El Niño, La Niña in a warmer world

Source: By Aswathi Pacha: The Indian Express

El Niño and La Niña, the two natural climate phenomena occurring across the tropical Pacific Ocean, influences the weather conditions all over the world. While the El Niño period is characterised by warming or increased sea surface temperatures in the central and eastern tropical Pacific Ocean, a La Niña event causes the water in the eastern Pacific Ocean to be colder than usual. Together, they are called ENSO or El Niño-Southern Oscillation. There is a growing body of research suggesting that climate change can cause extreme and more frequent El Niño and La Niña events.

A paper published in September 2021 in Nature Climate Change noted that increasing atmospheric carbon dioxide can cause a “weakening of future simulated ENSO sea surface temperature variability.” They note that the intensity of the ENSO temperature cycle can weaken as CO2 increases.

Prof Axel Timmermann, the co-corresponding author, explained in a release: “Our research documents that unabated warming is likely to silence the world’s most powerful natural climate swing which has been operating for thousands of years. We don’t yet know the ecological consequences of this potential no-analog situation, but we are eager to find out.” Timmermann is the director of the IBS Center for Climate Physics (ICCP) at Pusan National University in South Korea.

The team used one of South Korea’s fastest supercomputers, Aleph. According to the ICCP, it would take a single human 45 million years to complete the calculations that the supercomputer can perform in one second.

Our supercomputer ran non-stop for over one year to complete a series of century-long simulations covering present-day climate and two different global warming levels. The model generated 2 quadrillion bytes of data; enough to fill up about 2,000 hard disks,” said one of the authors, Dr. Sun-Seon Lee, in a release.

The team conducted climate model simulations to understand ENSO’s response to global warming — what will happen when there are CO2-doubling (2×CO2) and CO2-quadrupling (4×CO2) scenarios? They noticed sea-surface temperature anomalies at CO2-doubling conditions and it became robust at CO2 quadrupling.

The team studied the movement of atmospheric heat to decode the collapse of the ENSO system. They explain that future El Niño events will lose heat to the atmosphere more quickly due to the evaporation of water vapour. Also, in the future there will be a reduced temperature difference between the eastern and western tropical Pacific, inhibiting the development of temperature extremes during the ENSO cycle.

The team also studied tropical instability waves, a prominent feature in the equatorial Pacific. They note that there can be a weakening of these waves in the projected future, which can cause a disruption of the La Niña event.

There is a tug-of-war between positive and negative feedback in the ENSO system, which tips over to the negative side in a warmer climate. This means future El Niño and La Niña events cannot develop their full amplitude anymore,” comments ICCP alumni Prof. Malte Stuecker, co-author of the study in a release.