Considering larger-scale weather phenomena and temporal variation on the global distribution of extreme heat events (EHEs) across 150 most populated metropolitan locations

Extreme heat events (EHE) can contribute to heat-related morbidity and mortality rates, especially in metropolitan locations. As EHE frequency increases due to climate change, metropolitan locations with higher population densities will become more vulnerable to extreme heat exposures. While EHEs can have a significant impact on public health at the global scale, the effect of ENSO-based EHEs around the world is less focused on epidemiological studies. So in this work, we aim to characterize EHE frequency over time and across different ENSO phases on EHEs in the 150 most populated metropolitan locations in the world. We extracted Global Historical Climatology Network (GHCN) data through the National Climate Data Center (NCDC) Data Portal between 1980 to 2017 for selected metropolitan locations. We used 20 years (01/1980-12/1999) of daily meteorological data (maximum temperature) to calculate calendar day and location-specific 95^th percentile thresholds to define EHEs. We characterized EHE frequency distribution across calendar seasons (Spring, Summer, Fall, Winter) and ENSO (EI Nino, La Nina, and Neutral) phases. We applied ordinary least squares regression to estimate temporal and ENSO-phase rate changes in EHE frequency for each location. We grouped studied locations as defined by the World Health Organization (WHO) and compared annual changes in EHE frequency and EHE-related exposures at the regional scale. Globally, we found an overall increasing trend with EHE frequency with a significantly accelerated rate after 1996. Metropolitan locations in the African Region had the highest EHE frequency in the 2000s ((10.5 EHEs/spring, IQR: 6.3-19.7 EHEs/spring), (9.3 EHEs/summer, IQR: 3.3-16.2 EHEs/summer), (12.3 EHEs/fall, IQR: 4.3-15.0 EHEs/fall, (10.8 EHEs/winter, IQR: 6.0-16.6 EHEs/winter)). Most of the Neutral phases revealed the lowest EHE frequency. Lastly, metropolitan locations within the Northern Hemisphere exhibited a higher rate in EHE frequency than the Southern Hemisphere. Our results characterized temporal variation and impact of ENSO on EHE frequency in the 150 most populated metropolitan locations. This finding suggests the consideration of future ENSO patterns when developing extreme heat preparedness and adaptation policies by public health agencies and researchers.