Almost a billion people live off food made possible by the Indian Monsoon, making its timing a matter of immense importance. Even just knowing when it can be expected offers enormous benefits, and researchers have an unexpected explanation for why some years' predictions are more accurate than others.
Intuitively, we would expect volcanic eruptions to mess with long-range weather forecasting. It's just one extra complication in the system, making everything harder to model. However, a German-Indian team have found that when it comes to the monsoon, the opposite is the case, with large eruptions improving predictability.
The reason, they explain in Science Advances, is that volcanoes tie the monsoon more tightly to El Niños, the events that influence weather around the world every five years or so. Although there is still an immense amount we don't understand about El Niños, their effects are so widespread they are closely monitored and studied, creating a body of knowledge that can be drawn on for anything closely related.
The authors reached their conclusion by combining centuries of observations with geological data stretching back much further, for example tree rings and coral cores. Although the findings initially contradicted expectations, they eventually made sense with help from global climate models.
"The tiny particles and gases that a large volcano blasts into the air enter into the stratosphere and remain there for a few years,” Dr R. Krishnan, from the Indian Institute of Tropical Meteorology, said in a statement. “While the volcanic matter in the stratosphere to some extent blocks sunshine from reaching the Earth's surface, the reduced solar forcing increases the probability of an El Niño event in the next year."
Although this confounds expectations, the drivers of El Niños are one of the most heavily studied questions in climatology. El Niños make some parts of the world hotter and others cooler than usual, but overall they tend to be associated with unusually high global temperatures. However, Krishnan explained, “less sunshine means less warmth and hence a change of temperature differences between the Northern and Southern hemisphere, which in turn affects the atmospheric large-scale circulation and precipitation dynamics.”
Less studied has been the more complicated relationship between eruptions, El Niños, and monsoons. “Large volcanic eruptions are more likely to promote the coincidence of warm El Niño events over the Pacific and Indian monsoon droughts – or, in contrast, cool La Niña events over the Pacific and Indian monsoon excess," Krishnan concluded.
In the short term, the work could help farmers anticipate both the timing and strength of monsoons and adapt their planting to match. In the longer term, the findings could yield a better understanding of the consequences of proposals to mitigate global heating by blocking some sunlight from reaching the lower atmosphere.