Onset speed of El Niño influences winter temperature — Kyushu University reproduces 100 different weather conditions over the past 61 years

From past observation data, it is known that Japan tends to experience a warm winter in years when El Niño (a phenomenon in which the sea surface temperature in the equatorial Pacific Ocean becomes higher) occurs. The warm winter of 2023−24 followed this trend. However, there have been many cases of cold winters in Japan during El Niño years, such as the record cold winter experienced during the El Niño occurrence of 2014−15. The mechanism that determines warm and cold winters during El Niño years remains unknown. A research group led by Post-doctoral Fellow Masahiro Shiozaki, Professor Hiroki Tokinaga, and Assistant Professor Masato Mori of the Research Institute for Applied Mechanics at Kyushu University, analyzed large-scale numerical simulation data that reproduced 100 different weather events over the past 61 years. They found that the difference between the early and late development of El Niño events from summer to winter greatly influences whether Japan tends to have a warm or a cold winter in that year and revealed the mechanism of the El Niño influence. The results were published in the Journal of Climate.

The research group analyzed large-scale numerical simulation data called d4PDF to evaluate the impact of El Niño events on the winters that Japan tends to have while controlling internal atmospheric variability. The simulation calculated changes in the atmosphere during 1,700 El Niño events, and by averaging them all together, the effect from internal atmospheric variability is reduced to as little as possible; thus, only the effect of El Niño events is extracted.

An analysis of this data showed that if a strong El Niño event develops early in the summer, sea surface temperatures in the tropical Indian Ocean will also increase significantly. Their synergistic effects suppress active precipitation activity off the east coast of the Philippines. This suppression of the precipitation activity creates high atmospheric pressure off the southeast coast of Japan, causing the prevailing westerly winds to meander significantly to the north. This will weaken the blowing of cold air into the East Asian region, including Japan, resulting in a warm winter being more likely. Conversely, even if an El Niño event does occur, the slow progression of its development will not result in a significant increase in water temperature in the tropical Indian Ocean. Consequently, the precipitation activity off the east coast of the Philippines is suppressed only slightly; hence, high atmospheric pressure does not form off the southeast coast of Japan.

Meanwhile, when a low-pressure system over the North Pacific extends to the vicinity of Japan, it strengthens the winter pressure pattern of high in the west and low in the east as well as the blowing of cold air, making it easier to have a rather cold winter tendency. This finding is expected to contribute to improving the accuracy of seasonal forecasts, such as three-month forecasts, by better reproducing the relation between El Niño events and Indian Ocean variations using numerical simulation models.

Journal Information
Publication: Journal of Climate
Title: What Determines the East Asian Winter Temperature during El Niño? — Role of the Early-Onset El Niño and Tropical Indian Ocean Warming
DOI: 10.1175/JCLI-D-23-0627.1