Scientists reveal that early onset of El Niño means warmer winters in certain areas of Earth

El Niño is a weather pattern characterised by warming of the tropical eastern Pacific Ocean caused by weakening equatorial trade winds blowing from the western coast of South America to the Philippines and Indonesia. This pattern affects winters in certain areas of the Earth

Representation of the 2023/24 El Niño phenomenon in the equatorial Pacific. By June 2023, its patterns were already evident. Credit: Kyushu University/Masahiro Shiozaki

Scientists have used historical data from Japan and shown that El Niño years tend to lead to warmer winters. This case was recently exemplified by the warm winter season of 2023-2024 in Japan.

However, there have also been cases of cold winters in Japan during El Niño years, such as the one recorded in 2014-2015. However, it was not clear why this was happening.

El Niño and the warmth of winters in certain land areas

In a paper published in the Journal of Climate, researchers from the Research Institute of Applied Mechanics at Kyushu University found that the early onset of El Niño around June leads to warm winter climates in East Asia, while the late onset El Niño is associated with colder winters. The team hopes their results can help better model winter weather patterns in East Asia during El Niño years and lead to more accurate long-term climate predictions.

What is El Niño?

El Niño is a weather pattern characterised by warming of the tropical eastern Pacific Ocean caused by weakening equatorial trade winds blowing from the western coast of South America to the Philippines and Indonesia. In normal years, these trade winds would push warmer ocean waters to the west, causing colder waters to rise in the east. These warmer western waters drive atmospheric convection, generating clouds and rain.

"Each El Niño is individually unique and no two El Niño are exactly alike," explains postdoctoral researcher Masahiro Shiozaki, author of the study.

"Naturally, these differences give rise to a variety of abnormal global weather patterns. To better predict regional weather and climate during El Niño, it is important to know how and where the atmosphere changes."

Study results

Shiozaki highlighted a case that occurred in Japan during the winter of 2023-2024. This was an El Niño year, and in Japan that usually means a warmer winter. And in fact, the winter of 2023-2024 was unusually warm. However it was not always so. In the El Niño year of 2014-2015, Japan's winter was colder than average.

"Japanese winters are also influenced by the Arctic, as well as the strong natural variability inherent in the atmosphere. Because of these various effects, it has been a challenge to identify how El Niño determines warm or cold winters in East Asia," Shiozaki continues.

"To address this question, we simulated climate patterns over the past 61 years in 100 different ways, adding perturbations to the climate patterns. In this simulation, 1,700 El Niño events occurred and we calculated how the atmosphere changed for each of them. "This method allowed us to reduce any atmospheric noise in the data, giving us a clearer view of the direct impact of El Niño."

The team's analysis concluded that it was not just El Niño, but also an anomalous warming of the tropical Indian Ocean that led to warmer winters in East Asia.

‘The early onset of El Niño around June effectively warmed the Indian Ocean from summer into winter. This ocean warming suppressed atmospheric convection over the tropical western Pacific, resulting in less rainfall and atmospheric warming.

"The resulting reduction in atmospheric warming excited atmospheric waves that propagated westward into the North Pacific, forming an anomalous anticyclonic circulation southeast of Japan," explains Shiozaki.

"The anomalous southeasterly winds of this circulation pattern weakened the continent's northwestern winter monsoon, resulting in warm winter weather in East Asia. Furthermore, cold Japanese winters are associated with a late onset of the Niño and without significant warming of the Indian Ocean."

The team hopes their new findings will be used by researchers and meteorologists to better predict weather patterns months in advance, especially during active El Niño phases.

"The influence of global warming is evident in the recent trend towards warmer winters around the world. The effects of rising water temperatures are especially evident in the Indian Ocean," concludes Professor Hiroki Tokinaga, who led the research team.

"More research is needed to determine how global warming and other tropical climate events will change future winters in East Asia."


Masahiro Shiozaki et al, What Determines the East Asian Winter Temperature during El Niño? — Role of the Early-Onset El Niño and Tropical Indian Ocean Warming, Journal of Climate (2024). DOI: 10.1175/JCLI-D-23-0627.1