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Earth's Balancing Act

Greenhouse warming makes it harder to predict future strong El Niño events

Earth's Balancing Act
Earth's Balancing Act

16/09/2019 - Trieste

Earth is a strictly interconnected system, in which what happens locally can have global effects. This is particularly true in climatology and atmospheric science, in which it is common to investigate how climate anomalies are related to each other, even at large distances.

A relevant example of this is the link between oceanic currents and winds in the atmosphere, known as the El Niño – Southern Oscillation (ENSO). This is an irregularly periodic fluctuation that affects the climate of the tropical and subtropical regions in the Pacific Ocean. Since tropical sea surface temperatures are rather easy to predict, this connection is an extremely useful aid for the predictability of extreme atmospheric events, such as strong rainfalls, floods or droughts, that affect the climate in tropical and also extra-tropical regions.

A few years ago, scientists began to investigate a new connection at a larger distance. A research group guided by Belen Rodriguez-Fonseca, from the University of Madrid, identified a link between anomalies in the Tropical Atlantic and those happening in the Eastern Pacific. Later works confirmed this connection, showing that there is actually significant evidence that some of the major El Niño events have a precursor counterpart in the Atlantic.

This correlation has been increasing since the 1960s, and researchers started to speculate if this increase was connected to global warming, given that this is a phenomenon whose relevance has become more important in recent decades. The results of this investigation were recently published in the journal Science Advances.

“This paper reviews all these previous works,” said ICTP research scientist Fred Kucharski, one of the paper’s authors. “The idea is that there are some events, like ENSO, that influence the global climate, and we can use climate models and simulations to explain, for example, what would happen in the Eastern Pacific after an event took place in the Atlantic.”

The connection’s mechanism is based on a cyclic movement of matter between atmosphere and oceans, in the form of air currents and ocean waves. “If we have a warm anomaly in the Atlantic, for example,” explained Kucharski, “the air becomes more unstable, and we get convective heating. Then this must be compensated by a rising motion on a large scale, that will have to sink somewhere, influencing climate in other regions. Essentially, we can say that what goes up must come down.”

In a world that becomes warmer, extreme weather events increase in size and frequency, and being able to predict them would be an advantage in risk prevention and management.

To test the influence of greenhouse warming on the Atlantic-Pacific connection, the authors of the paper used hundreds of already existing climate simulations. These were performed by the several research centres around the world that took part in the Intergovernmental Panel on Climate Change's (IPCC) big project of building projections on the impacts of global warming on climate. Basing on these projections, the authors tested the link between the Atlantic and Pacific Niño phenomena in a scenario of a warming Earth, but obtained an unexpected result. “The simulations showed exactly the opposite of what we expected,” said Kucharski. “The models pointed out clearly that in a global warming scenario, this Atlantic-Pacific connection is actually weaker.”

It is a very well-known consequence of global warming that the atmosphere cools less with height, and this has the effect of stabilizing it. “Now, overall the Earth is warming, and if there is a phenomenon like the tropical Atlantic warm anomaly, additional to the global warming, with a more stable atmosphere, this means that an increase of convective events is less likely,” explained Kucharski. “This reduces the anomalous vertical velocity and then the teleconnection is weakened.”

Moreover, extreme weather events in the tropics are projected to increase in a global warming scenario, due to the releasing in the atmosphere of more water vapour. “If we have more water vapour in the atmosphere,” said Kucharski, “it will have an overwhelming effect whenever there is a convective event. It will rain more, and more violently.”

“Since the Atlantic Niño phenomenon happens before the Pacific anomaly, we can use the inter-basin connection to predict what will happen in the tropical Pacific after several months,” said Kucharski. “The most relevant contribution of this work, I think, is the hint that with increasing greenhouse warming the ENSO predictability will become more tricky in the future.” But the ability to predict extreme events in the Pacific remains a major research question given its impact on seasonal climate.

“The climate is so complex that each event could be actually caused by anything else through any kind of relationship,” said Kucharski, and the complexity of climate science gives researchers endless inputs for further investigations. “This is still a very active research area” said Kucharski, “as many questions are still open. Why did this correlation increase in recent decades? And how are these connections influenced by climate variations? We still don’t know!”

 

---- Marina Menga

 

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