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KLIMAFORSK-Stort program klima

The role of the atmospheric energy transport in recent Arctic climate change

Alternative title: Den atmosfæriske energitransports rolle i de pågående klimaforandringene i Arktis

Awarded: NOK 8.6 mill.

Do changes in the atmospheric circulation lead to Arctic warming and to melting of the Greenland ice sheet? These are questions that we attempt to answer in the project. The Arctic shows some of the world's most significant signs of climate change. These signs include a strong negative trend in summer sea-ice cover, and a warming which is three times larger than the global average. A large concern for humanity is that the climate change in the polar regions will lead to significant melting of the ice sheets and glaciers and hereby to rising of the sea level. Through the history we have built cities and infrastructure at low altitude close to the sea; human habitation at many of these locations will be challenged given a sea-level rise of a few meters. The atmospheric circulation plays a crucial role for the Arctic climate. For instance, in the the high Arctic, where solar radiation is absent in the winter months, the temperature would have been far below -100 °C during this season, had it not been for the atmospheric circulation and ocean currents bringing warm air and water from the south to these northerly latitudes. In addition the atmospheric circulation can bring warm air in over the Greenland ice sheet leading to ice melt. The role of the atmospheric circulation in recent Arctic climate change and in the recent strong melt of the Greenland ice sheet have attracted little attention. Based on a combination of analysis of observations, statistical methods, and modelling approaches, this project has attempted to provide a deeper understanding of these questions. Science work within the project reveals that humid winds cause considerable more Arctic warming than previously known. As the climate warms due to enhanced concentrations of greenhouse gases in the atmosphere, more humid air is transported northward contributing to warming and melting of the sea ice in the Arctic. The humid air increases the Arctic cloudiness, and both the clouds and the humid air enhance the greenhouse effect over the Arctic inducing more absorption of surface radiation by the atmosphere and reemission back to the surface, which leads to increased surface warming (Gaversen and Langen, J. Climate, 2019). But the humid winds not only warm the Arctic, they can also cause strong sea-ice melt. In 2007 the Arctic experienced a strong ice melt during the summer, which was unprecedented during modern history. A part of the sea ice always melts during summer, but this summer an additional area as large Western Europe became ice free in the Arctic. The same occurred in 2012, and the underlaying causes of these extreme events appear in both cases to be humid winds that during the late spring brought water vapour from the south in over the Arctic sea ice. As explained above, the water vapour lead to an enhanced greenhouse effect and increases radiation to the surface, which melted the ice (Graversen m.fl., Naturen, 2021). A breakthrough within the project is the development of methods for estimating contribution to atmospheric energy transport from different atmospheric phenomenon as for example cyclones and Rossby wave which are atmospheric waves spanning ten thousands of kilometres. These methods can be used to identify processes playing a key role for Arctic climate, and to provide an understanding of how altering of these processes lead to Arctic climate change. Studies within the project present such mathematical methods, and investigate their advantages and weaknesses (Heiskanen et al., Q. J. Royal Meteo. Soc., 2020; Heiskanen et al., Atm. Sci. Lett., in review; Stoll et al., Weath. Clim. Dyn., in review). It has long been known that the atmospheric humidity transport into the Arctic has changed only little over the last 40 years, a period where we have had good data for estimating the transport. As mentioned above, a development within the project is methods for decomposing the atmospheric energy transport into components associated with known physical phenomena, such as cyclones and Rossby waves. Studies within the project, where these methods are applied, show that even though the total humidity transport has stayed unchanged, the part caused by the Rossby waves has increased, whereas the that of the cyclones has decreased. This is important because Rossby waves can transport water vapour much deeper into the Arctic, hereby causing a much larger Pan-Arctic warming, than the cyclones are capable of doing. The conclusion is therefore that even though the total humidity transport is unchanged, changes in the atmospheric circulation patterns imply that the humidity transport contributes to Arctic warming (Rydsaa et al., Q. J. Royal Meteo. Soc., 2021). Studies also show that Rossby waves play a role in melting of Arctic sea ice (Hofsteenge et al., Clim. Dyn., 2022), and for melting of the Greenland ice sheet in later years (Heiskanen et al., Weath. Clim. Dyn., in review).

The project concerns the impact of the mid-latitude atmospheric circulation on Arctic climate. In order to quantitatively investigate these linkages, many of the project studies have focused on atmospheric energy transport. An analytical method, the Wave Method, was applied, which decomposes the energy transport into atmospheric wave types that can be assigned to known circulation patterns of the atmosphere. – The project deliver a data set based on the Wave Method that can be applied to investigate impact of atmospheric circulation on many climate aspects. Hence the data set can – and is already – used in application far beyond the objectives of the project. – Major scientific results from the project include: 1) Energy transported by the atmospheric circulation to the Arctic has a much stronger potential to impact Arctic weather an climate when transported in the form of water vapour than in the form of dry-static energy. 2) The energy transport into the Arctic contributes to enhanced Arctic warming although the transport is decreasing. This is due to a shift between the transport component so that water vapour transport to the Arctic increases. 3) Large scale Rossby-wave contribute to warming and sea-ice melt in the Arctic whereas cyclones play a minor role. 4) A shift of Rossby waves over Greenland around year 2000 has contributed to bring the Greenland ice-sheet into a melting state. – The project has provided a full PhD education for a student. It has given postdoctoral fellow opportunities to two young scientists, 2 years and 7 months for the one fellow and 5 months for the other. The project provided master thesis opportunities for two master project, where the one of these became the best master thesis of the year in 2018 at the Faculty of Science and Technology, UiT. Three internship students have obtained science-work experience within the project, two from The Netherlands and one from France. – The core team of the project is leading 7 first-author publications in international journals of which 4 are published and 3 are at different stages in the review process. The core team has also published 2 popular science articles, and the PhD student a PhD thesis. In addition, work associated with the project led by external partners, but with contributions from the core team, have led to 6 journal publications, all published or accepted for publication. – International cooperation has been supported by the project. 1) Four months of Visiting Scientists grants were given to three external researchers, two from The Netherlands and one from Italy. All contributed in articles associated with the project. 2) The project also financed shorter visits from other scientist and PhD students. 3) The project held a workshop in 2021 entitled “Dynamical interaction between the climate change in the Arctic and the mid-latitudes”. The workshop included 21 participants of which 13 were international, from Finland, US, The Netherlands, Sweden, and Italy.

The Arctic region shows some of the world's most significant signs of climate change. These signs include a negative trend in summer sea-ice cover of around 15 % per decade, and a surface-air warming which is three times larger than the global average. A large concern for humanity is that the climate change in the polar regions will lead to significant melting of the ice sheets and glaciers. In fact the discharge water from the Greenland ice sheet has in recent decades increased to the extent that this ice sheet is now one of the major contributors to sea-level rise. The atmospheric northward energy transport plays a crucial role for the Arctic climate; in fact this transport brings to the Arctic an amount of energy comparable to that provided directly by the sun. There has been little emphasis on the role of the energy transport in recent Arctic climate change, and only during the last years it has been acknowledged that different forms of the energy transport affect the Arctic climate differently. This project will significantly contribute with new knowledge on two of the most important questions concerning Arctic climate change: 1) What is the role of atmospheric circulation changes in recent Arctic warming? 2) The Greenland ice-sheet has in recent decades rapidly come into a state of significant mass-loss, will that state continue throughout the century? We approach these questions based on a combination of statistical analyses of observational data and idealised experiments with numerical climate models and an ice-sheet model. A completely new method for studying the energy transport by atmospheric waves will be applied. The results from the project will improve our understanding of the future changes of the Arctic climate, and of the future contribution to sea-level rise from the Greenland ice sheet. The results will especially be important for decision making on emissions of greenhouse gases, and on adaptation of society to climate change.

Publications from Cristin

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Funding scheme:

KLIMAFORSK-Stort program klima