Unseasonable weather conditions in Japan in August 2014
– Summary of analysis by the TCC Advisory Panel on Extreme Climatic Events –In an extraordinary session held at the Japan Meteorological Agency on 3 September 2014, the TCC
Advisory Panel on Extreme Climatic Events1 issued a statement on primary factors causing the cloudy andrainy conditions observed in Japan in August 2014.
 In August 2014, western Japan experienced record-high precipitation and record-low sunshine durations.
From 30 July to 26 August, heavy rainfall events were observed throughout the country.
 These weather conditions were caused by continuous southerly warm moist air flow in association with
two typhoons moving northward near or over western Japan from late July to early August and with thesouthward shift and significant meandering of the jet stream around Japan from mid- to late August. This
meandering is considered partially attributable to suppressed Asian monsoon activity in association withabove-normal sea surface temperatures in the eastern Pacific and the eastern Indian Ocean as well as
tropical intra-seasonal oscillation.2
1. Climatic characteristics (Figures 1 and 2, Table 1)
After 30 July 2014, cloudy and rainy conditions were prominent nationwide except in Okinawa/Amami
and the Kanto region.
The monthly precipitation ratio to the normal averaged over the Pacific side of western Japan for August
2014 was 301%, which was the highest since 1946 when collection of the area-averaged statistical data
referenced here began. Monthly precipitation ratios were also significantly above normal in northern Japanand on the Sea of Japan side of eastern and western Japan. Heavy rain was brought by two typhoons, fronts
and moist air flow, resulting in record-high precipitation at observation stations from Hokkaido in the north toKyushu in the south.
The monthly sunshine duration ratio to the normal averaged over the Pacific side of western Japan was54%, which was the lowest since 1946. The ratio averaged over the Sea of Japan side of western Japan was42%, which was the second lowest after 1980, and the figure was also significantly below normal overeastern Japan.

1The Advisory Panel, consisting of prominent experts on climate science from universities and research institutes, was established inJune 2007 by JMA to investigate extreme climate events based on up-to-date information and findings. See TCC News No. 9 and No.
28.
2
In the tropics, enhanced and suppressed areas of large-scale cumulus convection appear alternately in cycles of 30 to 60 days.

Table 1 Regional averages and related rankings of monthly precipitation ratios and monthly sunshine duration ratios forAugust 2014
Precipitation ratio (%)
Record high2014(year)

AugustSea of Japan side
Pacific side
Sea of Japan side
Eastern Japan
Pacific side
Sea of Japan side
WesternJapan
Pacific side
Okinawa and Amami
NorthernJapan

171
156
234
12124230176[3]
[3]
[3]
[1]

264
222279209301231252(1981)(1998)(1976)(1982)(1980)(2004)(2012)Sunshine duration ratio (%)
Record low
2014(year)
9494
58754254
93

[2]
[1]

61584954
415773(1998)(1998)(1993)(1980)(1980)(1980)(1960)*The numbers in square brackets indicate the ranking for August 2014 since 1946. Rankings of precipitation ratios and sunshine
duration ratios are shown in descending and ascending order, respectively.

Precipitation ratios (%)

Ogasawara Islands

Sunshine duration ratios (%)

Ogasawara Islands

Figure 1 Precipitation ratios and sunshine duration ratios for the period from 30 July to 31 August 2014
The base period for the normal is 1981 – 2010.

Seven-day(a) running-mean precipitation ratios for the Sea of Japan side of western(b)
Seven-day running-mean precipitation ratios for the Pacific side of western Japan

Japan(c)Seven-day running-mean sunshine duration ratios for the Sea of Japan side of western Japan

Seven-day running-mean sunshine duration ratios for the Pacific side of western Japan(d)

Figure 2 (a) Seven-day running mean precipitation ratios for the Sea of Japan side of western Japan. (b) As per (a), but for thePacific side of western Japan. (c) As per (a), but for sunshine duration ratios. (d) As per (b), but for sunshine duration ratios.
The red horizontal lines indicate the ratio of 100% to the normal. The purple lines indicate 30 July. The base period for the normal is 1981
– 2010.

2. Characteristic atmospheric circulation causing Japan’s unseasonable weather conditions (Figure 3)(1) General characteristics from late July to August 2014
From late July to August 2014, the Pacific High was enhanced to the southeast of mainland Japan and its
westward expansion was weaker than normal, which brought continuous southerly warm moist air flow tothe country.
From late July to early August 2014, Typhoon Nakri and Typhoon Halong approached Japan in quick
succession. After early August, fronts that tended to linger around the nation’s mainland brought
above-normal precipitation amounts and below-normal sunshine durations over a wide area of western Japanand other parts of the country, and heavy rainfall events were observed nationwide.
(2) Late July to early August 2014
The subtropical jet stream was shifted northward of its normal position around Japan. As a result, Typhoon
Nakri did not turn eastward after approaching Japan due to the jet stream’s diminished effect. The lasting
effects of the typhoon were felt in western and other parts of Japan even after its transformation into an
extratropical cyclone.
The Pacific High was enhanced to the southeast of mainland of Japan, which facilitated warm moist air
flow over the country along the periphery of the Pacific High. Heightened convective activity around thePhilippines is thought to have contributed to the enhanced Pacific High.

(3) Mid- to late August 2014
Typhoon Halong moved slowly northward in the area to the south of western Japan and made landfall
over Shikoku on 10 August. After passing over western Japan, it continued northward over the Sea of Japan.
In early August, the subtropical jet stream began to shift southward of its normal position around Japan before
meandering significantly (i.e., southward meandering to the west of Japan and northward meandering to theeast of Japan) toward the middle of the month. In association, fronts tended to linger near the nation’smainland. While the Pacific High was enhanced to the southeast of Japan, its westward expansion was weak.
Suppressed cumulus convection over the Asian monsoon region (Southeast Asia and South Asia) inassociation with the tropical intra-seasonal oscillation is considered to have contributed to the southward shift
and meandering of the jet stream.
While convective activity around the Philippines was enhanced during early August, it subsequently
became suppressed and continued in this condition. A contributory factor to this situation is considered to be
the influence of downward air flow over the western part of the Pacific including the Philippines inassociation with upward air flow caused by enhanced convective activity in association with above-normal
sea surface temperatures in the eastern Pacific and the eastern Indian Ocean. The influence of tropicalintra-seasonal oscillations is seen as another contributory factor.

Figure 3 Primary factors contributing to the unseasonable weather conditions of August 2014 in Japan

In association with positive anomalies of lower tropospheric pressure related to suppressed convective
activity around the Philippines, southwesterly warm moist air flow in the lower troposphere from the SouthChina Sea to the East China Sea was predominant, while westerly flow from the South China Sea to thePacific east of the Philippines is seen in normal years. This southwesterly flow is considered to havecontributed to the enhancement of warm moist air inflow over Japan.
3. Long-term trends of extreme precipitation events (Figures 4 and 5)
JMA operationally observes precipitation at around 1,300 unmanned regional meteorological observation
stations all over Japan (collectively known as the Automated Meteorological Data Acquisition System, orAMeDAS). The annual 3 and monthly (August) numbers of events 4 with precipitation exceeding 50mm/hour and 80 mm/hour observed at AMeDAS stations are likely to have increased (Figure 4). Data onwater vapor presence in Japan based on upper-air observation also show an increasing trend (Figure 5).
According to the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5),extreme precipitation events over most mid-latitude land masses will very likely become more intense andmore frequent, and the amount of water vapor in the air will likely increase by 5 to 25% by the end of thiscentury as global mean surface temperatures rise.
From the above, it can be inferred that the increasing trend seen in the number of extreme precipitation
events may be linked to global warming. As the period covered by observation records is still relatively short,the addition of future data series is expected to allow further clarification regarding the increasing trend’s

AMeDAS: number of events with precipitation ≥ 50 mm/hour inAugustTrend =4.9events/10 years/10year

JMA

Number of precipitation events (per 1,000 points) mm/hour

Number of precipitation events (per 1,000 points) mm/hour

relationship with global warming.
AMeDAS: number of events with precipitation ≥ 80 mm/hour inAugustTrend =0.6events/10 years

JMA/10year

Figure 4 Number of events with precipitation greater than or equal to 50 mm/hour (left) and 80 mm/hour (right) in Augustfrom 1976 to 2014 (per 1,000 Automated Meteorological Data Acquisition System (AMeDAS) points)
The green bars indicate values for each year, and the straight red lines indicate the long-term linear trend (statistically significant at aconfidence level of 90%).

3

See Figure 2.2-7 of the Climate Change Monitoring Report 2013 http://www.data.jma.go.jp/cpdinfo/monitor/2013/pdf/ccmr2013_all.pdf.
There were around 800 AMeDAS stations in 1976, gradually increasing to around 1,300 by 2012. To account for these numerical differences, thenumber of precipitation events is converted to a per-1,000-station basis.
4

Ratio against 1981 – 2010 average (%)

Specific humidity at 850-hPa height for August

Figure 5 Time-series representations of specific humidity ratios for summer (June – August) at 850 hPa from 1981 to 2014 inJapanThe data are presented as ratios against the baseline (the 1981 – 2010 average).
Note: The term specific humidity refers to the mass of water vapor in a unit mass of moist air (g/kg). The data used in this analysis werebased on observations made using radiosondes (balloon-borne instrument platforms with a radio-transmitting device) at 13 upper-air
observation stations in Japan. The dots show the averages of the data for the 13 stations. The thick blue line indicates the five-year
running mean, and the straight red line indicates the long-term linear trend (statistically significant at a confidence level of 95%). The
red triangles indicate changes of observation instruments.

4. Outlook
For the coming two weeks, the weather will be changeable in eastern and western Japan, and cloudy/rainy
conditions will be more frequent than normal for this time of year on the Pacific side and elsewhere.

Climatological regions of JapanThe country has four divisions (northern, eastern, western Japan and Okinawa/Amami) and eleven subdivisions (Hokkaido, Tohoku,Kanto-koshin, Hokuriku, Tokai, Kinki, Chugoku, Shikoku, northern Kyushu, southern Kyushu and Okinawa).