2023/07/15-25 Cerberus Heatwave in Southern Europe
High temperatures in Cerberus Heatwave strengthened by both human-driven climate change and natural variability
Press Summary (First published 2023/08/25, last updated 2023/11/22)
Heatwaves similar to Cerberus are between 1 ºC and 4 ºC hotter in the present than they would have been in the past across much of Southern Europe.
Cerberus was a very uncommon event.
Natural climate variability likely played a role in driving the pressure pattern linked Cerberus.
Event Description
Starting from the 10th July, Southern Europe experienced unusually high temperatures, peaking in the two following weeks. Although the European temperature record from 2021 was not broken, extremely high temperatures were recorded at many locations, for example temperatures in excess of 45 ºC in Catalunya and other locations in Spain, and Rome in Italy recording its highest ever temperature reading (yet to be officially confirmed) of 40.8 ºC. Northern Africa also experienced extremely high temperatures. The high temperatures, which coincided with dry and windy conditions, further favored extensive wildfires, notably in Greece.
During Cerberus we observe generally negative Surface Pressure Anomalies with respect to the climatology, while the Temperature Anomalies are over 10 ºC higher than the climatology. The heatwave developed on the background of the warmest month ever in the instrumental record, and of a positive phase of the El Niño–Southern Oscillation.
Climate and Data Background for the Analysis
According to the IPCC AR6 report, the “surface temperature in the Mediterranean region is now 1.5 °C above the pre-industrial level, with a corresponding increase in high-temperature extreme events (high confidence)”. The IPCC report further states that: “A growing number of observed impacts across the entire basin are now being attributed to climate change, along with major roles of other forcing of environmental change (high confidence). These impacts include multiple consequences of longer and/or more intensive heat waves.”
Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For Cerberus, we have medium-low confidence in the robustness of our approach given the available climate data, as the event is unusual in the data record.
ClimaMeter Analysis
We analyse here (see methodology for more details) how events similar to Cerberus have changed in the present (2001–2022) compared to what they would have looked like if they had occurred in the past (1979–2000) in the region [0°E 27°E 33°N 55°N]. Surface Pressure Changes over almost all of the considered domain show positive values, favouring a more intense hot temperature advection and warming due to solar radiation and vertical motions of the air. Temperature Changes show that similar events produce temperatures which are between 1 ºC and 4 ºC hotter than what they would have been in the past. This coincided with temperatures in Rome (Italy), Catania (Italy) and Tunis (Tunisia) being hotter than what they would have been in the past. We also note that Similar Past Events have become more common in the canonical summer months, while they previously occurred exclusively in September. This has likely contributed to the hotter temperatures discussed above.
Finally, we find that sources of natural climate variability, notably Atlantic Multidecadal Oscillation, may have heavily influenced the event. This means that the changes we see in the event compared to the past may be primarily due to natural climate variability.
Conclusion
Based on the above, we conclude that heatwaves similar to Cerberus are showing increasing pressure and between 1 ºC and 4 ºC warmer temperatures in the present than in the past. We interpret this heatwave as an unusual event for which natural climate variability likely played a role.
Contact Author
Gabriele Messori, Uppsala University, 📨gabriele.messori@geo.uu.se 🗣️Swedish, French, Italian, English
Davide Faranda, IPSL-CNRS, France 📨davide.faranda@lsce.ipsl.fr 🗣️French, Italian, English
Additional Information : Complete Output of the Analysis
NB1: The following output is specifically intended for scientists and contain details that are fully understandable only by reading the methodology described in Faranda, D., Bourdin, S., Ginesta, M., Krouma, M., Noyelle, R., Pons, F., Yiou, P., and Messori, G.: A climate-change attribution retrospective of some impactful weather extremes of 2021, Weather Clim. Dynam., 3, 1311–1340, https://doi.org/10.5194/wcd-3-1311-2022, 2022.
NB2: Colorscales may vary from the ClimaMeter figure presented above.
The figure shows the average of surface pressure anomaly (msl) (a), average 2-meter temperatures anomalies (t2m) (e), cumulated total precipitation (tp) (i), and average wind-speed (wspd) in the period of the event. Average of the surface pressure analogs found in the counterfactual [1979-2000] (b) and factual periods [2001-2022] (c), along with corresponding 2-meter temperatures (f, g), cumulated precipitation (j, k), and wind speed (n, o). Changes between present and past analogues are presented for surface pressure ∆slp (d), 2 meter temperatures ∆t2m (h), total precipitation ∆tp (i), and windspeed ∆wspd (p): color-filled areas indicate significant anomalies with respect to the bootstrap procedure. Violin plots for past (blue) and present (orange) periods for Quality Q analogs (q), Predictability Index D (r), Persistence Index Θ (s), and distribution of analogs in each month (t). Violin plots for past (blue) and present (orange) periods for ENSO (u), AMO (v) and PDO (w). Number of the Analogues occurring in each subperiod (blue) and linear trend (black). Values for the peak day of the extreme event are marked by a blue dot. Horizontal bars in panels (q,r,s,u,v,w) correspond to the mean (black) and median (red) of the distributions.