechanism involved, yet is too short for definitive conclusions.
So there’s no smoking gun on the table yet – but quite telling fingerprints all over the place.