Professor Wendisch, you are co-author of the comprehensive article recently published in Nature Climate Change on the subject of Arctic amplification. The title refers to the influence of this phenomenon on severe winter weather in the middle latitudes. And yet, we don’t notice this at all in Germany. What does the title of the article mean exactly?
It is true that in recent years we have had fairly mild winters in Germany, and indeed throughout Western Europe, rather than extremely cold ones. But if we look at Arctic amplification, meaning the mechanisms that cause the climate in the Arctic to change more rapidly than in other regions, then we notice significant winter cooling across the large continents of the mid-latitudes. Specifically, this means North America and Siberia, or North Asia. In winter, for example, we can often observe an area of high pressure over Siberia that is intensified by Arctic amplification; this in turn causes increased cooling over the Siberian continent. And Western Europe is on the warm side of that high, so to speak. If you then consider that many scientists from the US and Canada were involved in the article, this explains the title’s emphasis on extreme winter weather.
How does this influence make itself felt?
We believe there is a close connection between severe winter weather and Arctic amplification. At least the measurements have relatively clearly shown this to be the case. We have only a limited understanding of how this connection actually comes about, and which processes play a key role in it, as well as of Arctic amplification itself. There is a theoretical chain of explanation for this, albeit one that is still rather shaky because the vast majority of climate model calculations have so far been unable to verify it to a satisfactorily convincing degree. According to established models, continental cooling in winter should be much weaker, and in some places you would even expect warming. One important factor, which the models do not yet consider in a sufficiently realistic way, are inflows of warm air into the Arctic, as well as cold air eruptions from the Arctic into our geographical regions. It is important to reproduce these processes realistically in the models in order to be able to understand all interactions and then also make predictions – and this is of course what people expect of science. But at the moment there is still a lot of work to be done to achieve this goal.
The second funding phase of the Collaborative Research Centre “Arctic Amplification (AC)³”, of which you are the spokesperson, has just begun. Its members also include the Universities of Bremen and Cologne, the Alfred Wegener Institute, the Helmholtz Centre for Polar and Marine Research (AWI), and the Leibniz Institute for Tropospheric Research (TROPOS). What did you achieve in phase one, and what are your aims for phase two?
Our Collaborative Research Centre investigates above all the role of clouds in Arctic amplification. We made significant progress in this respect during phase one. Previously, there had been little research into its influence, especially over different ground conditions and above sea ice and the open ocean. We essentially conducted complex surveys and then merged different measurements and calculations. This enabled us to show, for instance, that models have so far often depicted clouds too thinly. Our surveys were of great help in identifying the specific causes behind the models’ shortcomings.
In the first phase, we concentrated on local processes – those that take place at a fixed location. We focused measurements from ships, aircraft, ground stations and satellites on a single area, where we investigated and quantified the influence of clouds on the warming or cooling of the ground-level air temperature. We did this over the Arctic sea ice, over the edge of the ice to the open sea, and over the water. This has never before been done with such precision. In the second phase, we will focus on the long-range effects, meaning those that do not originate directly in the Arctic, as well as those that originate in the Arctic and have an impact elsewhere in the mid-latitudes. We will also have a whole year as an observation period. Our basic hypothesis is still that clouds play a major role in Arctic amplification.
The MOSAiC expedition is currently under way, with the German icebreaker Polarstern frozen in ice for 14 months. Scientists from your Collaborative Research Centre are on board, and you yourself will be heading out there in March. What have you got planned?
From mid-March to mid-April and then again in August, in cooperation with the Alfred Wegener Institute and other partners within the Collaborative Research Centre we will be overseeing the deployment of the aircraft Polar 5 and Polar 6. We will take off with the planes from Svalbard and then, after stopping to refuel on Greenland, fly along the air masses moving towards the research vessel. This will give the local measurements taken on the Polarstern a regional connection, so to speak. The polar aircraft will use a landing strip on the ice very close to the Polarstern. The runway is also intended to transport goods and personnel on Russian transport planes – in the period when it might become difficult for other icebreakers to make such deliveries to the Polarstern by sea. In addition, our junior professor Heike Kalesse and one of my doctoral students as well as colleagues from the Leibniz Institute for Tropospheric Research will spend several months aboard the Polarstern. Their main task will be to conduct measurements with a tethered balloon.
For our work in the Collaborative Research Centre, however, we absolutely need the German research aircraft HALO as well, because we have to fly at higher altitudes and over longer distances than is possible with the polar aircraft. Water vapour, for example, as one of the most important greenhouse gases, can extend for up to five kilometres and more. We will be conducting our major Arctic survey campaign with HALO in the spring of next year.
The publication in Nature Climate Change:
Cohen, J., Zhang, X., Francis, J. et al. Divergent consensuses on Arctic amplification influence on midlatitude severe winter weather. Nat. Clim. Chang. 10, 20–29 (2020) doi:10.1038/s41558-019-0662-y