Rahm, Tabea (2023) How will the dominant weather regimes change under the influence of climate warming?. (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 64 pp.

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Abstract

Weather regimes are quasi-stationary and persistent atmospheric circulation patterns. They have been proven useful in describing the mid-latitude wintertime circulation and are commonly identified by applying a clustering algorithm to the mid-tropospheric geopotential height field. The four wintertime regimes that are typically recognised in the Euro-Atlantic sector are the positive and negative phases of the North Atlantic Oscillation (NAO), the Scandinavian Blocking, and the Atlantic Ridge regime. By impacting the large-scale atmospheric flow, the individual regimes have considerable in uence on surface weather and extreme events. In this thesis, future atmospheric circulation changes in the Euro-Atlantic region are investigated within the weather regime framework. For this purpose, simulations performed by the Swedish Meteorological and Hydrological Institute's Large Ensemble (SMHI-LENS) with the global climate model EC-Earth3 are explored. The regime patterns arising from clustering geopotential height anomalies at the 500 hPa level are calculated for two ensemble simulations, one with historical and one with SSP5-8.5 scenario forcing, and compared to ERA5 reanalysis data. Firstly, four weather regimes are identified in the SMHI-LENS historical simulation. Three of these resemble the regimes identified in ERA5 and in previous studies reasonably well. Only the positive NAO phase is not captured by the ensemble. Under the strong global warming scenario in the SMHI-LENS, the regime patterns get weaker by the end of the century, with the exception of the NAO{ regime, which experiences an enhancement of the geopotential height gradient. Composited near-surface temperature anomalies and extremes weaken, while precipitation anomalies and extremes intensify in response to global warming. The general patterns of the geopotential height, temperature, and precipitation anomaly composites, however, remain practically unchanged under the strong forcing scenario. The future changes in weather regimes simulated with the SMHI-LENS are seen to agree with the general shifts expected under global warming. Despite the disagreement between the observed and the simulated NAO+ regime, the SMHI-LENS as a single-model initial-condition large ensemble is shown to be a useful tool in robustly identifying independent circulation regimes and studying the accompanying surface weather conditions.

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