16 September 2010
Japan Meteorological Agency

Primary Factors of Extremely Hot Summer 2010 in Japan
Analyzed by Advisory Panel on Extreme Climate Events

1. Temperature for summer (June – August) 2010 in Japan- Three-month mean temperature for June – August in Japan 1 was the highest
on record since 1898.
- Three-month mean temperatures for June – August in Northern and EasternJapan were the highest on record since 1946.
- Monthly mean temperatures for August in Northern, Eastern and WesternJapan were the highest on record since 1946.
1The observatory stations that represent the average temperature of Japan are selected fromthose deemed to be least influenced by urban heat island .

2. Characteristic atmospheric circulations causing Japan’s hot summer and theirfactors (Figure 1)(1) Zonally-averaged tropospheric air temperature in the mid-latitudes of theNorthern Hemisphere was the highest for June – August since 1979.
As researches so far indicate, it is known that tropospheric airtemperatures increase on global scale after El Niño events and then persist
higher-than-normal for several months. Examining past La Niña events, there
is a tendency that tropospheric air temperature anomalies in the mid-latitudes of
the Northern Hemisphere were relatively higher than those in the tropics during
the summer when a La Niña event occurred. An El Niño event, which took
place in summer 2009, ended spring (March – May) 2010, and a La Niña event
occurred in summer 2010.
Therefore, there is a possibility thatzonally-averaged tropospheric air temperature in the mid-latitudes of theNorthern Hemisphere was extremely high in summer 2010 in the aftermath of
the El Niño event and partly due to the effect of the La Niña event.
It can be identified that the zonally-averaged tropospheric air temperaturein the mid-latitudes of the Northern Hemisphere has a warming trend. There isa possibility that the warming trend is associated with global warming due to thebuildup of anthropogenic greenhouse gases.
(2) Remarkably strong anticyclone persisted over Japan.
In the period from the second half of July to August, the subtropical jetstream near Japan shifted northward from its normal position with a frequent
northward meander (ridge of high pressure). Associated with these features ofthe subtropical jet and extension of the Tibetan High to Japan, quasi-barotropic
highs developed and persisted over Japan. It seems that the northward shift ofthe subtropical jet in the vicinity of Japan was associated with active
convections in and around the Indian Ocean. There is a possibility that active
convections from the northern South China Sea to northeast off the Philippines
were partly responsible for the strength of anticyclones around Japan,
especially from the second half of August to early September.
(3) Japan experienced less-than-usual influence of the Okhotsk High (a cool
stationary anticyclone).
In June, warm anticyclones frequently covered Japan, particularly its
northern parts. In the period from June to the first half of July, the Okhotsk
High hardly developed this year, while the Okhotsk High occasionally brings
cool air to the Pacific side of Northern and Eastern Japan in normal years. In
the second half of July, the Okhotsk High temporarily took place, but Japan
experienced little influence of the Okhotsk High due to northward shift of thesubtropical jet near Japan and a strong Pacific High to the east of Japan.

Figure 1 Schematic chart of primary factors of extremely hot summer (June– August) 2010 in Japan.
The numbers of (1), (2) and (3) in this chart correspond to those in Section
2 of the main text, respectively.