A global comparison of heatwave magnitude indices using ERA5 reanalysis data

Heatwaves are one of the most harmful extreme weather events in terms of their impact on human health, infrastructure and natural ecosystems. What is extreme under present-day conditions will likely become more common in the near-future, under global warming. A proper detection and characterization of heatwave events is therefore crucial for advancing the understanding of their drivers and predictability, allowing for the development of early warning systems and timely adaptation and mitigation policies.

One of the most important features of heatwaves, in terms of impact, is their intensity. Accurately assessing and comparing the magnitude of heatwaves at different locations poses many challenges. Numerous metrics for estimating heatwave magnitude have been developed and applied, although many studies have called for a more unified and consistent use of these metrics. A comprehensive assessment of heatwave magnitude and intensity, possibly allowing for a more robust comparison of events, is still missing.  

In this study, a set of 4 previously proposed indices for the characterization of heatwave intensity are applied to high-resolution daily maximum temperatures (Tmax) derived from ERA5 reanalysis data at the global scale, for the period 1950-2020. The main goal is to detect differences between the different metrics, focusing on the effect of estimators based on averaged versus cumulative values, and highlighting possible strong or weak points of each of them. Additionally, the study aims to conduct, for the first time, an assessment of the reliability of ERA5 in terms of Tmax, against other datasets at the global scale. Finally, the results are used for analyzing the trends of heatwave intensity over the period 1950-2020, for different regions around the globe.

Differences in daily Tmax, calculated between ERA5 and the other considered datasets, are relatively small over a large part of the globe, except for Greenland and Antarctica in all seasons, and northern North America in June-July-August. The pattern of the heatwave magnitude of the most extreme events occurring in the period 1950-2020 is considerably different between estimators based on cumulative and averaged values of temperature. Also, the year when these maxima occur changes completely in the two cases, showing that the use of different metrics might lead to an underestimation/overestimation of the magnitude of specific events, with possible implications for impact and attribution studies.  In particular, the results show that heatwave magnitude indices based on cumulative values should be preferable to the ones based on averaged values. Finally, trends in the magnitude of heatwaves for the period 1950-2020 show that in the last 30 years, heatwave events doubled not only in magnitude, but also in number with respect to the first decades of the considered period, confirming findings of previous works.