A Characterisation of Extreme‐Heat Occurrence and Its Trends in Mexico Based on a High Resolution Dataset

ABSTRACT The occurrence of extreme‐heat days during the warmest months in Mexico is analysed using daily maximum temperature ( T max ) from the ERA5 agrometeorological indicators dataset v2.0 (1979–2024; daily resolution; 0.1° × 0.1°). Extreme‐heat days were defined as those in which T max exceeded the 95th percentile; periods in which at least three of these days occurred consecutively were defined as heatwaves. Following the geographic differences in the occurrence of the warmest month, two climatological seasons were analysed: Apr–May and Jun–Aug. The comparison of AgERA5 T max against observations shows significant cold biases, but it also shows that the observed interannual variability and multiannual trends are adequately captured. Based on this finding, the analysis focused on variability and trends rather than total values. It is shown that regions with higher frequency in heatwaves include central Mexico, southeastern Mexico, and parts of Baja California, particularly during the Jun–Aug season. The longest‐lasting heatwaves occur also during Jun–Aug, mainly in central, southern and southwestern Mexico. Positive multidecadal trends in the duration of extreme‐heat days are found over most parts of Mexico; these trends are associated with more frequent heatwave events, not longer ones, and are more evident in the Jun–Aug season, over southern, northern and northeastern Mexico. The combination of positive trends in the occurrence of extreme‐heat days, and climatologically high T max values is worrisome; regions with this combination include southern Mexico the region near the City of Mexicali, parts of western Tabasco and the southern Campeche, as well as parts of northeastern Mexico. With regard to the causes, a linear regression analysis suggests that, the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO) can be associated with the observed positive trend in the occurrence of extreme‐heat days over relatively small regions, but none of these modes can explain the nation‐wide trend pattern. The latter result provides indirect support for the hypothesis that such trends might be rooted in anthropogenic causes, suggesting that positive trends in the occurrence of extreme‐heat days are likely to continue in the foreseeable future.