El Niño and Heatwaves: Why the World Gets Hotter During ENSO Warm Phase

Published: July 7, 2026 · 8 min read

The Physics: How El Niño Heats the Planet

During La Niña and neutral conditions, the tropical Pacific absorbs heat from the atmosphere and stores it in the subsurface ocean. The strong easterly trade winds push warm surface water westward, where it piles up and sinks, sequestering heat below the thermocline. It's like the ocean holding its breath.

El Niño is the exhale. The trade winds weaken. The warm subsurface water rises to the surface and spreads eastward across the equatorial Pacific. Vast amounts of heat — stored over months or years — are released to the atmosphere. Global average surface temperature rises. The effect is measurable: strong El Niño events add roughly 0.1-0.2 °C to the global annual mean temperature, making El Niño years some of the warmest on record.

But the global number masks the regional extremes. The tropical Pacific itself can warm 2-4 °C above normal. The heat spreads through atmospheric teleconnections, amplifying heatwaves thousands of kilometers from the source.

Marine Heatwaves: The Hidden Crisis

Ocean heatwaves don't make headlines like land heatwaves, but they're arguably more consequential. Warm water kills coral reefs, disrupts fisheries, fuels stronger tropical cyclones, and alters marine food webs from phytoplankton to whales. And unlike land heatwaves that break when a cold front passes, marine heatwaves can persist for months.

The 2023-24 El Niño produced a North Atlantic marine heatwave so far outside the historical range that scientists initially questioned their instruments. Sea surface temperatures in parts of the North Atlantic ran 4-5 °C above normal — not a marginal record, a statistical impossibility under pre-climate-change conditions. The event contributed to the earliest-forming Category 5 hurricane on record (Beryl, July 2024) and mass coral bleaching across the Caribbean.

For 2026-27, global ocean heat content is already at record levels entering the event. El Niño will add more heat on top. The tropical Pacific, Coral Sea, and Indian Ocean are all at elevated risk of marine heatwaves. Coral reefs that have barely begun recovering from 2023-24 bleaching face another hit.

Human Health and Mortality

Heatwaves are the deadliest weather phenomenon in most countries — killing more people than floods, hurricanes, or tornadoes combined. El Niño years see elevated heat-related mortality across South Asia, the Middle East, southern Europe, and Australia. The 2015-16 El Niño contributed to an estimated 2,500+ heat-related deaths in India alone. The 2023-24 event saw heat deaths surge across southern Europe (60,000+ excess deaths attributed to heat in summer 2023) and North America.

The risk compounds in regions without widespread air conditioning, in dense urban areas (urban heat island effect), and among elderly populations. Wet-bulb temperatures — a measure that combines heat and humidity — are the critical metric. When wet-bulb temperatures exceed 35 °C, the human body can no longer cool itself through sweating, and healthy people can die within hours. During the 2015-16 El Niño, parts of the Persian Gulf and South Asia approached this threshold. In a +1.3 °C world during the 2026-27 El Niño, temporary exceedances of survivability thresholds in the hottest regions are a genuine possibility.

2026-2027 Projections

WMO's Global Seasonal Climate Update for July-September 2026 shows "nearly global dominance of above-normal land surface temperatures" — strong wording for a typically cautious institution. The combination of a strong El Niño and ~+1.3 °C of background warming means global temperature records set in 2023-24 are likely to be challenged in 2027 (accounting for the temperature lag). The regions of highest concern: South Asia (pre-monsoon heat April-June 2027), Australia (summer 2026-27), the Middle East, southern Europe, and the southern United States. The 2026-27 event will be a stress test of heat adaptation infrastructure — cooling centers, early warning systems, power grid resilience — across the world's hottest regions.

Regional Economic Impact Comparison

The economic toll of El Niño isn't evenly distributed. Some regions absorb glancing blows while others take direct hits. The map below shows how heatwaves varies across the most vulnerable regions — and why preparedness investments produce vastly different returns depending on where you are.

The most vulnerable countries are those where agriculture accounts for a large share of GDP AND the climate is strongly teleconnected to ENSO. A country like Peru, where the fishing industry alone represents ~2% of GDP and is directly disrupted by El Niño warming, feels the impact faster and harder than a diversified economy with weaker ENSO links.

For the 2026-2027 event, the economic exposure is compounded by already-strained fiscal positions in many developing countries following the pandemic recovery period. Limited fiscal space means less capacity to absorb shocks through government spending — making early warning and preparedness even more critical.

Explore more at the El Niño Guide — comprehensive climate science explained.