Changes in North Atlantic atmospheric circulation in a warmer climate favor winter flooding and summer drought over Europe
Recent winter floods and summer droughts in Europe are consistent with the changes we find using a climate model with increased carbon dioxide concentrations
Publication
E. Rousi, F. Selten, S. Rahmstorf, D. Coumou “Changes in North Atlantic atmospheric circulation in a warmer climate favor winter flooding and summer drought over Europe”, Journal of Climate, 2021, https://doi.org/10.1175/JCLI-D-20-0311.1
Methods
In this paper we assess changes in atmospheric circulation patterns over the North Atlantic under an experiment of increasing carbon dioxide concentrations (CO2) of a very high resolution climate model (GFDL CM2.6):
The high resolution of the model allows for mesoscale oceanic eddies to be resolved and thus for oceanic currents, such as the Gulf Stream and the Atlantic Meridional Overturning Circulation (AMOC), to be realistically represented. This is important for the ocean-atmosphere interactions that play a significant role in forming the dominant atmospheric circulation patterns.
In order to study changes in atmospheric circulation due to CO2 increase we compared 2 different experiments of the model: one with stable pre-industrial CO2 concentrations (PI) and one with increasing CO2 by 1% per year (2xCO2):
By applying a clustering algorithm (Self-Organizing Maps) and strict subsampling to account for the internal variability in the model we find robust changes in certain atmospheric circulation patterns over the North Atlantic. The following figure shows a graphical representation of the four commonly used North Atlantic circulation patterns or weather regimes:
Robust changes in amospheric circulation
We find robust changes in atmospheric circulation patterns over the North Atlantic under increasing carbon dioxide concetrations in all months. In winter (and particularly in February), zonal regimes (NAO+ like) are increasing by ~30% in frequency in the 2xCO2. In summer (and particularly in August), Atlantic Ridge, a high-pressure system off the UK coast, is increasing by ~60%:
Impacts
Those robust changes in a warmer world may have great implications for European climate. In winter the increase of zonal circulation is linked to increased rainfall over northwestern Europe, increasing the risk of flooding in the area:
In summer, the increase of the “Atlantic Ridge” regime is linked to less precipitation and longer dry spells over western and central Europe, contributing to higher drought risk:
Those dynamical changes act supplementary to the thermodynamic effects of a warming world, exacerbating the impacts.
To summarize, studying dynamical changes in atmospheric circulation is tricky due to model biases in its representation and to the sensitivity to the method used to define circulation patterns which is also linked to the large internal atmospheric variability. In this work, the combination of a very high resolution model and a strict method that accounts for the uncertainties gives high confidence in the results. The robust changes we find are in agreement with previous studies documenting similar changes in a warming world.
