Effects of rapid Arctic climate change on jet streams and extreme weather (Ref 4606)

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Project BackgroundThe lower atmospheric warming as a result of manmade CO2 emissions is several times larger in the Arctic than in other parts of the world (see figure 1), a phenomenon known as Arctic Amplification. The importance of this amplified warming could extend well beyond the Arctic, however. The midlatitude jet streams (figure 2), and their associated storm tracks, are effectively powered by the temperature difference between the warmer low latitudes and the colder high latitudes. Arctic amplification decreases this temperature gradient in the lowermost atmosphere, potentially impacting the jet stream and storms. Such changes could lead to an increase in extreme weather, which is a particular concern for the lives and livelihoods of the billions of people who live under the path of the jet stream [e.g., Cohen et al., 2014]. Arctic amplification is a robust phenomenon that appears clearly in state-of-the art climate models, and is relatively well understood. However, the response of the jet stream to Arctic amplification is far more uncertain, and varies substantially between different climate model experiments (Screen et al., 2018; Screen and Blackport, 2019). Until we better understand the ways that the jet stream is affected by Arctic amplification, we cannot skilfully predict future changes in extreme weather, which is a significant problem. This lack of a robust jet stream response across models may reflect differences in how they simulate the present-day climate. For example, the average location of the jet stream is further north in some models than others, which might affect how strongly the jet stream is affected by Arctic amplification in a one model compared to another (Smith et al., 2017). In addition, Arctic amplification occurs alongside other robust features of climate change, including the tropical upper-tropospheric warming that will also influence the behaviour of the midlatitude jet. Understanding how these processes interact, and what their combined effect is on the jet stream is of significant interest. By understanding what causes models to differ, we seek to reduce uncertainty in future projections of midlatitude climate and extreme weather.Project Aims and MethodsWe will conduct experiments using a new, relatively simple but highly configurable global climate model known as Isca, developed by Dr Thomson and others at Exeter (execlim.github.io/IscaWebsite/). Isca has the ability, unlike most climate models, to be able to turn relevant processes on and off, and to 'nudge' the atmosphere to different background states. By using these features, we can investigate how jet stream responds to Arctic amplification in a controlled manner, and ascertain which processes are the most important. We will also make use of output of new experiments already run using sophisticated climate models as part of the Polar Amplification Model Intercomparison Project (PAMIP; Smith et al., 2019) lead by Dr Smith and Prof Screen. We will combine the data from PAMIP and results from more traditional all-forcing experiments from CMIP6, as well as results from Isca to understand better how biases in present day climate can influence predictions of jet stream properties, and the impacts on extreme weather.