New computer model explains faltering jet stream
Scientists in Germany have applied machine learning to model the weakening of the jet stream that is partly responsible for extreme weather events, including the long cold winter of 2019 that held much of North America and Europe in an icy grip.
It's a strange irony that the warming of the Arctic has been bringing colder winters to regions farther south. This is partly due to a change in the jet stream, a river of air flowing at hundreds of km/h high up in the stratosphere that runs all the way around the northern hemisphere. It marks the boundary between the mass of cold Arctic air that caps the top of our planet and the warmer air around the middle.
Battleground between hot and cold air
The jet stream is the battleground between these two air masses as the warm air tries to move north and the colder pushes south. This boundary line is constantly moving, and like all rivers, the jet stream meanders, sometimes wandering south, allowing the cold air to drop down to the mid-latitudes, which happened last winter, or it can curve northwards, bringing warm air up from the tropics, producing hot, dry periods.
The jet stream can also do the opposite, depending on when the meandering happens. If it flows north in January, we get a warmer winter. And if it dips south in July, we have a cool, wet summer.
Storm systems also tend to follow the jet stream, so it has a strong effect on the weather patterns for the entire continent.
A meandering jet stream
In recent years, the weakening of the jet stream has been in part due to increased warming in the Arctic, so there is less temperature difference between the northern and southern air masses.
This has also made the stream meander more north and south — producing extra-long cold snaps and hot summers to the mid-latitudes. The effect has been known for some time, but predicting how it will change in the future has been difficult.
Scientists at the Alfred Wegener Institute in Germany have developed a new computer model to understand the complex interactions in the atmosphere that are affecting the jet stream. The model incorporates the warming effect of ice loss in the Arctic, as well as chemical interactions with the ozone layer.
As white ice melts away, the exposed land and sea water absorb sunlight more easily, releasing heat into the air close to the ground. Meanwhile, at the top of the atmosphere, chemical reactions are taking place as sunlight breaks apart oxygen to produce ozone, our natural filter from the sun's ultraviolet light.
In recent times, there has been more mixing in the atmospheric layers so the ozone effect is amplifying the warming that occurs on the ground at higher altitudes. In other words, the atmosphere in the Arctic is being warmed from the top and the bottom, and that is somehow weakening the jet stream, affecting weather to the south.
Climate model that incorporates machine learning
Climate models in the past did not include the ozone amplifying effect, so the new system will provide a more accurate representation by using an artificial intelligence algorithm called SWIFT that takes into account the complex chemical and physical effects taking place in the ozone layer.
The goal is to better predict the movement of the jet stream and how it might evolve in the future. The shape of the jet stream has impacted many aspects of our weather, from forest fire seasons in the west to tornadoes in the south. These effects are predicted to continue into the future, so a better understanding of the behaviour of the invisible river of air flowing above our heads may help us adapt.