Global changes in low-level circulation types under future anthropogenic forcing
Authors: Fernandez de la Granja Juan Antonio , Casanueva Ana , Bedia Joaquín , Brands Swen , Fernández Jesús 
Published: March 2024

Large-scale atmospheric circulation determines regional near-surface climate and, ultimately, causes diverse impacts on ecosystems and societies. Possible modifications of such large-scale features due to global warming would inevitably lead to changes in the weather regimes, affecting the local-to-regional climate response. Weather Type (WT) classification methods, such as the one proposed by Jenkinson and Collison (1977), provide a way to summarize mid-latitude, low-level circulation at a regional-scale (Jones et al, 2013; Fern&#225;ndez-Granja et al, 2023). JC-WTs classify sea-level pressure into 27 WTs spanning different local air flow directions and shear vorticities. This methodology can be used to evaluate Global Climate Models (GCMs), which stand as a key tool in the study of past and future climate evolution. Despite obvious biases, historical GCM simulations show a reasonable representation of the frequency of WTs worldwide (Brands 2022; Brands et al, 2023) and the newest model generation shows consistent improvements in the representation of WT occurrence (Fern&#225;ndez-Granja et al, 2021). This leads to the question of how WTs will evolve along the century under anthropogenic forcing. In this work, we address this question by considering an ensemble of GCMs from the CMIP5 and CMIP6 initiatives under different emission scenarios. We focused on evaluating the variations of the JC-WT frequencies under climate change, considering how they emerge from natural variability. Also, we analyzed the consistency between CMIP5 and CMIP6 WT projections and their associated uncertainties. The JC-WT classification was applied globally, so our findings can inform any climate impact research where changes in large-scale circulation play a fundamental role.Acknowledgement: This work is part of project CORDyS (PID2020-116595RB-I00) funded by MCIN/AEI/10.13039/501100011033. J.A.F. acknowledges support from project ATLAS (PID2019-111481RB-I00) and grant PRE2020-094728 funded by MCIN/AEI/10.13039/501100011033 and ESF investing in your future.References:Brands, S., 2022. A circulation-based performance atlas of the CMIP5 and 6 models for regional climate studies in the Northern Hemisphere mid-to-high latitudes. Geoscientific Model Development 15, 1375&#8211;1411. DOI: 10.5194/gmd-15-1375-2022Brands S, Fern&#225;ndez-Granja JA, Bedia J, et al (2023) A global climate model performance atlas for the southern hemisphere extratropics based on regional atmospheric circulation patterns. Geophysical Research Letters 50(10). DOI: 10.1029/2023GL103531Fern&#225;ndez-Granja JA, Casanueva A, Bedia J, et al (2021b) Improved atmospheric circulation over Europe by the new generation of CMIP6 earth system models. Climate Dynamics 56:3527&#8211;3450. DOI: 10.1007/s00382-021-05652-9Fern&#225;ndez-Granja, J. A., Brands, S., Bedia, J., et al (2023) Exploring the limits of the Jenkinson&#8211;Collison weather types classification scheme: a global assessment based on various reanalyses. Climate Dynamics. DOI: 10.1007/s00382-022-06658-7Jones P.D., Harpham C., Briffa K.R. (2013) Lamb weather types derived from reanalysis products. International Journal of Climatology 33(5):1129&#8211;1139. DOI: 10.1002/joc.3498Jenkinson A., Collison F. (1977) An initial climatology of gales over the north sea. synoptic climatology branch memorandum. Meteorological Office, 62Keywords: Jenkinson&#8211;Collison classification, weather types, global climate models, climate change.