JCDP

Omiwatari appeared in this winter !

Takehiko Mikami

A couple of straight lines of sharp upheaval of ice, called Omiwatari on the frozen surface of Lake Suwa in central Japan, appeared for the first time in 5 years on February 2, 2018. This is a follow-up report of my column on December 27, 2017.

Snow-covered frozen Lake Suwa viewing from “Tateishi” Park (Photo taken by T.Mikami on 4 February, 2018)

In this winter, we have a very cold weather here in Japan, and Omiwatari phenomena occurred after almost complete freezing of the lake. Daily mean temperature at Suwa observatory in this January was -1.3 degrees, which is around same as normal means. Also the number of days with lowest temperatures below -10 degrees, which is indispensable for Omiwatari occurrence, was omly two days.

Omiwatari crossing on Lake Suwa (Photo taken at Akasunazaki by T.Mikami on 4 February, 2018)

Omiwatari crossing on Lake Suwa (Photo taken at Takagi by T.Mikami on 4 February, 2018)

According to legend, Omiwatari is said to be the footprints of God “Takeminakata-no-kami” of the Upper Suwa Shrine, who walks on the frozen lake to visit Goddess “Yasakatome-no-kami” of the Lower Shrine. The routes of Omiwatari would be different in either case due to the changes in temperature and thickness of lake ice.

Omiwatari viewing ceremony was held by “Yatsurugi” Shinto shrine priests on 5th February, and harvests and economy in this year were predicted by referring to the past documentary data for Omiwatari crossing routes.

Additional Information

The range of Omiwatari was further pushed up due to the subsequent cold weather conditions at Suwa, where daily minimum temperatures dropped down to -10.9 degrees on 8 February and -11.3 degrees on 9 February.

Omiwatari crossing on Lake Suwa (Photo taken at Akasunazaki by T.Mikami on 10 February, 2018)

02/06/2018

Topics

Anomaly maps in “Climate Atlas”

　　　　　　　　　　　　　　　　　　　　　　　
Junpei Hirano

In climatology, we use values called “anomalies” to understand the average state of climatic elements (atmospheric pressure, temperature, precipitation, sunshine duration, and solar radiation, etc.). The “normal” is defined as the average value of climatic factors during the past 30 years. Since the average value changes due to climate change such as global warming, the normal is updated every ten years. For example, for the 10-year period from 2011 to 2020, the 1981-2010 average was used as the “normal,” but it was updated in 2021, and for the 10-year period from 2021 to 2030, the 1991-2020 average will be used as the “new normal.

In weather forecasting, the term “warmer (colder) than normal” is sometimes used, but in this case, “normal” refers to the “normal” value of temperature. In other words, the “normal value” of temperature is the standard for evaluating the warmth (coldness), etc. of each year. The difference between the temperature of each year and the normal is called the ” anomaly”. In the case of temperature, a positive anomaly indicates a warmer than normal year, while a negative anomaly indicates a colder than normal year. A map of temperature anomalies shows the distribution of areas with warmer-than-normal temperatures (positive anomaly areas) and areas with colder-than-normal temperatures (negative anomaly areas).

Similar to temperature, precipitation can also be plotted based on the “normal” value. However, in the case of precipitation, anomalies are generally expressed as the ratio of precipitation to the normal (ratio of normal to precipitation). For example, if you draw a precipitation anomaly map for a year with an El Niño or La Niña event, you can see areas with more or less rainfall than normal. Global precipitation anomaly maps for summer (June-August) and winter (December-February) of El Niño events are available on the Climate Atlas “World” page. However, as the basis for the anomalies, we have used the 43-year average for the period 1979-2021, for which precipitation data are available, instead of the 30-year “normal. Green-colored areas indicate areas of higher-than-normal rainfall, while brown-colored areas indicate areas of lower-than-normal rainfall. It can be seen that in El Niño years, there is more rainfall in the eastern tropical Pacific and less in the western tropical Pacific.

Furthermore, by comparing the precipitation anomaly map with the SST anomaly map, we can see the relationship between SST anomalies and precipitation anomalies. For example, in the “Winter DJF SST anomaly map for El Niño years”, the eastern tropical Pacific is colored red, indicating that SSTs are higher than normal. In an El Niño year, the sea surface temperature in this region is high, which activates cumulus convection and leads to heavy rainfall.

Global precipitation anomaly maps for summer (June – August) and winter (December – February) La Niña years show the opposite pattern (heavy rainfall in the eastern tropical Pacific and less rainfall in the western tropical Pacific). In La Niña years, sea surface temperatures (SSTs) drop below normal in the eastern tropical Pacific (see SST anomaly distribution in La Niña years), which suppresses cumulus convection and results in less rainfall.

The JCDP “Climate Atlas” will include various climate anomaly maps in the future. We recommend that you use the anomaly maps in the JCDP Climate Atlas when explaining “extreme weather” and its spatial distribution in high school geography and geology courses and in university liberal arts courses.