Written by Etienne Dunn-Sigouin, postdoc at the Bjerknes Centre and the Geophysical Institute at the University of Bergen
During the winter, stratospheric winds form a fast moving vortex around the North Pole, roughly 10 kilometers above the surface, where commercial airplanes fly. On some days, rapidly growing waves that span thousands of kilometres disturb the polar vortex and can even destroy it. We collectively call these extreme stratospheric events.
The events impact the weather in the troposphere, the layer of the atmosphere where we live below, and are used by scientists to make better weather forecasts.
Scientists have debated the cause of extreme stratospheric events since the 1970s without consensus. Some theories argue tropospheric waves move upward into the stratosphere, break like to waves on a beach and disturb the polar vortex. Other theories argue that the stratospheric polar vortex can provide favourable conditions for waves to grow and destroy itself.
It is clear that stratospheric waves grow and disrupt the polar vortex, however, a key question remains: does the troposphere or stratosphere cause the events?
A new look at an old problem
New research, published in the Journal of the Atmospheric Sciences written by myself and a colleague at the University of Chicago, argues that the troposphere causes extreme stratospheric events.
We performed experiments using a simplified atmospheric model to understand events in the real world, similar to how scientists experiment with mice to understand humans.
Specifically, we forced the tropospheric waves to be similar to what happens during the events but let the stratospheric polar vortex free to evolve and vice versa. The goal was to see which experiment causes waves to grow in stratosphere.
The results are clear: the troposphere reproduces most of the events while the stratosphere does not supporting the view on the troposphere in the research debate.
The experiments are important because they overcome problems in previous research which tested only one piece of the puzzle (the troposphere or stratosphere), used models that are too simple (a fly instead of a mouse) or made conclusions based only on observations (correlation does not equal causation).
A guide towards improved weather forecasts
Weather forecasts tend to be better after extreme stratospheric events, but the events themselves are still hard to predict. Understanding the processes that cause the events could help extend the duration of better forecasts.
Our study suggests forecast centres should focus on improving the mechanisms producing tropospheric waves that move upward into the stratosphere, rather than focusing on the stratosphere itself, to improve weather forecasts.
Our results are important for Norway. A recent study suggests the February 12th 2018 event (pictured above) caused cold weather for several weeks afterwards resulting in above average snowfall and a record setting year for the Norwegian ski industry.
Dunn-Sigouin, E. and T. Shaw, 2020: Dynamics of Anomalous Stratospheric Eddy Heat Flux Events in an Idealized Model. J. Atmos. Sci., 77, 2187–2202, https://doi.org/10.1175/JAS-D-19-0231.1