Extremely hot conditions in Japan in midsummer 2013
13 August 2013
Tokyo Climate Center, Japan Meteorological Agency
http://ds.data.jma.go.jp/tcc/tcc/index.html

In summer 2013, many parts in Japan, especially in western Japan, experience very hightemperatures, while precipitation amounts are below normal on the Pacific side in eastern andwestern Japan as well as in Okinawa/Amami. Four stations recorded temperatures reaching or
exceeding 40˚C, among which Ekawasaki (Kochi Prefecture) hit 41.0˚C, a new country record
high temperature, on 12 August.
1. Surface climate conditions
In summer 2013, temperatures continue to remain above normal in many parts of Japan,
especially in western Japan (Fig. 1). Ten-day mean temperatures averaged over western Japan were
the third highest on record for three consecutive ten-day periods from the middle of July (11 – 20
July, 21 – 31 July and 1 – 10 August) in the JMA records since 1961 (Table 1). Since around 8
August, temperatures have been well above normal almost all over Japan and more than 3˚C above
normal at a large number of observation stations mainly on the Pacific side in eastern and westernJapan (Fig. 2). In particular, from 10 to 12 August, the Pacific side in eastern and western Japan
experienced extremely high temperatures and Ekawasaki (Shimanto-city, Kochi Prefecture) marked
the highest temperature on record in the country at 41.0˚C on the afternoon of 12 August, exceeding
the previous record of 40.9˚C logged at Kumagaya (Saitama Prefecture) and Tajimi (Gifu Prefecture)
on 16 August 2007. As of 12 August, a total of 106 observation stations recorded the highest
temperatures. Daily minimum temperatures were also quite high at many places and 68 stations
recorded the highest daily minimum temperatures, as of 12 August. On 11 August, the daily
minimum temperature at Tokyo was as high as 30.4˚C. It was the first time for the temperature inTokyo to remain over 30˚C for all day long since the daily observation started in 1875.
Since July, precipitation amounts continue to be below normal on the Pacific side in eastern andwestern Japan and in Okinawa/Amami. The precipitation amount from 1 July through 12 August
was as little as 0.0 mm in Naze (Kagoshima Prefecture).

Fig. 1 Time series of five-day running mean temperature anomalies (unit: °C)
for four divisions of Japan from 1 June to 12 August, 2013
Anomalies are deviations from the 1981 – 2010 average, based on daily observations made at 154
surface meteorological stations.
Table 1 10-day mean temperature anomalies (unit: °C) averaged over four divisions of Japan fromearly July to early August, 2013 and rankings since 1961 (shown only within top five)
Anomalies are deviations from the 1981 – 2010 average, based on daily observations made at 154 surface
meteorological stations.

Fig. 2 Five-day mean temperature anomalies (8 – 12 August, 2013)
Anomalies are deviations from the 1981 – 2010 averageFig. 3 43-day total precipitation amount ratio (% of the 1981 – 2010 average)(1 July – 12 August, 2013)
Total precipitation amount ratios are derived from daily observations made at 154 surface
meteorological stations.

2. Characteristic atmospheric circulation causing Japan’s hot conditions
During the last several days, both lower-level anticyclone (the Pacific High) and upper-level
anticyclone (the Tibetan High) were enhanced around Japan. Many parts of Japan experienced
significant high temperatures due to predominant sunny conditions (i.e., above-normal amounts ofsolar radiation) and prevailing downward motion of air associated with high pressure systems.
The enhanced Pacific High remained over southern China and to the south of the main islands ofJapan from early July. The Pacific High was enhanced due to predominant downward motion of air
originating from Southeast Asia where upward motion of air prevailed associated with enhanced
convective activity.
The Tibetan High, which expands to the main islands of Japan during midsummer of normal
years, was enhanced over the country in line with northward meandering of the westerly jet stream.
The northward meandering flow over Japan formed as a part of wave trains that propagated eastward
along the Eurasian subtropical jet stream.

Fig. 4 Primary factors contributing to the hot conditions in Japan (8 – 11 August 2013 )3. Outlook
The North Pacific High is predicted to cover the main island of Japan in the coming two weeks,
thus sunny and hot weather is likely to continue. Precipitation amounts are predicted to remain below
normal on the Pacific side in eastern and western Japan. JMA urges healthcare including heat stroke
prevention, careful crop management, and water saving*.
On the other hand, cloudy and/or rainy weather is expected in Okinawa after later this week due
to wet air flow.
* Information on water shortage can be found at
http://www.mlit.go.jp/tochimizushigen/mizsei/kassui/kassui.html (in Japanese)