A Mexican City’s Heat Island Doubled as El Niño’s Grip Grew Stronger

In the valley city of Tepic, in western Mexico, the climate is changing in a way that scientists say should worry every rapidly growing city in the developing world. A new study published in the journal One Ecosystem has disentangled two forces reshaping the city’s weather: the natural rhythm of the El Niño-Southern Oscillation, the vast Pacific Ocean cycle that sways temperatures and rainfall across the tropics, and the urban heat island, the bubble of trapped warmth that swells as cities pave over forests. What the researchers found is striking. Between the historical period of 1980-1999 and the recent period of 2000-2018, Tepic warmed by a full degree Celsius on average. Its urban heat island expanded by more than 60 percent in area and grew roughly 8 degrees hotter at its extreme. And as the city spread, El Niño’s influence on local temperature doubled, rising from 10 percent to 20 percent of the variation.

The study, led by researchers analyzing nearly four decades of weather station records, satellite imagery and ocean indices, set out to answer a deceptively simple question: when a local climate shifts, how much of that shift belongs to the planet’s natural cycles, and how much belongs to us? To do this, the team contrasted two twenty-year windows, using a single weather station in central Tepic with continuous data from 1980 to 2018 and only 2.6 percent missing readings. They standardized monthly temperature and precipitation anomalies and correlated them with the multivariate El Niño index, a composite measure built from sea-level pressure, winds, sea surface temperature and cloudiness over the tropical Pacific. They then layered on a Bayesian network analysis, a probabilistic technique that estimates the chance of a climatic outcome given an ENSO phase, even when no strict cause-and-effect relationship exists.

The temperature signal was unambiguous. Average temperature climbed from 21.9 degrees Celsius in the historical period to 22.9 degrees in the recent one. Maximum temperatures rose by about 0.8 degrees, but the most dramatic change came at night: minimum temperatures jumped by roughly 1.3 degrees, and during the autumn and winter months of October through January, the increase exceeded 2 degrees. Rising nighttime minimums are a classic fingerprint of urbanization, because asphalt and concrete absorb heat all day and release it slowly after dark. They are also among the most dangerous manifestations of warming for human health, depriving bodies of the overnight recovery window during heat waves and straining the cardiovascular systems of the elderly and children.

Precipitation told a more turbulent story. The annual total barely moved, hovering near 1,130 to 1,160 millimeters across both periods, but the seasonal rhythm shifted sharply. Summer rainfall declined by between 2 and 6 percent, while autumn and winter rains surged by 13 to 20 percent. That combination, drier summers and wetter autumns, is a recipe for both water stress and flooding, and Tepic is poorly equipped for the latter. The city’s drainage infrastructure is more than fifty years old, its natural watercourses have been obstructed by construction, and buildings have spread into flood-prone zones. The researchers warn that increasingly intense autumn storms will overwhelm sewers and urban rivers, a problem they trace directly to decades of unplanned growth.

The ENSO analysis revealed something unexpected: the Pacific’s grip on Tepic’s climate is not static, and it is moving in opposite directions for temperature and rain. El Niño’s influence on temperature variation doubled, from 10 percent in the historical period to 20 percent in the recent one, with warm Niño events now reliably associated with temperature increases and cool Niña events with decreases. The correlation between the ocean index and minimum temperature flipped entirely, from an inverse relationship of minus 10 percent to a positive one of 19 percent. In winter and spring months, the probability that a Niño event would raise minimum temperatures leapt from 8 percent historically to 34 percent in the current period. Meanwhile, ENSO’s influence on precipitation weakened, falling from 17 percent to 8 percent, yet the anomalies it produces grew more extreme, with the amplitude of wet and dry swings widening in the recent record.

Those shifting probabilities matter for forecasting. During summer El Niño events, the probability of reduced rainfall doubled from 10 percent to 20 percent, and the phenomenon’s seasonal footprint contracted from July through September to June through August, heightening the risk of drought precisely when crops and reservoirs need water most. In winter, Niña events now carry a 33 percent probability of precipitation decrease, up from 24 percent, threatening the severe droughts that historically struck between March and May. On the wet side, autumn El Niño events still bring torrential rains, though with diminished probability, 14 percent versus 31 percent historically. The Bayesian analysis also showed anomaly ranges stretching from severely dry to extremely wet conditions, a sign that when ENSO does act on Tepic’s rain, it acts harder.

To measure the human hand directly, the team turned to satellites. They processed thermal infrared imagery from Landsat 5, 7 and 8 for March of 1986, 2000 and 2018, converting raw radiance into surface temperature through standard brightness-temperature transformations calibrated across the different sensors. The picture that emerged was of a city cooking itself. In 1986, surface temperatures across Tepic ranged from 20.1 to 36.9 degrees Celsius. By 2000 the maximum had climbed to 43.9 degrees, and by 2018 it reached 45.3 degrees, roughly 8 degrees hotter than the historical ceiling. The heat island itself, defined as the zone of very high surface temperatures, covered 9.2 percent of the city in 1986, shrank in relative terms to 6.8 percent in 2000 and 6.1 percent in 2018, but in absolute area it exploded: the 2018 footprint amounted to 345.5 hectares against 217.1 hectares in 2000, a 62.8 percent expansion.

The geography of that expansion tells its own story. In 2000 the heat island was concentrated in the historic center. By 2018, scorching zones had appeared in the southeast, precisely where the city’s fastest growth occurred. The urban footprint doubled between the mid-1980s and 2015, and grew another 43.6 percent between 2000 and 2018 alone, much of it at the expense of the forests surrounding the city, including the Sierra de San Juan, a state-level biosphere reserve that supplies critical ecosystem services. The region lost about 30 percent of its adjacent forest cover over the study period. Vegetation absorbs thermal radiation and releases moisture, creating a microclimate buffer against planetary-scale phenomena; strip it away, the researchers argue, and the city loses its natural shield, leaving local temperatures more exposed to swings driven by the Pacific.

That loss of buffering may explain the study’s most provocative finding: the apparent link between heat island growth and ENSO’s doubled influence on temperature. The relationship remains descriptive rather than statistically proven, and the authors are careful to say so. They relied on a single weather station, made only three satellite observations across four decades, and could not perform a formal statistical correlation of heat island area against climate variability. They call for annual monitoring, in-situ temperature measurements of different urban surfaces, an inventory of green spaces, and the inclusion of future climate scenarios. They also suggest examining other ocean-atmosphere drivers, from the Pacific Decadal Oscillation to the North American monsoon and hurricanes, that could be shaping Tepic’s climate alongside ENSO.

Even with those caveats, the implications reach far beyond one mid-sized Mexican city. Tepic’s population of nearly half a million in its metropolitan area, hosting over 40 percent of the state of Nayarit’s people, makes it a template for hundreds of cities across the developing world where urban sprawl is outpacing planning. The consequences documented here, higher electricity demand and greenhouse emissions, water stress, intensified heat waves, flooding from aging drainage, and risks to public health ranging from viral disease to psychological strain, are the standard costs of unplanned growth. The researchers’ prescription is blunt: expand meteorological monitoring networks, invest in climate action research, and treat urban planning as an essential instrument for building climate-resilient cities. As global temperatures rise and the Pacific keeps oscillating, the cities that protect their forests and green space, the study suggests, will be the ones best armored against both.

Subject of Research: The influence of ENSO and the urban heat island on temperature and precipitation variation in Tepic, western Mexico

Article Title: Influence of ENSO and the urban heat island on climate variation in a growing city of the western Mexico

Article References: Nájera González, O., Nájera González, A., Marceleño Flores, S., & Flores Vilchez, F. (2024). Influence of ENSO and the urban heat island on climate variation in a growing city of the western Mexico. One Ecosystem, 9, Article e125302. https://doi.org/10.3897/oneeco.9.e125302

Image Credits: AI Generated

DOI: 10.3897/oneeco.9.e125302

Keywords: ENSO, El Niño, urban heat island, climate change, Tepic, Mexico, urbanization, deforestation, Landsat, precipitation, Bayesian networks, urban planning