El Niño-Southern Oscillation and global warming
The El Niño-Southern Oscillation (ENSO) is a complex climate pattern involving periodic warming (El Niño) and cooling (La Niña) of sea surface temperatures in the tropical Pacific Ocean, along with associated shifts in atmospheric pressure. This phenomenon has significant ramifications not only within the tropics but globally, influencing weather patterns and climate conditions across various regions. During El Niño events, the weakening of the trade winds leads to warmer ocean temperatures and a change in atmospheric circulation, while La Niña represents the opposite scenario, characterized by cooler sea surface temperatures and stronger trade winds.
ENSO is closely connected to discussions of global warming, as increased sea surface temperatures during El Niño can contribute to a rise in global average temperatures and extreme weather events. Recent observations suggest that El Niños have become more frequent, while La Niñas have become less common, sparking interest in the potential impacts of climate change on these oscillations. Although the exact relationship remains complex and uncertain, many scientists assert that the influence of anthropogenic climate change may amplify the intensity and frequency of future El Niño events. As ongoing studies explore these interactions, the interconnectedness of ENSO dynamics and global climate change continues to be a focal point for researchers.
El Niño-Southern Oscillation and global warming
El Niño and the Southern Oscillation are linked atmosphere-ocean phenomena that occur in the tropical Pacific, but their influence on climate can be seen globally. They represent a cyclical recurrence of warm ocean currents that cause large-scale changes in Earth’s weather.
Background
El Niño is a sporadic warming of sea surface water in the central and eastern equatorial Pacific Ocean, adjacent to the Peruvian coast. This warming is part of a cycle, and the cycle’s opposite, cooling phase is called La Niña. The Southern Oscillation (SO) is a “seesaw” of air pressure and air circulation between the eastern Pacific and the Indonesian region. The terms El Niño and La Niña are used to denote the extremes of the oscillation. The broader term El Niño-Southern Oscillation (ENSO) describes the range of atmospheric and oceanic processes and their accompanying changes. Although ENSO is based in the tropics, it influences weather throughout the Northern Hemisphere and possibly the globe as a whole. Because of its effects on global temperature averages, ENSO is often discussed in connection to global warming.
Characteristics of ENSO
Definitions of ENSO vary, but a common aspect of El Niño is the irregular warming of sea surface water off the coasts of Ecuador, northern Peru, and occasionally Chile. This warming is linked to irregular changes in air pressure at sea level across the Pacific Ocean. During El Niño conditions, the westward-flowing trade winds slacken. During La Niña conditions, by contrast, the westward-flowing trade winds are stronger than normal. A common measure of the SO is the Southern Oscillation Index (SOI), which is usually based on changes in sea-level air pressure at locations on opposite sides of the tropical Pacific. The most common basis for the SOI is the mean sea-level air pressure difference between Tahiti and Darwin, Australia, expressed as the long-term difference of their monthly pressures.
Normally, there is a low-pressure zone of warm air in the western Pacific and a high-pressure zone of cool air in the eastern Pacific. This pressure differential drives a loop of warm air from over the western Pacific that rises just east of Indonesia, travels eastward, and descends over the eastern Pacific. The loop then flows in a westerly direction at the surface back toward the west. The strength of this circulation, known as the Walker circulation, is heavily influenced by the seesaw-like sea-level pressure differences between the eastern and western Pacific. The pattern is named after Gilbert Walker, whose work led to the discovery of the Southern Oscillation.
The formation and breakdown of the Walker circulation cell is reflected in the pressure difference across the Pacific: When pressure is low in Tahiti, it is high in Darwin, and vice versa. This periodic yearly-to-decadal seesaw of atmospheric and oceanic circulation is the SO. When the SOI is negative, sea surface temperatures (SSTs) are warmer than usual in the eastern equatorial Pacific, off the coast of Ecuador and northern Peru (and occasionally Chile). A negative SOI is associated with El Niño conditions. When the SOI is positive, SSTs are cooler than usual in the eastern equatorial Pacific. A positive SOI is associated with La Niña conditions. During La Niña events, the east-west movement of air in the Walker circulation is enhanced with well-defined and vigorous rising and sinking branches.
Links to Global Climate Change
During El Niño events, there is an increase in Hadley cell circulation, the circulation cell of air that rises over the equator and descends in the subtropical latitudes on both sides of the equator. A more vigorous overturning of the Hadley cell circulation leads to an increase in heat transfer from tropical to higher latitudes in both hemispheres. Often, the climatic effects of increased Hadley circulation can be seen globally as above-average temperatures and extreme precipitation. Various other climatic consequences of ENSO events have also been reported: Warmer ocean waters have bleached coral reefs and vigorous atmospheric circulation has driven ocean currents northward, warming the Arctic Ocean and decreasing the amount of sea ice there.
The longer-term relationship between ENSO and global climate change is not fully understood and continues to be studied. While records since the 1980s indicate that El Niños have become more frequent and El Niñas less frequent, there is not enough data to effectively interpret the trend. However, scientists increasingly believe that global warming causes more extreme and more volatile El Niño events, at least in the short term. For example, 2015–16's El Niño was considered the most powerful since record keeping began, and the 2023–2024 El Niño was one of the five strongest on record, according to the World Meteorological Organization. Regardless of the exact interrelation, most scientists are convinced that ENSO and climate change are inextricably linked.
Assumptions about ENSO and Anthropogenic Global Warming
The Intergovernmental Panel on Climate Change (IPCC) has commented on the possibility of connections between ENSO and anthropogenic increases in atmospheric greenhouse gases. Based on the output of complex but unvalidated global climate models, the IPCC indicates that as temperatures increase, the average Pacific climate could more consistently emulate El Niño conditions. However, the IPCC accepts that some climate models point to a more La Niña-like response to global warming, because Hadley cell circulation may decrease with increasing global temperature. Paleoclimatic studies support the view that global warming is likely to foster weaker and less frequent El Niño events. However, the level of scientific uncertainty and the existence of conflicting results are such that reliable prediction of future climate is not possible at this time.
Some climate change skeptics even claim that the natural temperature variation created by ENSO is responsible for the observed trend of increasing global average temperatures, discounting the influence of anthropogenic climate change. However, virtually all mainstream scientists reject this point of view and accept the influence of humans on global warming. For example, while scientists from NASA and the US Oceanic and Atmospheric Administration (NOAA) acknowledge that El Niño played a role in the record-setting high temperature average in 2015, they suggested that the record would have been reached regardless of the presence of ENSO that year. It is generally accepted that ENSO's influence on temperature averages plays out on a relatively short scale, while global warming—including that caused by human activity—is an independently observable and longer-scale phenomenon. Still, scientists remain interested in the interconnection between ENSO and climate change and continue to investigate the subject. As another El Niño event developed in the summer of 2023, climate experts concluded that 2024 was the warmest year on record, resulting from a combination of the long-term climate change trend and the developing El Niño cycle.
Context
Owing to the complexity and uncertainty surrounding global climate and climate change, neither the mean annual values of ENSO nor the interannual variability of ENSO can be reliably simulated in global climate models. Despite this, projections have been made about future trends in precipitation extremes linked to ENSO. ENSO has a noticeable influence on mean global temperature, and shifts in temperature are consistent with shifts in the SOI, but the relationship between temperature and ENSO effects has not been consistently strong. On one hand, strong El Niño events create significant spikes in mean global air temperature. On the other hand, there is evidence that long-term warming depresses El Niño activity. For this reason, the mutual effects of climate change and ENSO upon each other remain difficult to predict.
Key Concepts
- Hadley cell: an atmospheric circulation system of air rising near the equator, flowing poleward, descending in the subtropics, and then flowing back toward the equator
- La Niña: the cooling half of the cycle of which El Niño is the warming half
- ocean-atmosphere coupling: the interaction between the sea surface and the lower atmosphere that drives many patterns and changes in Earth’s weather systems
- paleoclimates: climates of the distant past
- Walker circulation: an atmospheric circulation pattern in the Pacific and elsewhere in which hot, moist air rises, travels eastward, cools and dries, descends, and returns westward
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