Long-term trends in heatwave in the São Paulo megacity using the excess heat factor

Abstract

Extreme weather events have been reported more frequently, with heatwaves being responsible for thousands of deaths around the world every year. The Excess Heat Factor (EHF) is an indicator developed to identify and classify heatwaves according to their severity, based on two indices, significance and acclimatization, which refer to how hot a three-day period was in relation to a historical reference and the past thirty days, respectively. Adaptable for each region, the EHF allows the separation of events into moderate, severe, and extreme. This metric was used in the present study to identify and characterize heatwaves in the megacity of São Paulo between 1936 and 2023. The historical reference was defined as 1961-1990, the official normal period defined by the World Meteorological Organization (WMO). The results showed that approximately 75% of the severe or extreme events occurred during summer, revealing that severe and extreme heatwaves are not restricted to the typical warmest season. According to the EHF metrics, the city of São Paulo experienced 84 severe heatwave events, with some lasting over 20 days. Only 4 extreme events were registered: one in 1972, one in 2020, and two in 2023. The November 2023 extreme heatwave was the strongest ever registered in São Paulo. Positive and significant (p=0.05) historical trends were found for the number of heatwave events and also for maximum severity and the duration of the events, and as a consequence for the thermal load, especially after the 1970s.