For billions of people across South and Southeast Asia, the summer monsoon is life itself. It fills rivers, grows crops, and determines whether a year will bring plenty or just drought.
But why do these rains swing so wildly from decade to decade?
A new study by Dr Wenmin Man at the Institute of Atmospheric Physics, Chinese Academy of Sciences, has found a surprising answer.
The study, published in Geophysical Research Letters, has found that volcanic eruptions play a hidden role, one that has long been tangled up with natural ocean cycles.
WHAT IS THE RELATION BETWEEN VOLCANOES AND RAIN?
Analysing climate data and computer simulations spanning the past 1,000 years, the team mapped a striking connection in rainfall patterns across Asia.
In simpler terms, it was found that when South Asia gets wetter, Southeast Asia dries up, while northern East Asia tends to mirror South Asia’s conditions.
This seesaw, the study confirmed, is mainly driven by a natural ocean cycle called the Interdecadal Pacific Oscillation (IPO). It is a slow-moving cousin of El Nino that redistributes heat across the Pacific Ocean every few decades, pulling monsoon rains along with it.
But there is a twist. Major volcanic eruptions produce nearly identical rainfall patterns.
When volcanoes blast tiny particles into the upper atmosphere, they cool the ocean’s surface in a way that closely mimics the IPO, causing the same Asia-wide rainfall shifts even when the IPO itself isn’t at work.
“Volcanic forcing can ‘project’ onto the internal variability mode that naturally drives these precipitation patterns,” said Dr Man.
But despite the resemblance, the two are not identical.
IPO-driven ocean temperature changes are roughly symmetric above and below the equator. Volcanic cooling, by contrast, is lopsided; it’s stronger in one hemisphere depending on where and when the eruption happened.
That asymmetry gives scientists a way to tell the two apart in historical climate records.
HOW DOES IT MATTER?
The findings carry urgent relevance for proposed climate intervention strategies, specifically stratospheric aerosol injection.
The injection is a controversial idea to cool the Earth by deliberately releasing particles into the upper atmosphere, mimicking a volcanic eruption.
“If we’re considering stratospheric aerosol injection as a potential tool, we need to understand exactly how such interventions might affect regional rainfall patterns,” warned Dr Man. “Our research suggests they wouldn’t simply counteract warming uniformly—they’d interact with the climate system in complex ways, potentially amplifying or modifying the natural variability that billions of people depend on for their water supply.”
