Spring 2024: unprecedented atmospheric heatwaves in Mexico
Tereza Cavazos
Abstract
In the spring of 2024, most of Mexico was has been under a mid-tropospheric heat dome that has broken temperature records (between 30 and 45°C) in many cities with devastating impacts in the population and ecosystems.Between 13 April and early June 2024, most of Mexico experienced three unprecedented long-lasting heatwaves that broke temperature records (between 30 and 45°C) in more than a dozen cities across the country, including Mexico City that has a temperate climate. The city has the longest temperature records in Mexico, which began in 1877 in the National Astronomical Observatory of Tacubaya. According to the National Weather Service maximum temperature in Mexico City washas been broken several times during this spring, reaching a new record of 33.8°C for June 3. The three heatwaves have affected the wellbeing of the population and ecosystems and created a chain of effects that spanned from human and animal deaths to problems of urban water availability, power energy outages, discomfort, and health issues. The impacts were exacerbated by the current droughts in Mexico, drying dams, forest fires, and the seasonal drying and warming conditions typical before the summer rainy season.Mexico City, the 5 th largest city in the world with an estimated population of 22 million, wasis near to reaching water "day zero" as one of its main reservoir supply systems (the Cutzamala) wasis running dry. Lake Patzcuaro, a touristic destination in the central state of Michoacan, dried out this spring; this affected the local population as fisheries and tourism are their main economic activities. Drying and warmer than usual conditions are also affecting the export of binational waters in the US-Mexico border region. Mexico owes water to the United States from the Rio Grande/Rio Bravo, but the United States also owes water to Mexico from the Colorado River basin. According to the North American Drought Monitor, both sides of the border werehave been under severe droughts this spring, but large areas of Mexico wereare under (stronger) exceptional drought since last year. Compound events of droughts with the recent heatwaves feedbacked to each other enhancing their negative impacts.On June 5, Mexico´s Health Ministry (Secretaría de Salud, 2024) reported 1937 heat-related cases and 90 deaths due to heat strokes and dehydration during the three heatwaves. Each event lasted more than a week and their impacts have been reported in many national and international news, including the speech of the United Nations Secretary-General António Guterres in his message on World Environmental Day (June 5), who called for urgent action, as record-breaking temperatures wereare scorching several cities around the world. Heat stress poses a major threat to human health and increases mortality risk, particularly in urban areas (Burke et al., 2018;Vargas and Magaña, 2020) and regions under the influence of large humidity (Raymond et al., 2020;Russo et al., 2017).As the oceans also continue to warm more than ever recorded (Cheng et al., 2024), such as the Gulf of Mexico and the North Atlantic tropical cyclone development region, near the Caribbean sea (Fig. 1a), and with the upcoming of La Niña event, the country wasis anxiously expecting the rains based on NOAA´s forecast above normal occurrence of hurricanes on the Atlantic basin, a season that began the 1 st of June. Fresh air wasis much needed now, but no tropical cyclone washas formed yet as of June 6, and the region continueds under a semi-permanent anticyclone since mid-April. Air conditioning iswas not enough in many cities under heat island conditions, where the type of housing and building materials are not adapted to extreme heatwaves lasting several continuous days and nights, and with problems of energy supply and water availability. However, for most of the population air conditioning is not even an option.Relative humidity washas been below 40% from mid-April to Mayduring the past month and a half, and in large parts of Mexico temperatures were3-5°C above average (Fig. 1b). The only regions that have escaped from the extreme effects of these anomalous heatwaves wereare the Pacific coasts of Baja California and California, which wereare under the influence of the cold California current (Fig. 1a). According to the Climate Shift Index (a climate change attribution index), the temperatures during this period in Mexico, southern Texas, southern Florida, the Caribbean Islands, Central America, and northern South America were at least 4 to 5 times more likely due to climate change. A new report of the World Weather Attribution stated that 5-day maximum temperatures during the third heatwave (end of May and beginning of June) are now expected to occur every 15 years, while at the beginning of this century were expected every 60 years (Pinto et al., 2024). This is clearly a large-scale unprecedented event.Last year was a year of temperature breaking records around the globe. Surface air temperature warmed almost 1.5°C above the 1850 pre-industrial level (Copernicus, 2024), a threshold expected to be reached by the mid or late 2030s according to the Intergovernmental Panel on Climate Change (IPCC, 2018). In July 2023, the UN (2023) Secretary-General António Guterres said "the era of global boiling has arrived". Possibly this statement is a bit extreme, but for the level of adaptation (or mal adaptation) that most populations have, it is partially true. Moreover, the unprecedented warming of the oceans produced marine heatwaves in parts of the Gulf of Mexico, the Caribbean, the Mediterranean, and the North Pacific (Copernicus, 2024). In 2024, the anomalous ocean warming has continueds, except for the eastern Equatorial Pacific where the cold waters related to La Niña event are already evident in Fig. 1a, and along the California Current off the coasts of California and Baja California which are further related to a strong North Pacific subtropical anticyclone (Fig. 1). The combined effect of the cold California current and the warm air aloft, due to the North Pacific anticyclone, resulted in a thermal inversion that favors the formation of low-level marine stratus clouds and fog. These mechanisms, which are common in spring and early summer, have prevented the arrival of hot weather to the coastal regions of California and Baja California in spring and early summer.Apart from the background warming of most of the oceans and the continents, during the spring of 2024 the tropical and subtropical regions surrounding Mexico werehave been under the remaining influence of El Niño conditions. The westerly winds of the subtropical jet stream hadhave not migrated north in early June, as it is typical in spring. MoreoverHowever, the jet stream washas been 12 m/s stronger than average in northern Mexico (Fig. 1c); consequently, the wind shear washas been also stronger than average, a condition that during the summers of El Niño events is an important ingredient that reduces the probability of tropical cyclone and hurricane formation (Martinez-Sanchez and Cavazos, 2014). Therefore, this westerly wind anomaly needs to debilitate (with a northward or northwestward migration of the jet stream) for tropical cyclones and easterly waves, once formed, can enter the Gulf of Mexico.During spring, surface warming in eastern Mexico and over the Gulf of Mexico and the Caribbean produced an anomalous low pressure in the lower troposphere (below 850 hPa; Fig. 1d). The North Atlantic subtropical high (NASH) was weaker than normal, further reducing the entrance of easterly winds to the Gulf of Mexico, which exacerbated the lack of fresh air into the region. Most importantly, in the mid and upper troposphere, an anticyclone and sinking air produced adiabatic heating, clear skies, and more entrance of solar radiation, which warmed up and dried out the surface over the continent. A heat-dome-like has dominated central and southern Mexico since mid-April to May due to the presence of a semipermanent anticyclone (Fig. 1c,1e). Heatwaves in other regions of the world have been also associated with upper-level anticyclonic stationary waves and dry soils (Chen et al., 2023). Some studies have suggested that the NASH could intensify and move westward during the summer under warming scenarios due to an increased land-sea thermal contrast (Li et al., 2012;Cherchi et al., 2018); this would be the case when the continent is warmer than the ocean, but on intraseasonal timescales when the ocean is warmer or warms up at the same rate as the continent, the NASH could weaken, like this spring. The SST anomalies in the Gulf of Mexico and parts of the North Atlantic basin during the third heatwave were more than 2°C warmer than normal (Fig. 1a).Amid this extreme warming in the tropical and subtropical regions, the cold waters in the equatorial Pacific associated to La Niña condition are already evident and the event is expected to be fully formed by the end of the summer. La Niña´s atmospheric teleconnection tends to push the subtropical jet stream to the north weakening the vertical wind shear in the Gulf of Mexico; this condition commonly favors above normal summer rains in Mexico, the Caribbean and Central America because of regional convection and more frequent hurricanes. The summer rains wereare much expected in Mexico and in the whole region to alleviate the population with cool and humid air, and with the precious water to fill rivers and dams and for the thirsty ecosystems. However, more extreme weather conditions may occur this summer, as intense hurricanes have been forecasted due to La Niña and the anomalous warming in the tropical North Atlantic basin and the arriving of La Niña event. Indeed, the first tropical storm of the season occurred at the end of spring; tropical storm Alberto formed inside the anomalously warm waters of the Gulf of Mexico and produced large amounts of rainfall in eastern Mexico during 19-20 June, enough to fill out several dams in the region. Government officials from several states agreed that ending the drought was a much larger benefit from the storm than other material damages.This is the first time Mexico experiences and records the compounding impacts of several long-lasting extreme heatwaves and droughts. This is just a taste of what can occur more often due to climate change; the future scenarios are already here. Thus, immediate action is needed to develop adaptation measures with a long-term vision; financial support should not be wasted in reactive measures in Mexico norand in any other country. However, adaptation actions to reduce climate risks need the support of transdisciplinary collaborations and different types of knowledge, local and international financial support, and political will (e.g., Cavazos et al., 2024). The role of scientist Claudia Sheinbaum, the first appointed female Mexican President, who participated in the Intergovernmental Panel on Climate Change (IPCC) Reports, and her new appointed Secretary of the Environment (SEMARNAT) Alicia Bárcenaformer Executive Secretary of CEPAL, Economic Commission for Latin America and the Caribbean, are expected to strongly support and endorse these actions at municipal, state and national levels for the wellbeing of the country.