North Atlantic climate variability in observations and model simulations

The North Atlantic climate variability is mainly regulated by the North Atlantic Oscillation (NAO), one of the most important patterns of atmospheric circulation variability. It has a strong influence on surface climate over this region with a marked impact on the environment, society and economy. Thus, it is of paramount importance to deepen our understanding of it and its governing mechanisms. In this work I have analysed both observed variability using ERA5 reanalysis (1950-2023) and the simulated variability through a set of twin simulations, provided by the meteorology group of the University of Barcelona, performed with the standard configuration of EC-EARTH3 global climate model (GCM) for CMIP6. In particular, the latter were used to assess the impact of air-sea coupling and radiative forcing on the NAO. As a first step, model biases were evaluated by comparing the climatology of different variables with observational data, and contrasting them with results from previous studies using also EC-EARTH. Although the initial conditions used for the different simulations are different, the model performance is qualitatively similar in terms of position and extent of the biases. Another result obtained from the analysis of model biases is that, in order to get a realistic representation of climate variability in the North Atlantic-European region, variability of sea ice concentration (SIC) throughout the year is required, hence suggesting that it is necessary to consider the effect of the ocean. This evidence is also confirmed by comparing coupled (atmosphere-ocean) and atmosphere-only simulations, performed to assess the effect of air-sea coupling. In fact, to fully explain the variability associated with the winter NAO, the coupled experiment is needed. On the other hand, air-sea coupling has no clear effect on the NAO dynamics in terms of structure and position (e.g. advection of temperature and latitudinal displacements of precipitation/storm-track): thus, it can be argued that the mechanisms involved in the NAO are predominantly dominated by internal atmospheric processes. The effect of radiative forcing has been evaluated by comparing simulations at present (year 2000) and future (year 2050) conditions, assessing also the effect of air-sea coupling. Since the future winter NAO and its impact on precipitation and temperature are similar in the corresponding coupled and atmosphere-only experiments, according to the literature, it has been found that also in this case the ocean does not play a key role in governing the NAO dynamics. However, to fully capture the radiatively-forced changes in climatology and variability of the climate system and to explain the amplitude of the anomalies linked to the NAO it is necessary to consider the effect of the ocean, as it was already the case for present conditions. The fact that the ocean is important in best representing the magnitude of NAO-related anomalies is a result that can help in defining the role of air-sea coupling and in increasing the predictability of NAO related climate variations. With this analysis, a general framework for the most important mode of climate variability in the North Atlantic-European region has been provided and the impact of air-sea coupling and radiative forcing was explored, also giving insights for further research.