Deliverable No. 3.2 Report on coordinated atmosphere-only multi- model assessment of the seasonal to inter-annual impact of Arctic sea ice decline on lower latitudes

Work package 3 within APPLICATE examines atmospheric and oceanic linkages with the objectives of (1) advancing our understanding of the mechanisms by which the mid-latitude weather and climate could respond to the substantial Arctic climate change that is expected in the coming decades and (2) coordinate a suite of novel multi-model experiments designed to identify the oceanic and atmospheric linkages between the Arctic region and the northern mid- latitudes. Deliverable 3.2 presents results from two of these experiments in which the impact of Arctic sea-ice loss is investigated using state-of-the-art climate models forced by sea-ice concentrations from the present-day climate and from a climate with reduced sea-ice concentrations in the Arctic. Investigating the difference between these model experiments allows us to assess the effect of Arctic sea-ice decline, focusing on lower latitudes. This is relevant for the project objectives of APPLICATE, which is to develop enhanced predictive capacity for weather and climate in the Arctic and beyond, and to determine the influence of Arctic climate change on Northern Hemisphere mid-latitudes, for the benefit of policy makers, businesses and society. Results show that Arctic sea-ice loss is associated with significant low-level warming over the Arctic region. The warming is strongest during boreal autumn and winter. In these seasons, the near-surface temperature response is largest in the regions where the sea ice is changing the most, over the Arctic Basin during autumn and more over the marginal seas during winter; but temperature changes are also occurring in regions in Eastern Europe, Siberia and North America. Most models show an equatorward shift of the Northern Hemisphere tropospheric jet stream during winter, but there is large variation in the magnitude of the response. Furthermore, even the sign of the response of the stratospheric jet is uncertain. We find a significant low- pressure anomaly residing over the central Arctic during fall. During winter, the response varies more from model to model, but there is generally an increase in pressure in the Icelandic region and decrease further south (though the significance varies between models), in line with a negative phase of the NAO and an equatorward shift of the tropospheric jet. A shift in the tropospheric jet has potential ramifications for extreme events. We find a significant decrease in surface winds and precipitation during winter over Northern Europe in HadGEM3-GA7.1, which corroborates results from the recent APPLICATE case study 1 which highlights the potential role of low autumn Arctic sea ice leading to extreme climate events at mid-latitudes the following winter. Extreme weather and Arctic amplification has also been linked to changes in the waviness of the atmospheric flow. We find a significant influence of Arctic sea-ice loss on planetary-scale waves during summer in CNRM-CM6. However, no significant differences are found during winter, nor for synoptic-scale waves for either season. These results are corroborated by results from ECHAM6.3, using a sinuosity index. The spread in model responses reported here is expected and desirable since it potentially allows the real-world situation to be diagnosed using the “emergent constraint” framework. A key part of this will be to understand the physical processes in detail and hence develop an observable metric to explain the differences between models and reduce the uncertainty. Other experiments within APPLICATE are aimed at understanding the physical processes, specifically using coupled models and testing the sensitivity to regional sea-ice changes and model biases. These will be reported in future deliverables D3.1 and D3.3. An important achievement of APPLICATE has been establishing the Polar Amplification Model Intercomparison Project (PAMIP), which includes the experiments described above and expands the set of models beyond those run by the APPLICATE partners. An overall synthesis of results from APPLICATE and PAMIP will be presented in D3.4