The longest-lasting 2023 western North American heat wave was fueled by the record-warm Atlantic Ocean

According to the World Meteorological Organization (WMO), 2023 was ranked as the warmest year in the global surface temperature record since 1850, setting new warm surface temperature records over more than 20% of the global land surface (WMO, 2023). In particular, the southwestern United States (US) and Northern Mexico experienced their longest stretch of record-breaking heat wave from late-June to mid-August, affecting over 100 million people, and causing over 200 deaths. This compounded drought and heatwave caused $14.5 billion in economic loss, the costliest natural hazard of 2023. Our analysis based on observational data indicates that the 2023 heat wave event was directly linked to a strong anticyclonic blocking pattern that persisted for more than six weeks across the western US. Regression analysis and dedicated atmospheric model simulations suggest that the anticyclonic blocking pattern was ultimately forced by the extremely warm sea surface temperature in the Atlantic. An extreme value analysis shows that the warm Atlantic along with a growing El Niño in the Pacific were responsible for doubling the heat wave number, tripling the heat wave days, and increasing the duration of heat wave events by about 50 percent in the region.