The Atlantic Niño weakens under global warming

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A new study shows a strong weakening of sea surface temperature variability.

The tropical Atlantic Ocean is enclosed between the Brazilian coast on one side and the West African coast on the east. As the ocean is a major player in climate systems, changes in the ocean affect local weather – they also change marine ecosystems and affect people living off the fish resources in the oceans.

As in the greater Pacific Ocean, the South Atlantic Ocean also experiences El Niño events occurring on interannual time scales.

In a new study published in Natural climate change, researchers are studying how the Atlantic Niño will be affected by global warming.

The Atlantic Niño

The El Niño phenomenon in the Pacific Ocean is a strong climatic fluctuation on time scales of a few years.

This phenomenon, El Niño – Southern Oscillation (ENSO), causes a significant warming of the eastern Pacific, accompanied by catastrophic rainfall over South America and droughts in the Indo-Pacific region. Powerful events have global effects that reach even to extratropical regions.

There is also a variant of El Niño in the Atlantic, called Atlantic Niño.

The Atlantic Niño affects precipitation in West Africa as well as the development of tropical cyclones over the eastern tropical Atlantic.

The Atlantic Niño, like its Pacific counterpart, exhibits a characteristic asymmetric pattern in changes in sea surface temperatures and surface winds from east to west, with the strongest warming occurring in the east . However, there are some differences: Atlantic events are smaller in scale, shorter in duration and less predictable.

A better understanding of the poorly studied little brother of the Pacific El Niño in the Atlantic could potentially improve climate predictions in the region.

(Source: Bjerknes Center for Climate Research)

The Atlantic Niño effects:

  • In the Gulf of Guinea: Warm sea surface temperature anomalies linked to the Atlantic Niño cause an increase in precipitation over the Gulf of Guinea and a decrease over the Sahel region. Opposite effect with Atlantic La Niña when the anomalies are negative.
  • Fishing, especially for the West African coast: When there is a weaker interaction between the deep layers and the upper layers of the ocean, the upwelling of nutrients should be less efficient, so the amount of available nutrients that are vital for marine ecosystems should decrease.
  • Rainfall in Brazil: Precipitation in Brazil is also influenced by the Atlantic Nino, but in a more complicated way because other phenomena also contribute to precipitation in this region.

Reduced variability in ocean surface temperatures

By studying a large ensemble of climate models and accounting for model errors, the researchers found a reduction in ocean surface temperature variability of up to 24-48% by the end of the century, under scenarios highest emissions.

According to lead author Lander Crespo, this study provides well-founded indications of how the Atlantic Niño will change.

“We were positively surprised to be able to find robust changes in the future variability of the tropical Atlantic, as it is well known that climate models do not work very well in this region. Therefore, no one has attempted to study climate change in the region before,” he says. “The projected weakening of the Atlantic Niño is large enough that we can expect large changes in ocean and atmospheric circulations with consequences for local fisheries as well as changes in drought and precipitation patterns. We now need to consider how these changes will affect the climate system and ultimately the consequences for society.

Crespo is a researcher at the University of Bergen and the Bjerknes Center for Climate Research.

Weakened coupling between the deep layers of the ocean and the surface

What happens is that in a warmer future climate, the upper ocean in the eastern equatorial Atlantic warms faster than the deeper ocean. Therefore, the thermocline becomes deeper. The thermocline marks a boundary between temperatures above and below 20°C and 23°C.

The illustration shows the temperature drop with depth. Note the rapid change between 100 and 1000 meters depth.

With a deeper thermocline, the ocean surface becomes less sensitive to deeper ocean anomalies. These anomalies include nutrient upwelling, an important phenomenon for the productivity of marine ecosystems.

A deeper thermocline means the ocean surface is farther away and less influenced by cold water below. In other words, surface waters are less sensitive to changes in deeper layers. This effect is what the researchers call thermocline feedback weakening.

“We propose the weakening of the thermocline feedback as the primary mechanism for future changes in Atlantic variability,” says Crespo.

This effect is remarkably different from driving mechanisms in the equatorial Pacific, the study shows.

“In the Pacific, changes in variability appear to be controlled by changes in the SST gradient between the eastern and western parts of the basin triggered by changes in equatorial winds. Although our mechanism is robust and presents a fairly good model of agreement, this does not seem to be the case in the Pacific where the uncertainties are larger and the models do not agree so well; some models predict a warmer eastern Pacific while others predict cooler anomalies,” Crespo explains.

Less variability makes predictions more difficult

One conclusion of the study’s researchers is that less variability in the ocean makes it harder to predict changes.

“When we say that there are fewer variations, in the study, we are talking about statistics over fifty years. Even if the overall variability decreases, we may still see strong El Niño events in the future with significant impacts for people living along the West African coast,” Crespo said.

Reference:

Crespo et al. Weakening of the variability of the Atlantic Niño under the effect of global warming, Natural climate change, flight. 12, 2022. DOI: 10.1038/s41558-022-01453-y

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