Past Evolution and Recent Changes in Western Europe Large-scale Circulation

Abstract

Abstract. Detecting trends in regional large-scale circulation (LSC) is an important challenge as LSC is a key driver of local weather conditions. In this work, we investigate the past evolution of Western Europe LSC based on the 500 hPa geopotential height fields from 20CRv2c (1851–2010), ERA20C (1900–2010) and ERA5 (1950–2010) reanalyses. We focus on the evolution of large-scale circulation characteristics using three atmospheric descriptors that are based on analogy – characterizing the geopotential shape stationarity and how well a geopotential shape is reproduced in the climatology – together with a non-analogy descriptor accounting for the intensity of the centers of action. These descriptors were shown relevant to study precipitation extremes and variability in the Northwestern Alps in previous studies. Even though LSC characteristics and trends are consistent among the three reanalyses after 1950, we find major differences between 20CRv2c and ERA20C from 1900 to 1950 in accordance with previous studies. Notably, ERA20C produces flatter geopotential shapes in the beginning of the 20th century and shows a reinforcement of the meridional pressure gradient that is not observed in 20CRv2c. We then focus on the recent changes in LSC from 1950 to 2019 using ERA5. We combine the four atmospheric descriptors with an existing weather pattern classification to study the recent changes in the main atmospheric influences over France and Western Europe (Atlantic, Mediterranean, Northeast, Anticyclonic). We show that little changes are found in Northeast circulations. However, we show that Atlantic circulations (zonal flows) tend to become more similar to known Atlantic circulations in winter. Anticyclonic conditions tend to become more stationary in summer – a change that can potentially affect summer heatwaves. Furthermore, Mediterranean circulations tend to become more stationary, more similar to known Mediterranean circulations and associated with stronger centers of action in autumn, which could have implications for autumn extreme precipitation in the Mediterranean-influenced regions of the Southwestern Alps.

Dynamical Properties of Weather Regime Transitions

“…We interpret this as a hint that small-scale organization may be necessary to large-scale transitions. Other local indicators also based on analogues such as the ones used by [24] and [25] shall also be considered in an attempt to predict transitions.…”

Unmasking the Effects of Aerosols on Greenhouse Warming Over Europe

“…Despite the change in the rate of decrease of AOT, the total warming in Central and Eastern Europe has however increased at more or less the same rate after 1980, although somewhat faster for clear-sky condition compared to the all-sky situation. Beside enhanced greenhouse effect and reduced impacts from the aerosols, previous studies suggest changes in atmospheric circulation (Blanc et al, 2022;Sfică et al, 2020), which may have contributed to the rapid warming. The present finding of a substantial warming in Europe due to decreases in aerosols in addition to greenhouse warming is important and relevant when considering other regions on Earth in the future.…”

“…Blanc et al. (2022) investigated possible changes in large‐scale circulation over the period 1950–2019 for Western Europe using European Centre for Medium‐range Weather Forecasts (ECMWF, Hersbach et al., 2019) ERA5 reanalysis. They found that anticyclonic conditions tend to become more stationary in summer.…”

“…Their results relied on five data sets: four reanalysis sets and the fifth based on satellite retrievals from the Advanced Very High-Resolution Radiometers. Blanc et al (2022) investigated possible changes in large-scale circulation over the period 1950-2019 for Western Europe using European Centre for Medium-range Weather Forecasts (ECMWF, Hersbach et al, 2019) ERA5 reanalysis. They found that anticyclonic conditions tend to become more stationary in summer.…”

Unmasking the Effects of Aerosols on Greenhouse Warming Over Europe

“…Their results relied on five data sets: four reanalysis sets and the fifth based on satellite retrievals from the Advanced Very High-Resolution Radiometers. Blanc et al (2022) (Dong et al, 2017). A set of experiments using the atmospheric component of the HadGEM3 global climate model were analyzed to understand the causes of the warming.…”