Projected changes in synoptic circulations over Europe and their implications for summer precipitation: A CMIP6 perspective

Abstract

Projected changes in summer precipitation deficits partly depend on alterations in synoptic circulations. Here, the automated Jenkinson–Collison classification is used to assess the ability of 21 global climate models (GCMs) to capture the frequency of recurring circulation types (CTs) and their implications for European daily precipitation amounts in summer (JJA). The ability of the GCMs to reproduce the observed present‐day climate features is evaluated first. Most GCMs capture the observed links between the mean CTs directional flow characteristics and the occurrence of dry days and related dry months. The most robust relationships are found for anticyclonic and easterly CTs which are generally associated with higher‐than‐average occurrences of dry conditions. Future changes in summer CTs' frequencies are estimated in the high‐emission SSP5‐8.5 scenario for the sake of a high signal‐to‐noise ratio. Our results reveal consistent changes, mainly in the zonal CTs. A robust decrease in frequency of the westerlies and an increase in the frequency of easterly CTs favour more continental, dry and warm air masses over central Europe. These dynamical changes are shown to enhance the projected summer drying over central and southern Europe.

European Summer Synoptic Circulations and Their Observed 2022 and Projected Influence on Hot Extremes and Dry Spells

“…This comes as no surprise given the stronger relative influence that this CT has around the Mediterranean. Here, LF conditions dominate during half of the summer days as subtropical weaker pressure gradients extend their influence polewards (Herrera‐Lormendez et al., 2023; Otero et al., 2018). However, the overwhelming predominance of LF conditions over the south is also a result of the classification's limited ability to properly capture the extent of subtropical anticyclones over near‐equatorial latitudes due to the weaker pressure gradients (Fernández‐Granja et al., 2023).…”

“…These last two conditions are calculated independently over every grid point and regardless of the CT using 1981-2010 as the climatic reference period. • To assess the contribution of the synoptic circulation on the S22 anomalous occurrence of Tmax p95 and DDs, we employ a simple decomposition inspired by Cattiaux et al (2013) and already applied in Herrera-Lormendez et al (2023). This simple method allows us to determine how a variable X (daily Tmax anomalies relative to Tmax p95 and the number of DDs) during S22 has been influenced by a synoptic-scale dynamical effect (i.e., between-class effect diagnosed as the effect due to changes in RF) versus other sources (i.e., within-class effects diagnosed without any change in CT frequency).…”

“…(2013) and already applied in Herrera‐Lormendez et al. (2023). This simple method allows us to determine how a variable X (daily Tmax anomalies relative to Tmax p 95 and the number of DDs) during S22 has been influenced by a synoptic‐scale dynamical effect (i.e., between‐class effect diagnosed as the effect due to changes in RF) versus other sources (i.e., within‐class effects diagnosed without any change in CT frequency).…”

“…The BC term represents the part of the changes in the variable that can be attributed to the variations in the frequency of occurrence of the individual CTs. The WC term refers to the contributions of the variability of anomalies within the CTs which include thermodynamical processes (Herrera‐Lormendez et al., 2023). In other words, the modification in the link between the CTs and HDs and DDs considering no change in the CTs' frequencies.…”

“…We do not include a discussion on the climatological influence of CTs on summer DDs. However, a detailed discussion has been addressed in our previous work (Herrera-Lormendez et al, 2023). We also do not comment on the RES term since its contribution to the S22 extremes is marginal.…”