Retreating winter and strengthening autumn Mediterranean influence on extreme precipitation in the Southwestern Alps over the last 60 years

Abstract

This article analyzes the large-scale circulations producing daily precipitation extremes in the Southwestern Alps and their trends from 1958 to 2017. We consider a high-resolution precipitation data set of 1 × 1 km2 and the weather patterns associated to the precipitation seasonal maxima at each grid point. The high-resolution allows us to analyze in details the atmospheric influences triggering seasonal maxima. Four influences are considered—the Atlantic influence, the Mediterranean influence, the northeast circulation and the Anticyclonic situation. We show that influences on maxima are very well organized in space but their organization depends on the season. Maxima are very mainly triggered by two types of influences in the region—the Atlantic influence and the Mediterranean influence. Trends in weather patterns producing maxima are also organized in space, with opposite trends for the Atlantic and the Mediterranean influences. The Mediterranean influence retreated very significantly over the period in winter and spring, while the Atlantic influence significantly extended further south. In autumn the Mediterranean influence strengthened where it was already dominant.

Linking Large‐Scale Circulation Descriptors to Precipitation Variability in the Northern French Alps

“…This classification into eight weather patterns was established to link daily rainfall field shapes over Southern France with synoptic situations. In order to focus on the main influences, we aggregate the eight weather patterns into four atmospheric influences according to the origin of the air flow reaching the French Alps (see the arrows in Figure 3 of Garavaglia et al [2010]; see also Blanchet et al [2021]). We end up with four atmospheric influences: the Atlantic influence (aggregation of Atlantic Wave, Steady Oceanic, Southwest Circulation), the Mediterranean influence (aggregation of South Circulation, East Return, Central Depression), the Northeast circulations, and Anticyclonic conditions (Figure 1).…”

Elevation-dependent trends in extreme snowfall in the French Alps from 1959 to 2019

“…This observation might be due to a stronger increasing trend in extreme precipitation for the Mediterranean circulation than for the Atlantic circulation. Indeed, precipitation maxima in the north of the French Alps are frequently triggered by the Atlantic circulation, while maxima in the south are often due to the Mediterranean circulation (Blanchet et al, 2020). Furthermore, increasing trends in extreme snowfall have already been observed in the proximity of the Mediterranean Sea.…”

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

“…Specific LSC patterns also drive extreme weather events over Europe and the Mediterranean region, including extreme precipitation (Giannakaki and Martius, 2016), floods (Stucki et al, 2012), extreme snowfall (Scherrer and Appenzeller, 2006), or heatwaves (Jézéquel et al, 2018). Extreme precipitation in the Western Mediterranean basin, Southwestern Europe, and in the southern slopes of the Alps mainly occur in autumn and they are associated with low pressure systems from the Atlantic to the Iberic Peninsula driving southwesterlies and strong southerly flows (Blanchet et al, 2021b;Horton et al, 2012;Mastrantonas et al, 2021;. Extreme precipitation in the Northwestern, Northern and Central Alps are associated with low amplitude through over the UK, zonally oriented flows and East Atlantic ridge driving southwesterlies-to-northwesterlies towards the region (Blanchet et al, 2021b;Giannakaki and Martius, 2016;Horton et al, 2012;.…”

“…Extreme precipitation in the Western Mediterranean basin, Southwestern Europe, and in the southern slopes of the Alps mainly occur in autumn and they are associated with low pressure systems from the Atlantic to the Iberic Peninsula driving southwesterlies and strong southerly flows (Blanchet et al, 2021b;Horton et al, 2012;Mastrantonas et al, 2021;. Extreme precipitation in the Northwestern, Northern and Central Alps are associated with low amplitude through over the UK, zonally oriented flows and East Atlantic ridge driving southwesterlies-to-northwesterlies towards the region (Blanchet et al, 2021b;Giannakaki and Martius, 2016;Horton et al, 2012;. Over Central Europe, summer floods and extreme precipitation appear to be associated with quasi-stationary low pressure systems over the region (Grams et al, 2014;James et al, 2004) -the extreme nature of these phenomena being related to the extremeness in several atmospheric predictors (Kašpar and Müller, 2014;Müller et al, 2009).…”

“…In this paper, we propose a contribution to fill this gap by employing four atmospheric descriptors that characterize the 500 hPa geopotential height field and that allow the consideration of dynamical trends within the main atmospheric influences. These descriptors were introduced in previous works, and they were shown to explain precipitation variability and extremes in the Northern French Alps (Blanc et al, 2021b;Blanchet and Creutin, 2020). They characterize i) the stationarity of a flow direction (celerity), ii) how well a flow direction is reproduced in the climatology (singularity, relative singularity), and iii) the intensity of the low and the high pressure systems (Maximum Pressure Difference).…”