Deforestation in Southeast Asia and Pacific Islands Could Make El Niño Southern Oscillation More Unpredictable, Study Finds

Every two to seven years, a climate phenomenon known as the El Niño-Southern Oscillation (ENSO) occurs in the tropical Pacific Ocean. ENSO is associated with air pressure changes from east to west, reported Phys.org.

El Niño events cause westerly trade winds blowing along the equator to become weaker, leading to wind speed and air pressure changes. These shifting patterns move warm sea surface water toward coastal South America from the western Pacific. This causes the depth at which the temperature of the ocean rapidly changes to become deeper, preventing the usual rise of cooler waters containing an abundance of nutrients. This can have devastating effects on marine food chains and the local communities that rely on them.

In South America, along with El Niño comes heavier and longer periods of rainfall, which increase the threat of flooding. In Indonesia and Australia, however, the weather phenomenon leads to drought, causing irrigation and water supply issues. These conditions trade places during La Niña.

“ENSO is one of the most important climate phenomena on Earth due to its ability to change the global atmospheric circulation, which in turn, influences temperature and precipitation across the globe,” the National Weather Service said.

A new study points to the likelihood that ENSO is strongly affected by deforestation on the Maritime Continent (MC), which is the region between the Pacific and Indian Oceans that includes Borneo, the Philippine Islands, Indonesia, New Guinea and the Malay Peninsula.

The study, “The Potential Influence of Maritime Continent Deforestation on El Niño-Southern Oscillation: Insights From Idealized Modeling Experiments,” was published in the journal Geophysical Research Letters.

When land use is altered dramatically, such as with deforestation, how much sunlight gets reflected by the surface of the planet, called “surface albedo,” is altered, and the natural process of evapotranspiration is also reduced, Phys.org reported. The combination has a warming effect on the surrounding environment and impacts interactions between the land, atmosphere and ocean, modifying the local climate.

The research team simulated future deforestation over a century using the Community Earth System Model, and changed native deciduous and broadleaf evergreen trees to C4 grass. Doing so was found to make ENSO stronger, and its associated events became more frequent and shifted toward the central Pacific. This happened due to winter sea surface temperatures at mid-latitude affecting the subtropical atmosphere during the subsequent spring and summer, called the “seasonal footprinting mechanism,” the authors of the study said.

On the other hand, air pressure falling over the western Pacific tropics had the effect of suppressing atmospheric convection with atypical contrasts in temperature between land and sea. Colder water spread toward the poles, resulting in a positive sea level pressure shift in the subtropical northeastern Pacific. This, along with high atmospheric pressure during the boreal winter from December to February, lead to stronger ENSO events.

Because of the reactivation of the seasonal footprinting mechanism by atmospheric high pressure in the subtropical north Pacific, La Niña conditions could happen concurrently for years in a row, rather than alternating with El Niño.

The researchers said La Niña events happening multiple years in a row are more likely to occur in future decades if deforestation continues, at a rate of 13.8 percent, the simulations suggested. There have already been three multi-year La Niña events this century: 2010 to 2012; 2016 to 2018; and 2022 to last year.

The occurrence of events similar to ENSO in the central Pacific could be caused by a shift in northeastern Pacific trade winds, which cooled off the ocean surface with increased localized wind speeds.

The model showed that El Niño events have an 11.7 percent increased likelihood of occuring due to deforestation, while La Niña events had a 14.6 percent increase.

“Previous studies have attributed the changing ENSO properties to global warming and decadal climate variability. Our study adds to this by suggesting that deforestation in the MC may also contribute to the growing complexity of ENSO by boosting the significance of subtropical ENSO dynamics,” the authors wrote in the study.

Shifts to the occurrence of more frequent multi-year ENSO events raises concerns about communities having time to prepare.

“Although our deforestation experiments are idealized and not realistic, they demonstrate the possibility that deforestation in the MC could increase the complexity of El Niño, making El Niño events more complex and harder to predict,” the authors concluded.