A research group at Kyushu University has discovered that whether Japan experiences warm or cold winters depends on whether the El Niño phenomenon develops quickly or slowly.

 The El Niño phenomenon, which refers to a remarkable increase in sea surface temperature in the Pacific Ocean region east of the equator, is a cause of abnormal weather and abnormal weather around the world. It has been pointed out that Japan tends to have warm winters in years when El Niño occurs. However, there are many cases in which Japan experienced cold winters even in years when El Niño occurred, and the factors that differentiate warm winters from cold winters in years when El Niño occurred were unclear.

 This research group analyzed large-scale numerical simulation data to evaluate only the effects of the El Niño phenomenon. The following was discovered from atmospheric simulation data (61 years in total) that reproduced 100 weather patterns over the past 6,100 years.

 When Japan experiences a warm winter, the sea surface temperature in the equatorial eastern Pacific Ocean, which is an indicator of the El Niño phenomenon, first exceeds the standard value in June, and high water temperatures persist until February of the following year. It was recognized that it has the characteristics of being very fast and long lasting. As a result, seawater temperatures in the Indian Ocean will rise significantly, which will suppress active precipitation off the coast of the Philippines. Then, a high-pressure system forms off the southeast coast of Japan, causing westerly winds to meander northwards, weakening the outflow of cold air into Japan, and leading to a warm winter.

 Conversely, when Japan experiences a cold winter, the El Niño phenomenon is characterized by a very slow development and short duration, with sea surface temperatures in the Pacific Ocean in the eastern equatorial region exceeding the standard value for the first time in November. As a result of the small warming of the Indian Ocean and the small suppression of precipitation activity off the coast of the Philippines, an anticyclone will not appear off the southeast coast of Japan. The low pressure system (Aleutian Low pressure system) over the North Pacific Ocean is protruding, strengthening the winter-like pressure pattern of high in the west and low in the east, and the outflow of cold air, making winter more likely.

 This result, which elucidates the mechanism behind warm and cold winters in years when El Niño occurs, is expected to contribute to improving the accuracy of abnormal weather forecasts several months in advance, such as three-month forecasts, as well as longer-term seasonal forecasts.

Paper information:[Journal of Climate] What Determines the East Asian Winter Temperature during El Niño?— Role of the Early-Onset El Niño and Tropical Indian Ocean Warming

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