Stratospheric winds may affect climate change unexpectedly

Climate change is often assumed to happen linearly, however this may not be the case according to a recent study that found out that westerly winds over Europe will not change much until the surface temperature is risen by 2 degrees.

New research found out that during winter these westerly winds will not change much until the average Earth's surface temperature is risen by approximately 2°C. Only after that do winds start to shift north and strengthen over Northern Europe.

One of expected consequences of the global warming is an increase in the temperature difference between the Tropics and the Polar Regions at altitudes of about 5-15 kilometres. Climate models usually predict that this change will push the belt of westerly winds, which crosses North Atlantic and Europe, further north, potentially bringing more winter storms to Northern Europe and less rains to Mediterranean.

"The reason for interesting, non-linear, behaviour of the winds that we find originates in the stratosphere. During the first period the stratospheric polar vortex, a system of strong westerly winds at altitudes 10-50 km, is projected to weaken, and this weakening slows down westerly winds all the way down to the surface, cancelling out the effect of the increasing temperature difference between the Tropics and the Pole", explains researcher Alexey Karpechko from FMI. However during the second period, the stratospheric polar vortex is projected to strengthen. In combination with increased temperature difference, this strengthening will give rise to stronger westerly winds over northern Europe while at the same time weaken westerly winds over Mediterranean.

More research needed to understand the non-linear mechanism

Why exactly the stratospheric winds will change so non-linearly is currently unknown.

The most likely explanation, according to the authors, is the changes in the Arctic sea ice. As the Arctic warms the sea ice melts. Additional heat released to the atmosphere from the opening ocean will amplify the Arctic warming and generate atmospheric waves which ultimately slow down stratospheric winds. When the Earth has warmed by 2°C, the marginal Arctic seas will become ice free in autumn and early winter, and the amplification of the Arctic warming, as well as its influence on the atmospheric winds, will stop.

In order to obtain their result, the authors analysed an ensemble of 68 possible climate responses to the same idealized scenario of increased concentration of carbon dioxide, more than most climate studies up to date. While this data set is large, the authors nevertheless warn that even more climate projections by different climate models are needed to increase confidence in their result.

"Nonlinear Response of the Stratosphere and the North Atlantic-European Climate to Global Warming" –research was published in Geophysical Research Letters by a group of authors led by Dr. Elisa Manzini from Max Planck Institute for Meteorology in Germany.

More information:

Head of group Dr. Alexey Karpechko, tel. +358 0361 3901, Alexey.karpechko@fmi.fi

Reference: Manzini, E., Karpechko, A. Y., & Kornblueh, L. (2018). Nonlinear response of the stratosphere and the North Atlantic-European climate to global warming. Geophysical Research Letters,45. https://doi.org/10.1029/2018GL077826