Key Earth System processes to understand Arctic Climate Warming and Northern Latitude Hydrological Cycle Changes

The KeyCLIM project has created a wealth of new knowledge on the importance of coupled processes for the understanding of climate evolution in Arctic/Antarctic and northern high latitudes. The research involved in KeyCLIM has resulted in several new publications since the last year report; atmospheric rivers and extreme precipitation (Michel et al. 2021), transport through oceanic Arctic gateways (Shu et al. 2022), maintaining energy consistency and energy conservation in CMIP-class models of the atmosphere (Lauritzen et al. 2022), an extensive analysis of NorESMs cloud phase representation in the Arctic (Shaw et al. 2022), and several more are in preparation. Several major findings result from using multiple methods investigating the same phenomena. For instance, the quantification of heat and moisture transport into the Arctic, and localization of the most important source regions, has been done by combining Eulerian and Lagrangian analysis on data from NorESM and IFS reanalysis. A better understanding of the potentially huge impact of moisture transport on Arctic warming is crucial to constrain the large model spread seen in CMIP6 (Rantanen et al. 2022). Among the new and most prominent NorESM2 developments are an interactive land ice component for Greenland, a high-resolution ocean version, possibility to switch between hybrid (z-density) and sigma (density) vertical grids, more realistic mixed phase clouds, heat conductivity of snow on sea ice, full gas-phase chemistry in both the troposphere and stratosphere, improved coupling of bromoform between the ocean and atmosphere and enhanced parameterization of mesoscale eddies in the ocean. Several of the KeyCLIM sensitivity experiments have been completed, and the rest are in production. We highlight here experiments investigating the ocean planetary boundary layer, the ocean eddies, the Greenland ice sheet and the improved ice-cloud microphysics, which all were completed this year. The findings from these sensitivity experiments will, together with the Arctic sea-ice and the pre-industrial aerosol experiments described in last years report, contribute to a new updated version of the NorESM2 in 2023, a version important to prepare a comprehensive upgrade of NorESM for CMIP7. Along with the efforts to perform the sensitivity experiments, KeyCLIM organized several workshops this past year to inform all participants on model development and major results. Updates and news are kept on the new website for the KeyCLIM project; https://keyclim.met.no.

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The projected enhanced warming of the northern latitudes in the forthcoming decades will impact the region’s hydrological cycle, the cryosphere and biogeochemical cycles, impacts which are associated with forecasted, significant changes of the Earth System functioning. It is not clear how the northern world will look like with a summer ice-free Arctic Ocean, changed atmospheric and oceanic circulation patterns and the many specific responses of the Northern Earth system. It will require a powerful Earth System model to generate new understanding of the possible future climate evolution. Scandinavian and Norwegian climate will be in particular affected by Earth System changes in the North Atlantic, Arctic and adjacent land masses, thus this region is chosen to be key for this project. The core research community utilizing the Norwegian Earth System Model (NorESM) is assembled for this project proposal to improve our understanding on three levels: 1) to analyze bias and deficiencies in NorESM, engaging into complementary analysis of the CMIP6 model experiment archive and re-analysis data, from this to recommend on model development needs; 2) to study those key Earth System processes in the key region which are important for the northern climate evolution for the next 50-300 years, improving NorESM where needed; 3) isolate the role of sphere couplings and feedbacks for future projection of climate with a suite of specifically designed NorESM coupled simulations, supplementing CMIP6. Ultimately KeyCLIM shall achieve a much improved picture of the near and long term future climate for northern latitudes and Norway, providing open data to the climate mitigation and impact community. KeyCLIM studies of the interaction between the spheres and important feedbacks will allow the research community to assess the possibility of irreversible changes in the climate system.