Record January heavy snowfall may be linked to global warming — Hokkaido University releases preliminary analysis

The record heavy snowfall that hit eastern and northern Japan in late January may have been influenced by human-induced global warming. That is the finding of a preliminary analysis released by the "Weather Attribution Center Japan (WAC)." According to the analysis, during a period when strong cold air was covering the country, global warming increased precipitation over a wide area along the Japan Sea Polar Air Mass Convergence Zone (JPCZ).

WAC was established in May last year by extreme weather specialists, including Professor Masahiro Watanabe of the Atmosphere and Ocean Research Institute (AORI) at the University of Tokyo. The center employs a methodology known as "event attribution" to study and communicate the extent to which global warming contributes to various extreme weather events.

A research group led by Professor Tomonori Sato from the Faculty of Environmental Earth Science, Hokkaido University, a member of WAC, investigated the relationship between global warming and the heavy snowfall of more than 100 centimeters recorded at various locations along the Sea of Japan coast from Tohoku to Hokuriku between January 21 and 23.

On January 21, the Japan Meteorological Agency (JMA) issued its first "Significant Heavy Snowfall Advisory" of the season for Ishikawa Prefecture, followed by similar advisories for Shiga and Fukui prefectures, urging caution over record short-duration snowfall. According to WAC, there was a classic winter pattern of high pressure in the west / low pressure in the east during this period, and in this context strong cold air from the continent gathered moisture as it was warmed over the Sea of Japan, generating band-like cloud formations. Snow clouds also developed along the JPCZ.

Sato and his colleagues extracted two sets of 200 cases resembling the atmospheric pressure pattern around Japan from January 21-23 from a climate model simulation database—one set accounting for human-induced warming effects and another accounting for natural variation only—and compared and analyzed the changes in precipitation.

According to the analysis the research group posted on the WAC website on January 27, warming-enhanced precipitation along the JPCZ and on the Sea of Japan side of northern and eastern Japan may have contributed to the record snowfall totals. In Kawadani, Joetsu City and Tokamachi City in Niigata Prefecture, 72-hour precipitation exceeded 150 mm. The probability of such precipitation occurring was found to be 1.6% without warming factors, but was 4.8%, three times higher, when warming factors were included. It was also found that extreme precipitation on the Sea of Japan side of eastern Japan had increased by approximately 9.5% due to global warming.

In March last year, a JMA study group released its assessment of the previous winter's heavy snowfall. The group stated, "The westerly winds had dipped southward near Japan, causing a large-scale low-pressure system accompanied by cold air to split and move south. Warming-induced rises in air and sea surface temperatures increased atmospheric water vapor, leading to greater snowfall amounts."

The Hokkaido University research group explained: "While long-term warming-driven rises in air and sea surface temperatures tend to reduce the proportion of precipitation that falls as snow, the whole country was covered by strong cold air during the analysis period (late January), which made it harder for the proportion of snow to decrease—and this was one factor that amplified the impact of the snowfall."

Event attribution is a methodology that quantifies the influence of human activity on extreme weather events. Conventional approaches require one to two months because they involve running massive numbers of simulations on supercomputers; by contrast, WAC's approach draws on a big-data archive of past simulations and observations, enabling more accurate results within just a few days. In this case, Hokkaido University's analysis was released as a preliminary report in less than one week after the heavy snowfall occurred.

Because this method uses complex statistical equations that factor in Japan's specific meteorological and climatic characteristics, it can be considered a "Japan-specific" version of event attribution. In August last year, Watanabe and his colleagues issued a preliminary report stating, "The nationwide record-breaking heatwave in late July had reached a level that would have been impossible without human-induced warming."