Climate characteristics and factors behind record-high temperaturesin late June/early July 2022 and subsequent weather conditions
14 September 2022
Tokyo Climate Center (TCC), Japan Meteorological Agency (JMA)
https://www.data.jma.go.jp/tcc/tcc/

SummaryThe seasonal temperature in summer of 2022 was higher than normal across Japan.
Especially the period from late June to early July was characterized by record hightemperatures recorded in eastern and western parts of the country (Figure A1). Heavyrain occurred in many areas in mid-July. From late July to mid-August northern Japan
also experienced heavy rain, and high temperatures were observed in western Japanand elsewhere.

 The conditions observed are mainly attributed to the following: The record-high temperatures observed in late June and early July are attributed to themeandering northward of the upper-level subtropical jet stream (STJ) near Japan anda record strengthening of both the upper-level high and the Pacific High at the surface
for this time of year, combined with a persistent warming trend.
 Factors contributing to the subsequent heavy rainfall in various areas in mid-Julyinclude a persistent blocking high over northern Japan and a meandering of the STJ
southward near Japan, which made the area susceptible to upper-level cold air.
 From late July to mid-August, the STJ moved northward and tended to maintain a front
near northern Japan that produced heavy rainfall, while high temperatures persisted inwestern Japan and elsewhere.

１．Climate characteristics and factors behind record high temperaturesin late June and early July
(1) Characteristics of high temperatures and climate conditions
In late June, the northward extension of the Pacific High at the surface strengthened
and the Baiu front moved northward, resulting in record-high temperatures in eastern Japan(+4.0°C above the normal), western Japan (+3.2°C) and elsewhere (Figure 1-1). Northern
Japan observed the second-highest temperatures on record (+2.9°C above the normal).
Also in late June and early July, 24 of the 914 stations in Japan observed record-hightemperatures. In particular, Isesaki in Gunma Prefecture recorded daily maxima above 40°C
on June 25/29 and July 1, and Tokyo also observed daily maxima above 35°C for a record

nine consecutive days from June 25 onward. Figure 1-2 shows the cumulative number ofextremely hot days (with daily maximum temperatures above 35°C) observed at AMeDASstations nationwide from June 1 to August 31. The top three years with the highest averagesummer temperatures (2010, 2013 and 2018) are shown for comparison. During the same
period for 2022, the cumulative number of such stations increased significantly and earlier
than in the aforementioned three years.
To identify possible causative factors, JMA, with the TCC Advisory Panel on Extreme
Climatic Events (a JMA body staffed by prominent experts on climate science fromuniversities and research institutes), investigated factors considered to have contributed tothese extreme climatic conditions.

(2) Characteristics of large-scale atmospheric circulation causing record-hightemperatures
This section describes the characteristics of the large-scale atmospheric circulation thatcaused the record-high temperatures of late June/early July and factors contributing to thephenomenon (Figure 1-3). The circled numbers correspond to those in Figure 1-3.
 Near Japan, both the Pacific High at the surface (① ) and the upper-level high (② )were significantly strengthened for this time of year (Figure 1-4). A tall anticyclone
characterized by warm air covered the area around Japan, and enhanced solar
radiation associated with strong downdrafts and persistent stable clear skies broughtwarm conditions to the region. Combined with the influence of localized over-mountainair currents, this resulted in record-high temperatures exceeding 40°C in some areas.
 The strengthened extension of the upper-level anticyclone toward Japan and thePacific High at the surface were influenced by the continued significant northward
meandering of the upper-level sub-tropical jet (STJ) near Japan (③).
 The STJ continued to meander significantly from Eurasia to the seas east of Japan,
forming a warm and near-equivalent barotropic anticyclone near the country. This
meander may have been partly caused by a large meander of the jet stream over
the North Atlantic to Europe, and its influence extended downstream.
 Extremely enhanced cumulus convective activity near the Philippines from late June
onward (④) compared to the normal was also associated with the strengthening of thePacific High's extension toward Japan (Pacific-Japan (PJ) Pattern1).
 In addition to higher-than-normal sea surface temperatures in the vicinity of thePhilippines, the strengthening of an upper-level anticyclone near Japan (making theWhen cumulus convective activity near the Philippines is more active than normal, the low-pressurepart of the lower atmosphere (monsoon trough) and associated counterclockwise circulation seen fromSoutheast Asia to the Philippines during the summer season are stronger than normal, and air rising near
the Philippines descends near Japan, strengthening the Pacific High's extension over the country’s
Honshu mainland.
1area more susceptible to upper-level cold air intrusion into the subtropical region to
the south of the anticyclone (⑤)), may have contributed to increased cumulus
convective activity near the Philippines.
 In addition to the strengthened Pacific High and upper-level anticyclones, a continued
rising trend in global temperatures associated with global warming (⑥) andsignificantly higher tropospheric temperatures in the Northern Hemisphere
mid-latitudes since late 2020 (⑦) may have further contributed to the record-hightemperatures observed.
 Preliminary results from Event Attribution2, which assesses the impacts of global
warming, suggest that the probability of such high temperatures is considerably
higher than it would have been in the absence of global warming.
 The persistence of high tropospheric temperatures in the mid-latitudes of theNorthern Hemisphere is considered to have been influenced by the La Niña eventthat persisted from summer 2020 to spring 2021 and fall 2021 onward, resulting inlower temperatures in the troposphere over the tropics, while the STJ shifted
northward from its normal position throughout the Northern Hemisphere.

２．Weather characteristics and related factors from July to mid-August(1) Weather features
Weather conditions from July to mid-August showed large intra-seasonal variability.
Eastern and western Japan experienced record-high temperatures from late June to early
July, while northern Japan also continued its trend of extremely high temperatures from lateJune onward, with the average in early July being the highest since 1946 (Figure 1-1).
Okinawa/Amami also experienced the highest mid-July temperature on record.
Meanwhile, cloudy and rainy conditions from northern to western Japan in mid-Julyresulted in fewer hours of sunshine and above-normal precipitation amounts. Heavy rainfallwith record amounts of short-period precipitation occurred in many areas (Figure 2-1).
Precipitation totaling 111.0 mm over one hour and 360.0 mm over six hours was observed at
Hatoyama in Saitama Prefecture on July 12, and 191.5 mm over three hours was observedat Mitsushima in Nagasaki Prefecture on July 18, representing new highs for both locations.
The Pacific High extended over mainland Japan again in late July, and the number ofstations recording extremely high temperatures (above 35°C) increased (Figure 1-2). From
August 3 to 4, a front moved slowly southward over the Tohoku and Hokuriku regions,bringing record-heavy rainfall (Figure 2-2) that caused landslides, river flooding and otherissues. Shimoseki in Niigata Prefecture recorded precipitation of 149.0 mm over one hour,
A method for probabilistic estimation of how much the occurrence of individual phenomena haschanged due to global warming by comparing a number of simulation experiments conducted under pastclimate conditions using a climate model and simulations conducted under a scenario with no global
warming caused by human activity.
2323.5 mm over three hours and 560.0 mm over 24 hours on August 4, all of which wererecord-highs for the location.
From late to mid-August, a stagnant front persisted near northern Japan. The northernpart of Tohoku region experienced record-heavy rainfall, especially in Aomori Prefecture,with total precipitation exceeding 400 mm from August 8 to 14 (Figure 2-3). Due to Tropical
Storm (TS) Meari (T2208), the area of heavy rainfall extended to the Tokai and Kanto
regions.

(2) Characteristics of large-scale atmospheric circulation
The characteristics of the large-scale atmospheric circulation that produced the weather
conditions reported here are detailed below for each period.
(a) Mid-July (Figure 2-4)
 In upper levels, a blocking high formed to the north of Japan in early July and persisted
until mid-July. Over Japan to the south of this upper-level high, a cold trough (vortex)
persisted, causing unstable atmospheric conditions and contributing to record-heavy
short-period rainfall.
 The formation and persistence of the blocking high to the north of Japan was related
to the ongoing large meander of the polar-front jet stream.
 The persistence of the trough in the upper troposphere near Japan was also
influenced by the persistent large meander of the STJ over Eurasia and its ongoing
southward shift near Japan. Along with this, the Pacific High at the surface tended toextend westward over the seas south of Japan with a weak extension toward thecountry’s mainland.
 The large meander of the STJ over Eurasia may have been influenced by significantlyenhanced cumulus convective activity from the southeastern Arabian Peninsula to thearea near Pakistan in early July.
(b) Late July – first half of early August From late July to early August, the STJ meandered significantly northward near Japan,
and the Pacific High again strengthened its extension toward the country’s mainland,
resulting in higher-than-normal temperatures over eastern, western and other parts ofthe nation. TSs Songda (T2205) and Trases (T2206), which successively formed to thesouth of Japan, moved northward over the East China Sea along the fringe of thePacific High (Figure 2-5, bottom).
 On August 3 - 4, the northward meander of the STJ weakened, the Pacific Highextension was centered over western Japan, and the front moved slowly southward
over the Tohoku and Hokuriku regions.

Warm and moist lower-level air containing large amounts of water vapor associated withthese two TSs flowed over the Sea of Japan along the fringe of the Pacific High and over
the Tohoku and Hokuriku regions, where the front was stagnant (Figure 2-5, top),
contributing to heavy rainfall in these regions. Stationary linear mesoscale convective
systems also developed there, resulting in record amounts of precipitation.

(c) Late-early to early-mid August (Figure 2-6)
 While the front remained stagnant from the Sea of Japan to around northern Japan,
resulting in record-heavy rainfall in northern Japan, the area from eastern Japan toOkinawa/Amami was often covered by the Pacific High (except when TS Meari (T2208)
landed), resulting in higher-than-normal temperatures.
 Around northern Japan, a confluence of moisture inflow along the fringe of the PacificHigh and moisture inflow along the frontal zone persisted. These conditions werebrought about by the STJ moving northward again over northern Japan, while theupper-level trough meandered southward over Russia’s Primorskya Oblast and thePacific High remained stronger than normal over seas south of Japan.
 The stronger-than-normal STJ flowing northeastward around northern Japan and thefront tended to stagnate, while the upper-level trough strengthened upwelling andenhanced convective precipitation activity around the front, which may have been afactor behind the heavy rainfall.
 The significant meandering of the STJ over Eurasia and the strengthened
westward-extended Pacific High to the south of Japan may have been influenced byenhanced cumulus convective activity from the northern Indian Ocean to the South
China Sea.
 The enhanced cumulus convective activity in the region is thought to have enhanced
the meandering of the STJ over Eurasia, which further affected the area aroundJapan.
 In addition, lower-level winds blew toward the region of active cumulus convection,and the influence of suppressed cumulus convective activity due to the predominanceof downwelling over seas south of Japan is thought to have strengthened thewestward extension of the Pacific High.
(d) Effects of global warming on precipitation
Japan exhibits a long-term trend of increasing intensity in extremely heavy rainfall, withthe annual maximum 72-hour precipitation at AMeDAS stations showing an upward trend ofaround 10% over the past 30 years (Figure 2-7). One background factor to this is the amountof water vapor in the atmosphere, which has shown a long-term rise along with temperature
in association with global warming (Figure 2-8). An increase of 1°C in air temperature is
known to theoretically increase saturated water vapor content by around 7%.

Further research on heavy rainfall probability and other variables is required for fullquantitative evaluation in the field. However, it can be considered that the long-term increase
in atmospheric water vapor associated with global warming may have been a factor in theheavy summer rainfall.

Figure 1-1. Time-series representations of 5-day running mean temperature anomalies [°C] for
June-August 2022
The base period for the normal is 1991 - 2020. The red circled numbers and values indicate the rank
from highest average temperatures since 1946 and anomalies for each month and each ten-day
period (up to the top three).

Figure 1-2. Cumulative numbers of extremely hot days (daily maximum temperatures above35°C) observed at AMeDAS stations nationwide
Results for June 1 - August 31 (red line: 2022 up to August 21) and previous hot years (2010, 2013 and2018). The total number of AMeDAS stations as of June 1 was 919 for 2010, 927 for 2013 and 2018, and914 for 2022.

Figure 1-3. Characteristics of atmospheric circulation bringing record-high temperatures inlate June and early July

Figure 1-4. Trends in yearly intensity of high pressure around Japan (June 1 – August 31)
The upper panel shows the five-day running average of anticyclone intensity (sign-reversed relative
vorticity; unit: 10-4s-1) in the upper troposphere (around 12,000 m altitude) averaged from 35 to 45°N
and 125 to 150°E, and the lower panel shows the lower troposphere (around 1,500 m altitude)
averaged from 30 to 40°N and 125 to 150°E. The red line represents 2022 (until August 20), thegreen dashed line represents the normal (1991 – 2020 average), and the grey lines represent theyears from 1958 to 2021, with larger values indicating stronger anticyclones. Data are based on theJapanese 55-year Reanalysis (JRA-55).

Figure 2-1. Maximum 3-hour precipitation during July 11-20

Figure 2-2. Total precipitation for August 1 - 6
Record breaking status is not shown.

Figure 2-3. Total precipitation for August 8 - 14
Record breaking status is not shown.

Figure 2-4. Characteristics atmospheric circulation bringing the unusual weather conditions
in mid-July

Figure 2-5. Water vapor flow and surface weather maps for July 30, August 1, and August 3

Figure 2-6. Characteristics of atmospheric circulation bringing heavy rainfall centered overnorthern Japan in late-early to early-mid August

Figure 2-7. Annual maximum precipitation ratios in 72 hours from 1976 to 2022 in Japan
This graph is based on precipitation data from 637 AMeDAS stations which have been continuously
operated from 1976 to 2022. Bars indicate the ratio to the baseline (the 1991–2020 average), while theblue and red lines indicate the related five-year running mean and the long-term linear trend, respectively(statistically significant at a confidence level of 95%). The value for 2022 is preliminary as of August 21.
The red triangle marks the timing of a change in the observation method for precipitation (observed every
hour before 2003 and every 10 minutes thereafter).

Figure 2-8. Specific humidity ratio at 850hPa for August from 1981 to 2022 over JapanThe data are presented as ratios against the baseline (the 1991 –2020 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 were based on radiosonde observations (balloon-borne instrument platforms
with a radio-transmitting device) at 13upper-air observation stations in Japan (Wakkanai, Sapporo, Akita,Wajima, Tateno, Hachijojima, Shionomisaki, Fukuoka, Kagoshima, Naze, Ishigakijima, Minamidaitojima,and Chichijima). The values for 2022 are based on data through August 20. The thin black line indicatesthe averages of the data for the 13stations. The blue and red lines indicate the related five-year runningmean and the long-term linear trend, respectively(statistically significant at a confidence level of 99%).
Data from the period marked by the red triangles may include biases due to instrument changes.
Figure A1. Climatologicalregions of Japan
JMA’s seven regional divisions
for climate monitoring andforecasting (the Sea of Japanand Pacific sides of northern,eastern and western Japan,and Okinawa/Amami)