See full version: New anomalous jet stream pattern imperils global food supplies
A precursor study by Kornhuber and others earlier this year noted that during the 2018 northern heat waves, more southerly regions including the Balkans and Japan saw extraordinary rains and destruction from flooding and landslides. During a 2010 northern heat outbreak in Russia, concurrent flooding on the Indus River in Pakistan displaced millions and destroyed crops. [links]
The scientists showed that in years when these amplified waves occurred during two or more summer weeks, cereal production went down 4 percent when averaged across all the affected regions, and as much as 11 percent in a single affected region. Food-price spikes often followed. [links]
The waves have hit in 1983, 2003, 2006, 2012 and 2018, when many temperature records fell across the United States, Canada, Scandinavia and Siberia. In addition to killing crops, the waves have killed thousands of people, especially in Europe and Russia, where air conditioning is far less common than in North America. [links]
Kornhuber warned that the heat waves will almost certainly become worse in coming decades, as the world continues to warm. The meanders that cause them could also potentially become more pronounced, though this is less certain. Because food commodities are increasingly traded on a global scale, either effect could lead to food shortages even in regions far from those directly affected by heat waves.
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Among other effects, these atmospheric wobbles may pull frigid air masses from the polar regions, or hot ones from the subtropics, into the populous midlatitudes. The wobbles strongly influence daily weather. When they become particularly large, they can bring prolonged heat waves, droughts or floods in summer; or in colder seasons, abnormal cold spells. more
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Upper-level divergence is proportional to both the wavelength of troughs and ridges (aka the degree of curvature) as well as to wind shear.
Thus, on an upper-level plan view the ageostrophic vectors would look something like this for a general trough/ridge system: more
Here's a more realistic view of jet streak circulations, in which flow is induced along sloped isentropic surfaces (ignoring diabatic heating) toward the divergent regions.
The winds in the jet stream do not necessarily blow at a constant speed or in a straight line. Within this fast moving air there are accelerations and decelerations as the air speeds up, slows down or in fact changes direction. It is at these points in the atmosphere that high and low pressures starts to form, and either moves quickly in the wind flow, or develops into a bigger depression or anticyclone. These positive or negative acceleration points are very important to the weather forecaster and these occur at the entrance and exits of the jet stream.
A Rossby wave can therefore lead to a string of alternating high - and low-pressure systems, with the jet stream snaking around them from west to east. Like water waves, Rossby waves generally move relative to an observer on the ground, and this movement leads to changes in the weather from week to week. In fact, the Rossby waves themselves always move towards the west, which means they are always swimming upstream against the eastward-flowing jet. If conditions are right and the wave speed matches that of the jet, the wave will remain stationary. Then the high- and low-pressure systems are no longer moving relative to the ground, and a persistent weather regime is born. Summer 2007 was a good example of this: a low-pressure system remained stationary over the UK and led to widespread flooding, while just downstream a high-pressure system brought heatwaves and drought to the Mediterranean and Eastern Europe. The UK had the trough of the wave and Eastern Europe had the peak. When waves on the ocean surface become too large they overturn and break, resulting in very turbulent motion. When Rossby waves break, the resulting weather situation is known as blocking. [links]
The strong winds along the jet stream generally blow from west to east due to the rotation of the earth. That is why, especially in winter time, flights from the USA often land early in this country as they are blown along by these very strong winds. (Incidentally it is also the reason for some "bumpy" rides with clear air turbulence). Planes never land early going the other way. more
In this case the turbulent flow often becomes dominated by an anticyclonic air mass cut off from its origin in the subtropics. This high-pressure system blocks the normal passage of the jet stream, and a regime of dry, settled weather sets in. When this happens in winter, blocking leads to a bitterly cold spell, as the mild westerly winds are replaced by winds bringing cold continental air from the east. When it happens in summer the result is drought and heatwaves, and blocking contributed to the events seen this summer in Russia. At the same time, downstream of Russia, a Rossby wave trough remained and interacted with the monsoon system to bring flooding to Pakistan
The Jet stream also helps to steer our Atlantic depressions from their normal easterly movements. If the jet stream moves south of the UK then the cold wet depressions will then get "trapped" over the UK as happened in June / July 2012. While the North Atlantic jet stream generally points roughly due east, straight across the Atlantic, it often meanders north and south. These meanders are the signature of so-called Rossby waves, which are similar to the familiar waves on the surface of water. However, instead of water moving up and down, Rossby waves consist of large air masses, often thousands of kilometres across, moving north and south. When an air mass moves north it starts to spin clockwise (when viewed from above), forming what’s called an anticyclonic weather system – a high-pressure system associated with settled weather. When air moves south it spins anti-clockwise and becomes cyclonic – a low- pressure system which is prone to wet weather.
Jet streams are narrow fast flowing "rivers" of air. They are formed by temperature differences in the upper atmosphere, between the cold polar air and the warm tropical air. This abrupt change in temperature causes a large pressure difference, which forces the air to move. In our latitude the jet stream is generally found at around 35,000 feet and is called the Polar Front Jet Stream. The polar jet stream, as its name implies, separates the cold polar air to the north and the warm sub-tropical air to the south. With the temperature contrast of these air masses greatest in the winter time, the jet stream is stronger at this time of the year, reaching 300 miles per hour (but have been measured at over 400 miles an hour in southwest Scotland). Jet streams are typically thousands of miles long, hundreds of miles wide and a few miles deep. more