Norway sits at the tail of the very active North Atlantic storm track, a fact that those of us living on the west coast are well aware of. Between 60% and 90% of extreme precipitation events in Norway are linked to cyclones travelling within the storm track.
Storm tracks control many features of midlatitude weather, including extreme precipitation and wind events. On global scales, storm tracks maintain Earth’s habitable climate by transporting energy from the equator towards the poles. Their fate under greenhouse warming will have widespread consequences for some of the most heavily populated areas in the world, yet this fate remains highly uncertain.
“Any change in the storm tracks will undoubtedly affect Norway, and the storm tracks will undoubtedly change under global warming, but the million dollar question is what will these changes be?” says Camille Li, associate professor at UiB and the Bjerknes Centre for Climate Research.
She is part of a team of storm track researchers who have published a new review paper in Nature Geoscience. The paper grew out of a workshop held last year in Switzerland, of which the 11-strong author team from institutions in Europe, North America and Asia was the organizing committee. It focuses on recent advances on the dynamics of storm tracks and jet streams, as well as key questions related to their regional response to climate change.
Multiple environmental influences can pull both ways
“The future position and intensity of storm tracks depend on how temperature gradients will change as the Earth continues to warm. The complication is that there is no single, simple way that global warming affects temperature gradients. In fact, global warming causes changes in the climate system that can both strengthen and weaken temperature gradients, leading to a tug-of-war on the storm tracks”, says Li.
Clouds as a puzzle piece
In a press release from the University of Chicago, lead author and assistant professor Tiffany S. Shaw gives one example of how cloud changes are an important piece of the puzzle in setting Earth’s equator-to-pole temperature gradient.
In idealized and comprehensive climate model simulations, warming due to increased carbon dioxide concentrations in the atmosphere leads the clouds in high latitudes to reflect more solar radiation, thereby cooling the earth’s surface in those regions and increasing the temperature gradient between the equator and the poles. In insolation (the amount of solar energy reaching the Earth’s surface) this would lead to a poleward shift of the storm tracks. Meanwhile, those same clouds tend to enhance the greenhouse effect, thereby warming the Earth’s surface in those same regions and decreasing the temperature gradient between the equator and the poles, producing an opposite shift (also, in insolation).
This is but one example of the opposing influences noted by the authors. Other examples include the opposing influence of warming in the tropical upper atmosphere and Arctic amplification (enhanced surface warming in the Arctic), both of which occur in models in response to climate change.
One of the key conclusions of the review paper is that researchers still lack adequate knowledge of how global warming will shift the balance of processes that control the storm tracks.
According to Li, future progress depends on understanding and accurately quantifying the relative influence of physical processes on both sides of the tug-of-war. In addition to better observations, the paper argues that a key tool for studying and predicting storm track responses is a hierarchy of dynamical models of the atmosphere-ocean system.
Li participates in several research projects at UiB and the Bjerknes Centre for Climate Research dealing with precisely such processes, including jetSTREAM and DynAMiTe.
T. A. Shaw, M. Baldwin, E. A. Barnes, R. Caballero, C. I. Garfinkel, Y.-T. Hwang, C. Li, P. A. O'Gorman, G. Rivière, I. R. Simpson & A. Voigt, Storm track processes and the opposing influences of climate change, Nature Geoscience 9, 656–664 (2016) doi:10.1038/ngeo2783