New Delhi: The Japan Meteorological Agency (JMA) issued a tsunami warning for parts of the country’s east coast on Sunday after an earthquake struck in the Pacific Ocean. Both the agency and the US Geological Survey reported that the 6.7 magnitude quake struck about 78 miles east of the coastal city of Yamada, at a depth of a little more than six miles. A magnitude 6.76.76.7 earthquake struck off the east coast of Japan, prompting the RSMC Tokyo – Typhoon Center

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Factors behind record-high temperatures over Japan in summer 2025 and dry
conditions in July, and the large-scale atmospheric circulation behind heavy
rainfall in the first half of August
30 September 2025
Tokyo Climate Center (TCC), Japan Meteorological Agency (JMA)
https://www.data.jma.go.jp/tcc/tcc
Summary
Characteristics of record-high temperatures in June –August (summer) 2025 and dry conditions in July
in Japan:
⚫ Summer mean temperatures far exceeding previous records set in 2023 and 2024
⚫ A new national record-high daily maximum temperature and unprecedented cumulative numbers of weather stations observing daily maximum temperatures exceeding certain thresholds.
⚫ Early seasonal march with the earliest-ever withdrawal of the Baiu rainy season over many regions, and consequent record-dry conditions over Hokuriku region (Fig. A) in July Factors behind these characteristics:
⚫ Enhanced convection over the Asian monsoon region from the early summer monsoon period in association with above-normal ocean temperatures in the tropical western Pacific
➢ This contributed to a markedly poleward-shifted subtropical jet over Eurasia from June onward and the Tibetan High extension toward Japan.
➢ The North Pacific Subtropical High (NPSH) extended toward Japan in association with unprecedented intensification of cumulus convection to the east of the Philippines.
➢ Both the Tibetan High and the NPSH covered areas around Japan, causing temperature rise under a dominant anomalous descent.
⚫ A long-term tropospheric warming trend associated with global warming and pronounced above-normal tropospheric temperatures over the mid-latitude Northern Hemisphere in association with persistent remarkably high sea surface temperatures (SSTs) in the area in recent years the Japanese Reanalysis for Three Quarters of a Century (JRA-3Q; Kosaka et al. 2024) and Merged Satellite and In-situ Data Global Daily Sea Surface Temperature (MGDSST; Kurihara et al. 2006) datasets were used in this analysis for atmospheric circulation and SSTs, and NOAA Climate Prediction Center (CPC) Blended Outgoing Longwave Radiation (OLR) data were used for inference of tropical convective activity. In-situ station data on surface air temperature over Japan were obtained from the JMA Automated Meteorological Data Acquisition System (AMeDAS). The base period for the normal is 1991 to 2020. The term “anomaly” as used in this report refers to deviation from the normal.

Introduction
In summer 2025, Japan experienced sweltering heat with above-normal surface air temperatures in northern, eastern and western Japan, and significantly high temperatures from mid-June to early August. In July, record-dry conditions were observed over parts of the country due to an inactive Baiu front and a
suppressed supply of moisture. The nation’s Hokuriku and Kyushu regions experienced heavy rainfall in the first half of August. This report outlines discussions held by the JMA Advisory Panel on Extreme Climatic Eventsn(comprised of prominent climate science academics and researchers) on 5 September 2025 to summarize
primary factors behind these anomalous climate conditions (Fig. 0).The details provided are as of the time of the meeting. Figure 0 Large-scale factors behind record-high temperatures and dry conditions in summer 2025 “STJ,” “SST” and “NPSH” stand for subtropical jet, sea surface temperature and North Pacific Subtropical High, respectively. Dashed lines show normal positions. Climate conditions in summer 2025
1-1. Record-high temperatures
⚫ Surface air temperature anomalies over Japan in summer 2025 were significantly positive nationwide except in Okinawa/Amami, with values in northern, eastern and western Japan at their highest since. The record-high summer mean temperature anomaly over the country in 2025 was +2.36°C, far exceeding the previous 2023 and 2024 records (both +1.76°C) (Fig. 1-1).
⚫ On 5 August, a new national daily maximum temperature of 41.8°C was recorded at Isesaki in Gunman Prefecture, north of Tokyo, accompanied by unprecedented cumulative numbers of weather stations observing daily maximum temperatures exceeding 35 and 40°C(Fig. 1-2). 1-2. Dry conditions in July
⚫ In July, record-dry conditions were observed over parts of Japan (Fig. 1-3) due to an inactive Baiu front and a suppressed supply of moisture.
➢ Most regions experienced early onset and withdrawal of the Baiu rainy season in association with an early seasonal march. Many regions outside the Tohoku area experienced their earliest-ever withdrawal of the Baiu rainy season in June.
➢ Monthly precipitation amounts were significantly below normal on the Sea of Japan side. Those in hokuriku region were the lowest for July since 1946.
➢ Parts of the country experienced significantly above-normal monthly sunshine durations, with some recording the longest for July since 1946. Large-scale atmospheric circulation and related influence on climate conditions over Japan
⚫ The subtropical jet (STJ) around Japan significantly shifted poleward around Sakhalin from mid-June to July, with the shift persisting in August (Fig. 2-1). This caused Baiu front inactivity around Japan at the end of June, corresponding to the record-early withdrawal of the Baiu rainy season in eastern and western Japan. The Tibetan High south of the STJ markedly extended northeastward, causing higher
tropospheric temperatures around the country.
⚫ On an intraseasonal timescale, the STJ around Japan markedly meandered northward in association with Rossby wave propagation across Eurasia in a Silk Road pattern (SRP, Enomoto et al. 2003; Enomoto 2004; Kosaka et al. 2009) around mid-June, early July, and late July. The intraseasonal SRP intensification contributed to further extension of the Tibetan High toward Japan (Fig. 2-2).
⚫ Patches of upper-level cyclonic vortices with high-potential vorticity anomalies detaching from the intensified mid-Pacific trough moved westward toward the south of Japan, contributing to enhanced convection to the east of the Philippines(Section 3).
⚫ The low-level North Pacific Subtropical High (NPSH) extended toward areas from mainland to northern Japan (Fig. 2-1).Both associated anomalous descent and increased downward solar radiation under the NPSH contributed to high surface air temperatures over the country. On a local scale, intensified downslope winds associated with the Foehn effect also caused higher local surface air temperatures.
⚫ In the first half of August, the STJ meandered southward in association with the phase-shifted SRP (Fig. 2-2). A stationary front from China to mainland Japan was active under moist airflow originating from the Asian monsoon area. Record-heavy rainfall in Hokuriku and Kyushu regions of Japan was associated with intensified moist airflow along the periphery of the southwestward-extended NPSH toward the stationary front (Fig. 2-3). Tropical convection and related influence on large-scale atmospheric circulation.
⚫ The summer Asian monsoon onset was several weeks earlier than normal with persistent enhancement until mid-July, contributing to the intensified Tibetan High and a persistently poleward-shifted STJ from Eurasia to Japan (Figs. 2-1 and 3-1).
⚫ In July, record convection enhancement to the northeast of the Philippines(Fig. 3-2)triggered a Pacific- Japan (PJ) pattern (Fig. 3-3; Nitta 1987; Kosaka and Nakamura 2006, 2010).
⚫ Convective activity exhibited notable sub-monthly variations in association with boreal-summer intraseasonal oscillation (BSISO; Lee et al. 2013; Kikuchi 2021). From the end of July to early August, BSISO-related northward-moving enhanced convection was clearly seen from the tropics to the subtropics. To the south of this enhancement, anticyclonic circulation anomalies shifted northward from The philippines to the south of Japan, corresponding to the southwestward extension of the NPSH. Warm moist airflow along the periphery of the extended NPSH in addition to that originating from the asian monsoon area moving toward Japan in early August caused heavy rainfall in Hokuriku and kyushu regions of the country. Oceanographic conditions and related influence on large-scale atmospheric circulation
⚫ Summer means SST sin the tropical Pacific were above normal in western parts and below normal from central to eastern partsin a pattern exhibiting La Niña-like characteristics (Figs. 4-1 and 4-2), although
JMA’s conditions for the definition of a La Niña occurrence were not satisfied. This zonal contrast of SST anomalies possibly contributed to enhanced convection over the summer Asian monsoon area.
⚫ In the tropical western Pacific, particularly to the east of the Philippines, ocean temperatures to a depth of several hundred meters were significantly above normal, possibly contributing to the record enhancement of convection observed to the northeast of the Philippines (Fig. 3-2).
⚫ SSTs in the mid-latitude North Pacific were significantly above normal (Fig. 4-2), in addition to the aforementioned tropical SST anomalies, possibly contributing to the remarkably above-normal mid latitude tropospheric temperatures associated with the feedback effect of the air-sea interaction behind poleward-shifted STJ persistence (Fig. 4-1).
⚫ Positive SST anomalies to the east of Japan were further intensified from the second half of June to the first half of July in association with the northward-meandering STJ around Japan. The significantly above-normal surface air temperatures observed around northern Japan were possibly attributable to
the high SSTs observed around the country through an enhanced greenhouse effect due to increased moisture. Additionally, increased downward solar radiation owing to reduced low-level cloudiness related to the high SSTs(Norris and Leovy 1994) may be linked to the persistently above-normal SSTs and surface air temperatures observed around northern Japan.
⚫ The negative phase of the Pacific Decadal Oscillation (PDO; Mantua et al. 1997; Nakamura and Yamagata 1999; Newman et al. 2016), which has persisted since around 2020, is a possible factor behind these anomalous SSTs(Fig. 4-1).

5. Influence of global warming
   ⚫ A joint research team from the Ministry of Education, Culture, Sports, Science and Technology (MEXT)’s Advanced Studies of Climate Change Projection initiative and JMA’s Meteorological Research Institute assessed the influences of global warming using an event attribution (EA) approach (Table 5-1), with the following results:
   ✓ The record-high temperatures observe around Japan in summer 2025 would virtually never have happened with the assumption of no effects from global warming.
   ✓ Such high-temperature events can be expected to occur once every 60 years in the climate conditions of 2025 with global warming.
   ✓ The heavy rainfall observed around Kumamoto in Kyushu region from 10 to 11 August was approximately 25% greater with current climate conditions than with theoretical experimental conditions assuming no global warming.
   ⚫ The rate of temperature increase experienced under global warming has accelerated in recent years.
   ✓ Record-high summer mean surface air temperatures over Japan have been observed in three consecutive years (2023 to 2025), far exceeding the projected linear trend of temperatures based on the period from 1995 to 2024.
   ✓ Although there is still no unified view, recent studies have reported that the increase in the rate of temperature rise is attributable to some factors such as interdecadal-timescale atmospheric and oceanographic variability and reduced air pollution. Table 5-1 Expected frequency of temperatures observed in 2025 (EA approach) Period Expected frequency of temperatures exceeding 2025 levels
   July Summer (June –August) Pre-industrial Negligible Negligible Normal (1991 – 2020) Approx. once every 420 years Approx. once every 500 years 2025 (error margin) Approx. once every 70 years (from 30 to 100 years) Approx. once every in 60 years (from 30 to 180 years).

Table of Contents: TCC News No. 81
Climate Change in Japan 2025………………………………………………………………………………………………………………… 1
Upgrade of Reports on Global Extreme Climate Events…………………………………………………………………………………. 4
HTML Upgrade for Monthly and Seasonal Highlights on the Climate System………………………………………………………. 6
TCC contributions to the Report on the States of the Climate in Asia 2024 ………………………………………………………… 8
Upgrade Plan of the JMA’s Seasonal Ensemble Prediction System …………………………………………………………………… 9
Following the 2015 adoption of the Paris Agreement against a background of global and regional climate change, Japan’s Climate Change Adaptation Plan (based on the Climate Change Adaptation Act) was approved by the nation’s Cabinet in 2018.
In March 2025, the Japan Meteorological Agency (JMA) and Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT) published the Climate Change in Japan 2025 report to support related action by providing information with a physical science basis to the public, commercial enterprises and local/national government bodies

This series of reports provides information on the status of atmospheric greenhouse gases and observed/projected changes in climate variables in and around Japan. The 2025 edition includes high-resolution climate projection updates and information on how the frequency and intensity of extreme heavy rainfall and high temperatures are projected to change with global warming. Projections are based on the 2/4°C Warming Scenarios (RCP2.6 and RCP8.5 as referenced in the IPCC Fifth Assessment Report, respectively).
➢ 2°C Warming Scenario: potential climatic conditions with achievement of the Paris Agreement’s 2°C goal.
➢ 4°C Warming Scenario: potential climatic conditions with no future additional mitigation measures.
The information in the reports is used for risk assessment, policy formation, infrastructural development and
disaster mitigation.

1. Temperature
   Annual surface temperature is virtually certain to have risen at a rate of 1.40°C per century between 1898 and Since 1910, the annual numbers of days with maximum temperatures (Tmax) of ≥ 30°C and ≥ 35°C have increased (and since 1929 for days with minimum temperatures (Tmin) of ≥ 25°C), while the number of Tmin < 0°C days has decreased. Figure 1-1 shows changes in annual surface temperature as of the end of the 21st century relative to the end of the 20th century. Under both IPCC scenarios, annual surface temperature is projected to rise, with more Tmax ≥ 35°C / Tmin ≥ 25°C days and fewer Tmin < 0°C days projected in many regions.
2. Precipitation
   The frequency of heavy rainfall events has risen overall (Figure 1-2), with heavier instances showing a particularly high rate of increase. It is extremely likely that annual maximum daily precipitation has increased. Under both IPCC scenarios, the national average frequency of heavy rainfall events is projected to increase, along with annual maximum daily precipitation (i.e., increased frequency and intensity of extreme rainfall). In conditions with a 4°C rise, centennial extreme precipitation (daily) compared with pre-industrial conditions is projected to occur approximately 5.3 times, and centennial daily precipitation amounts are projected to increase by approximately 32%.
3. Snowfall
   The annual maximum snow depth is extremely likely to have decreased in each region on the country’s Sea of Japan side since 1962. While annual maximum snow depth and snowfall are projected to decrease under the 4°C
4. Warming Scenario, actual snowfall may increase during extreme events in some areas, such as mountainous regions of Honshu.
5. Areas with no symbols, those marked with [*], and those marked with [#] have virtually certain trends, extremely likely trends, and no discernible trend, respectively.
6. Sea levels
   It is extremely likely that a rising trend in mean sea level (MSL) has occurred since the 1980s along the coast of Japan, although some long-period variability (assumed to be natural) is predominant over the whole period (Figure 1-4). MSL is also projected to continue rising during the 21st century along the coast of Japan.

Upgrade of Reports on Global Extreme Climate Events

TCC monitors the global climate with CLIMAT and SYNOP reports from NMHSs and collects quality-checked data on temperature and precipitation, using the information to monitor extreme climate events around the world. Based on the data obtained, the Center issues weekly, monthly and seasonal monitoring reports on extreme climate conditions with overviews of disastrous events. On 14th March 2025, TCC upgraded these products as follows to enhance usability and support workflow in users’
data analysis:

• Revised maps of extreme climate events for weekly, monthly and seasonal reports to improve clarity and readability (Figure 2-1). The revision reduces ambiguity and helps to highlight critical points more efficiently.
• New data on weather stations recording extreme climate conditions in HTML and CSV format as part of monthly and seasonal reports (Figure 2-2).
  The data format is the same as that of weekly reports and provides granular data access, thereby clarifying the details of observation data and anomalies.

– In the tropical Pacific, remarkably positive sea surface temperature (SST) anomalies were observed in the western region and negative anomalies were observed along the equator from the central to eastern region, indicating a La Niña-like pattern. NINO.3 was -0.5°C. In the Indian Ocean, remarkably positive and negative SST anomalies were observed in the eastern region and in the Arabian Sea, respectively, indicating a negative Indian Ocean Dipole like pattern. Remarkably positive SST anomalies were also observed north of 20°N in the western North Pacific. In the East China Sea and around western Japan, these anomalies exceeded three times the climatological standard deviation.
– In the tropic, the large scale pattern of upper level divergence showed a wave number 1 and convective activities were remarkably more active than normal from the eastern Indian Ocean to the Maritime Continent while they were remarkably less active than normal around the date line and from Africa to the western Indian Ocean in the equator. Total activities of the Asian Monsoon were more active than normal besides a intra-seasonal oscillation such as BSISO was unclear. Paired anticyclonic circulation anomalies and paired cyclonic circulation anomalies were observed across both hemispheres in the upper and lower troposphere over the Indian Ocean, respectively. In contrast, reversed circulation anomalies were observed from the western to central Pacific. These anomalies indicate that the Walker circulation was stronger than normal in both the Pacific and Indian Oceans.
– In the Northern Hemisphere, wind speeds associated with the subtropical jet were generally stronger than normal. From East Asia to the North Pacific, the core of the subtropical jet shifted northward relative to its climatological position and exhibited a wavy pattern, which contributed to dominant anticyclonic anomalies over East Asia. In the subtropical region over the western North Pacific, pronounced anticyclonic anomalies were also observed.
– In the 500-hPa height fields, the polar vortex was located north of Eastern Siberia and was stronger than normal, while positive height anomalies were observed from Northern Europe to Western Siberia. A prominent wave-like anomaly pattern dominated, resulting in positive anomalies over Northern Canada, the Mediterranean region, East Asia, and south of the Aleutian Islands, and negative anomalies over areas south of Alaska, the eastern United States, west of England, and from the Caspian Sea to Mongolia. Sea level pressure anomalies were broadly negative across Eurasia, with particularly pronounced negative anomalies over Southern Siberia, while significantly positive anomalies were observed south of Japan.
– In September, Japan experienced significantly above-normal temperature throughout the country, with Okinawa/Amami recording the highest temperature anomalies for September since 1946. The monthly anomaly of the average surface temperature over Japan was +2.49°C which was the 3rd warmest for September since 1898. Precipitation was above normal in northern Japan and on the Sea of Japan side of eastern Japan, while below normal on the Pacific side of eastern Japan and in Okinawa/Amami, where it was significantly below normal. Sunshine duration ratios were above normal across most of Japan, except for the Sea of Japan side of western Japan. Significantly above-normal sunshine durations were observed on the Pacific side of northern Japan and in Okinawa/Amami. Monthly Report on Global Extreme Climate Events

Highlight

• Monthly mean temperatures were extremely high from eastern to southern East Asia, in southern Central Siberia, from northern Europe to around the Mediterranean Sea via eastern Europe and from central to western Canada.
• Monthly precipitation amounts were extremely high in and around Central Siberia, in and around Kalimantan, around the Caspian Sea, in central Europe and from Bolivia to northern Argentina.
• Monthly precipitation amounts were extremely low in and around Western Russia and in and around southern Canada.

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	Type	Area	Remarks
1	Warm	From eastern to southern East Asia, southern Central Siberia	The monthly mean temperature in Japan was the second highest on record for September since 1898 (Japan Meteorological Agency).The monthly mean temperature in the Republic of Korea was the second highest on record for September since 1973 (Korea Meteorological Administration).The monthly mean temperature in Hong Kong was the third highest on record for September since 1884 (Hong Kong Observatory).
2	Wet	In and around Central Siberia
3	Heavy Rain	From Nepal to Pakistan	It was reported that heavy rains caused a total of at least 200 fatalities in Nepal, India and Pakistan (European Commission, EM-DAT, the government of Nepal and the government of Pakistan).
4	Typhoon	From southeastern China to Indochina Peninsula, the Philippines	It was reported that Typhoon “RAGASA” caused a total of at least 50 fatalities in southeastern China, the Philippines and Thailand (EM-DAT).It was reported that Typhoon “BUALOI” caused a total of at least 120 fatalities in Viet Num, the Philippines, Thailand and Laos (EM-DAT).
5	Wet	In and around Kalimantan
6	Wet	Around the Caspian Sea
7	Dry	In and around Western Russia
8	Warm	From northern Europe to around the Mediterranean Sea via eastern Europe	The monthly mean temperature in Finland was the third highest on record for September since 1961 (Finnish Meteorological Institute).The monthly mean temperature in Norway was the third highest on record for September since 1901 (Norwegian Meteorological Institute).The monthly mean temperature in Lithuania was the third highest on record for September since 1961 (Lithuanian Hydrometeorological Service).The monthly mean temperature in Belarus was the third highest on record for September since 1945 (Belhydromet).
9	Wet	Central Europe
10	Warm	From central to western Canada
11	Dry	In and around southern Canada
12	Wet	From Bolivia to northern Argentina

The figure is based on CLIMAT report. Areas with missing or unreceived data are not shown.