Graphs showing anomalies in the zonal wind in different time periods
The top row shows anomalies of the zonal wind in November; the bottom row shows the same for December to March. When the polar stratosphere is well represented in a model, the upper-tropospheric tropical warming (bottom left panel) and Arctic amplification (bottom middle panel) add up (bottom right panel) to produce an equatorward shift of the jet in northern hemisphere winter. Credit: Peings et al. [2019], Figure 2
Source: Geophysical Research Letters

In recent years, the breakdown of the “polar vortex” has often made the evening news, to explain unusually cold snaps over North America. The vortex is a climatological feature of the atmospheric circulation that is associated with the temperature gradient, or difference, between the tropics (low latitudes) and the pole (high latitudes): in winter, when this temperature difference is largest, it makes for a strong mid-latitude jet, of winds blowing west to east, which corral the cold air near the pole.

Warming of the atmosphere can affect the jet in two contrasting ways, hence the reference to a “tug-of-war”. As the tropics warm and expand, they push the jet poleward. But because the Arctic is warming more than the rest of the hemisphere, a phenomenon known as “Arctic amplification”, the equator-to-pole temperature difference is overall reduced, weakening the jet.

Up until now, modelling studies were marred by a lack in consistency, both in terms of their experimental design and the responses identified. Peings et al. [2019] explore an important factor resolving this lack of consistency, which typically results in the “tug-of-war”: the polar stratosphere. With this key component appropriately represented in a climate model, they find that in fact, in wintertime, upper tropospheric tropical warming and Arctic amplification do not oppose each other, but rather, concur to weaken the jet.

Citation: Peings, Y., Cattiaux, J., & Magnusdottir, G. [2019]. The polar stratosphere as an arbiter of the projected tropical versus polar tug of war. Geophysical Research Letters, 46, 9261– 9270.

—Alessandra Giannini, Editor, Geophysical Research Letters

Text © 2019. The authors. CC BY-NC-ND 3.0
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