The Unprecedented Heatwave in Mexico and Texas during June 2023: Character, Context, and Physical Drivers

A severe and persistent heatwave affected Mexico and adjacent parts of Texas during June 2023, resulting in more than 130 deaths. Numerous all-time record maximum temperature (Tmax) records were broken, in some cities on multiple days and by up to 2°C. For example, San Angelo, Texas, USA (period of record 1907-present) tied or exceeded its previous all-time record Tmax (43.9°C) on five days between the 19^th-26^th, peaking at 45.6°C on the 20^th and 21^st. Chihuahua, Mexico, with a population of nearly one million, exceeded its previous all-time record Tmax three times in a span of four days, peaking at 42.0°C on the 22^nd and 24^th. Using ERA5 atmospheric reanalysis, we characterize the intensity, spatial extent, and duration of this heatwave and compare it to past heatwaves over the combined Mexico-Texas region (April-September, 1940-2023). Our results show that this heatwave was unprecedented in the reanalysis era based on all three characteristics. June 20^th, 2023 had the highest domain-averaged Tmax in this record (36.7°C), with the extent of daily standardized Tmax anomalies >2 at 1.49 million km² (~57% of domain). This was substantially larger than the previous peak daily extent of 943 000 km². In addition, it was the most persistent heatwave by a substantial margin: 11 consecutive days with >2 anomalies across at least 20% of the domain compared to the previous record of 3 days at these thresholds. To understand the local and remote physical drivers of this unprecedented event, we analyze the influence of sea surface temperature anomalies in the Pacific and Atlantic ocean basins, and antecedent precipitation and soil moisture anomalies. Further, we use self-organizing maps (SOMs) to examine the large-scale circulation patterns associated with this heatwave and evaluate dynamic and thermodynamic changes that may have increased the likelihood of this extreme event. Our results help place this heatwave into a historical context and elucidate its drivers and mechanisms to help inform future risk assessments of similar heatwaves.