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News Navigator: Why is winter snowfall on the Sea of Japan coast heavier than usual?

People clear snow on a road in the city of Fukui on Jan. 11, 2021. (Mainichi/Naohiro Yamada)

Amid numerous reports of cars stranded on roads and high death rates among people who engaged in clearing work, the Mainichi Shimbun answers some common questions readers may have about the heavier than usual snowfall affecting areas on the Sea of Japan coast.

    Question: It's very cold this winter, and significant amounts of snow have hit the Sea of Japan coast. Why is that?

    Answer: It's a combination of three factors that has led to more cold air from Siberia on the Eurasian continent being brought over to Japan than usual.

    Q: What are the three factors?

    A: First, the "polar front jet stream" westerlies, flowing over the northern hemisphere, meandered south and were disrupted by a high pressure system near western Siberia. Second, the "subtropical jet stream" westerlies, which blow south of the polar front jet stream, also drifted further south than in usual years due to the effects of "La Nina," a phenomenon that lowers ocean surface temperatures off Peru. Third, a low pressure system that usually stays in the Arctic moved southward toward Japan.

    Q: So, the two westerlies and a low pressure system brought cold air over Japan. How much chillier have they made it here?

    A: It was the coldest early January for 36 years, with average temperatures in northern Japan during the period coming in at 3.8 degrees Celsius lower than in normal years. Average temperatures in western Japan were also 3 C colder than usual.

    Q: I've heard a lot of mentions of "record snowfall," but what's meant by that?

    A: Between Dec. 14 last year and Jan. 11 this year, 72-hour periods of snowfall in 19 locations along the Sea of Japan coast broke records. As the winter pressure pattern -- a distribution of atmospheric pressure in which the high pressure area is to the west and the low pressure area is to the east -- continued because of the meandering westerlies, cold air easily covered Japan and brought heavy snowfall.

    Q: Are there causes for the significant snowfall besides cold air?

    A: There's a possibility that the warmer waters in the Japan Sea, which had a surface temperature 1 C higher than in usual years, increased the amount of vapor in the atmosphere and produced more snow clouds, causing greater snowfall.

    (Japanese original by Mayumi Nobuta, Science & Environment News Department)

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