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Extreme La Niña events occur when cold sea surface temperatures across the central Pacific Ocean create a strong temperature gradient to the Maritime continent in the west. This work projects an increase in frequency of La Niña events due to faster land warming relative to the ocean, and a greater chance of them occurring following extreme El Niño events.

Extreme El Niño events cause global disruption of weather patterns and affect ecosystems and agriculture through changes in rainfall. Model projections show that a doubling in the occurrence of such extreme episodes is caused by increased surface warming of the eastern equatorial Pacific Ocean, which results in the atmospheric conditions required for these event to occur.

The impacts of climate change on certain aspects of the El Niño/Southern Oscillation (ENSO) have been established. However, the change in sea surface temperature, commonly used to represent ENSO amplitude, remained uncertain. Now, the sea surface response is shown to be time-varying, with an increasing trend to 2040 followed by a decreasing trend. The previous uncertainty is attributed to the expectation of unidirectional behaviour and unrealistic model representations.

Climate models predict that by 2020, 20–55% of the three key ocean basins express an anthropogenic fingerprint of change. The well-ventilated Southern Ocean water masses are particularly sensitive, emerging as early as the 1980–1990s, consistent with observations of change over the past 30 years.

While present in palaeoclimate records, the drivers behind 20-year climate variability are poorly understood. Here, using climate simulations and in situand palaeo data, the authors present a possible link between volcanic eruptions, Great Salinity Anomalies and the Atlantic overturning circulation.

El Niño tends to follow 2 years after volcanic eruptions, but the physical mechanism behind this phenomenon is unclear. Here the authors use model simulations to show that a Pinatubo-like eruption cools tropical Africa and drives westerly wind anomalies in the Pacific favouring an El Niño response.

The El Niño propagation asymmetry (in which sea surface temperature anomalies propagate eastwards during an extreme El Niño event) is shown to be caused by the variations in upper ocean currents in the equatorial Pacific Ocean; increased occurrences of the propagation asymmetry may be a manifestation of global greenhouse warming.

Our current understanding of the spatio-temporal complexity of the El Niño–Southern Oscillation phenomenon is reviewed and a unifying framework that identifies the key factors for this complexity is proposed.

The El Niño–Southern Oscillation is a naturally occurring fluctuation that originates in the tropical Pacific region and affects the lives of millions of people worldwide. An overview of relevant research suggests that progress in our understanding of the impact of climate change on many of the processes that contribute to El Niño variability is considerable, but projections for the phenomenon itself are not yet possible.

This Review looks at the state of knowledge on the El Niño/Southern Oscillation (ENSO), a natural climate phenomenon. It discusses recent advances and insights into how climate change will affect this natural climate varibility cycle.