Deforestation is one of the human activities, aside from industrial carbon dioxide emission, that has profound climate impacts. Over the Maritime Continent of Southeast Asia, a high rate of deforestation has been observed during the past four decades. Due to its location at the core of the Indo-Pacific warm pool, deforestation in the Maritime Continent influences not only local temperature and precipitation but also has considerable teleconnections to other regions. For example, it can lead to precipitation reduction in southern China and temperature variations in high latitudes. It is of our particular interest to see if Maritime Continent deforestation could impact the properties of El Niño-Southern Oscillation (ENSO), which is the strongest interannual variability of Earth’s climate system. Although Maritime Continent deforestation is known to have the ability to change the Walker circulation and may have the potential to affect ENSO spatial pattern, its influence on ENSO is far from being well understood.

Observing that there is a change in the post-peak evolution of El Niño during the past four decades. Specifically, the El Niño events followed in the decaying year by an immediate La Niña event have increased, indicating that El Niño is more likely to be followed by La Niña. Coincidentally, this change occurred under Maritime Continent deforestation. We conducted the study to investigate whether the change in El Niño evolution is partly attributed to Maritime Continent deforestation.

We first showed that the transition from El Niño to La Niña is closely related to climate variation in the Maritime Continent. El Niño events with a transition to La Niña feature an enhanced convection over the Maritime Continent during the decaying summer. The enhanced Maritime Continent convection induces surface easterly anomalies in the western equatorial Pacific, which, by shoaling thermocline in eastern equatorial Pacific, favor the cold sea surface temperature anomalies to develop. Deforestation in the Maritime Continent leads to stronger local convection and precipitation, helpful for the transition from El Niño to La Niña due to a more effective resultant Bjerknes feedback.

We also conducted numerical experiments to demonstrate the influence of Maritime Continent deforestation. We conducted experiments using regional climate model and atmospheric general circulation model to demonstrate that Maritime Continent deforestation could cause more local precipitation and induce surface easterly anomalies in the western equatorial Pacific. To show directly that Maritime Continent deforestation has influence on the post-peak evolution of El Niño, we further conducted coupled general circulation model (CGCM) experiments. In the CGCM experiments, when forests in the Maritime Continent are cleared, the proportion of El Niño events followed in decaying winter by a La Niña is ~5% higher.

The study's findings are significant as they offer a new perspective on how deforestation may impact global climate. In addition to inducing more greenhouse gases emission, deforestation in the Maritime Continent can change the properties of the air-sea interaction phenomena in the tropical Pacific, which in turn influence the climate in remote regions through teleconnections. As deforestation in the tropics is likely to continue in the near future, further explorations of how it may shape global climate are essential.