SUWA, Nagano -- A natural phenomenon called "Omiwatari," in which ice forming on the surface of Lake Suwa in Nagano Prefecture splits to create a mountain rangelike crease, has not appeared for the seventh winter in a row, with climate change believed a factor.

According to records kept since the Muromachi period (1336 to 1573), the number of "ake no umi" winters without the phenomenon has risen dramatically since 1951. Kiyoshi Miyasaka, the 74-year-old chief priest of Yatsurugi Shrine in the prefectural city of Suwa, said that "Climate change and global warming are becoming evident on the lake."

Legend has it that Omiwatari is the route Takeminakata, the male god of the Suwa Taisha shrine complex's Kamisha Shrine in Suwa, used to visit the goddess Yasakatome of its Shimosha Shrine in the neighboring town of Shimosuwa. For that reason, it is also called the "path of love of the gods."

According to Miyasaka, the Omiwatari was first recorded in 1397. It was written about in documents passed down by the Moriya clan which once headed Suwa Taisha's Kamisha Shrine. This was the 583rd year its presence or lack thereof has been recorded, after it was once discontinued but has been kept up annually since 1443, in the mid-Muromachi period.

Including five years for which records are lost, "ake no umi" has occurred 81 times, including 40 times in the 75-year period since 1951, inclusively. The rate has picked up particularly since 2000, with 18 of the years since having lacked the Omiwatari phenomenon. It has not yet been seen since Japan entered the current Reiwa era in 2019. Worried, Miyasaka commented, "The views of Lake Suwa that have been around since long ago are disappearing. The time may soon arrive when the 'Omiwatari' will become a legend as an event of the past."

This year as well had a dearth of very cold days, with the mercury measured at 3.2 degrees Celsius for the water and 0 C for the air at a weather station on the south side of the lake on Jan. 20, which in Japan is traditionally thought of as the coldest day of the year. Both the numbers were on par with averages for late March. Although a blast of icy air on Feb. 9 and 10, plunging temperatures down to minus 10 C, made most of the lake surface freeze over, the next day much of it broke up. With no further waves of cold air expected, this winter was declared an "ake no umi."

Miyasaka declared the lack of an Omiwatari in front of his shrine on Feb. 15, but added lightheartedly afterward, "Though the Omiwatari was not possible, it's refreshing that spring has come."

Ake no umi winters occurred for the longest stretch of eight winters in a row from 1507 to 1514, during the Sengoku (Warring States) period of the fifteenth and sixteenth centuries. For each of these years, the lack of Omiwatari was noted in one-line accounts in records of the time. The current stretch is the second-longest, followed by the six winters of 1992 through 1997 inclusively.

Miyasaka surmised that the eight-year-period was during the chaos of wartime, when it may not have been the right time for the ritual. He added, "With this as the next-longest period, we can only think about (the effects of) global warming. We tend to think of climate change as if it were a fire on the other side of the river, but it must be recognized that even our familiar Lake Suwa is changing, as evidenced by the lack of ice formation on the lake. I hope this will get each of us to think about it."

(Japanese original by Kazunori Miyasaka)