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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 by comparing daily geopotential height fields to each other. They characterize the stationarity of geopotential shape and how well a geopotential shape is reproduced in the climatology. A non-analogy descriptor is also employed to account for the intensity of the centers of action. We then combine the four atmospheric descriptors with an existing weather pattern classification over the period 1950–2019 to study the recent changes in the two main atmospheric influences driving precipitation in the northern French Alps. They correspond to the Atlantic circulation pattern dominated by a zonal flow and the Mediterranean circulation pattern dominated by low-pressure anomalies over the near Atlantic, close to Portugal. 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. Over the period 1950–2019, we show that winter Atlantic circulations (zonal flows) tend to be shifted northward, and they become more similar to known Atlantic circulations. Mediterranean circulations tend to become more stationary, more similar to known Mediterranean circulations and associated with stronger centers of action in autumn, while an opposite behavior is observed in winter. Finally, we discuss the role of these LSC changes for seasonal and extreme precipitation in the northern French Alps. We show that these changes in LSC characteristics are linked to (a) the decreasing contribution of Mediterranean circulations to winter precipitation and (b) more circulations that are likely to generate extreme precipitation in autumn.
Highlights
By defining the direction and intensity of airflow and modifying stability and moisture availability, large-scale circulation (LSC) is a key driver of local weather conditions
As most of the descriptors rely on analogy in geopotential shapes, we start the analysis by checking whether the different reanalyses provide similar geopotential shapes over this period
Recalling that a Teweles–Wobus score (TWS) score of 0 represents two identical geopotential shapes, we observe that differences in geopotential shapes between reanalyses are weaker after 1950 than before (20CRv2c, ERA20C) and that differences remain quite steady from 1950 to 2010 (20CRv2c, ERA20C, ERA5)
Summary
By defining the direction and intensity of airflow and modifying stability and moisture availability, large-scale circulation (LSC) is a key driver of local weather conditions. LSC variability over the Euro-Atlantic sector influences precipitation anomalies over Europe and the Mediterranean region. The North Atlantic Oscillation (NAO) is the first mode of LSC variability over the North Atlantic (Barnston and Livezey, 1987). Other modes of LSC variability – such as the Euro-Atlantic blocking (EAB) or the East Atlantic/western Russia pattern (EA/WR) – better explain precipitation variability in central Europe, especially in the Alpine region which acts as a climatological barrier at the crossroads of different atmospheric influences (Auer et al, 2007; Bartolini et al, 2009; Beniston, 2005; Quadrelli et al, 2001; Scherrer et al, 2016). The northern flanks of the Alpine range experience wet conditions under the Atlantic ridge pattern, while the southern flanks experience wet conditions when low-pressure anomalies stand over the near Atlantic (Kotsias et al, 2019; Plaut and Simonnet, 2001)
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