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Monthly Highlights on the Climate System

'Monthly Highlights on the Climate System' has been issued since March 2007 as a monthly bulletin focusing on the monthly highlights of the monitoring results.

Notice: Products have been upgraded from PDF to HTML format starting from the issue of May 2025 for improved accessibility.

Highlights in August 2025

- In the tropical Pacific, negative SST anomalies were observed over the central part, and remarkably positive SST anomalies were observed in the western part. NINO.3 was –0.3°C. In the Indian Ocean, remarkably positive SST anomalies were observed in the tropical eastern part, and remarkably negative SST anomalies were observed in the Arabian Sea. In the North Pacific, remarkably positive SST anomalies were observed in the mid-latitudes.
- In the tropics, convective activity was enhanced around the Maritime Continent and along 10°S over the Indian Ocean, while suppressed over the equatorial Indian Ocean and in the tropical Pacific between 150°E and 150°W. In the subtropics, convective activity was enhanced from Northeast Africa to India while suppressed over the western part of the North Pacific. The overall activity over the Asian monsoon convection was weaker than normal in early August, and stronger than normal in middle to late of the month.
- In the lower troposphere, remarkably cyclonic circulation anomalies straddling the equator were seen from the Indian Ocean to New Guinea, while anti-cyclonic circulation anomalies were seen over the western part of the tropical Pacific. Remarkably anti-cyclonic circulation anomalies were seen over the subtropical western North Pacific, corresponding to the intensified southwestward extension of the North Pacific subtropical high. In the upper troposphere, zonally elongated anti-cyclonic circulation anomalies were seen over mid-latitude Eurasia, corresponding to the intensified northward extension of the Tibetan high.
- In the 500-hPa height field, the polar vortex was stronger than normal to the north of Eastern Siberia, while the significant positive anomalies were seen around the Kara Sea where the blocking high persisted. Over Eurasia, negative and significantly positive anomalies were seen in the latitudinal bands of 45-60°N and south of 45°N, respectively, which was corresponded to that the subtropical jet stream over Eurasia shifted slightly northward and was stronger than normal. At 850-hPa, above-normal temperatures were dominant in the mid-latitudes except the latitudinal bands of 50°N over Eurasia and the latitudinal bands of 40°N from North America to the Atlantic Ocean.
- In the sea level pressure field, significant negative anomalies were seen around the Sea of Okhotsk, while positive anomalies spread the south of Japan. The extension of the North Pacific subtropical high toward south of Japan was stronger than normal, covering eastern and western Japan.
- The characteristics of climate in Japan in this month were above normal temperature, above normal sunshine durations, above normal precipitation in the Sea of Japan side of Japan, and below normal precipitation in the Pacific side of Japan. The monthly anomaly of the average surface temperature over Japan was +1.84°C which was the 2nd warmest for August since 1898. The statistics of regional average in Japan showed significantly above normal temperatures in northern/eastern/western Japan, significantly above normal precipitations in the Sea of Japan side of eastern Japan, and significantly below normal precipitations and significantly above normal sunshine durations in Okinawa/Amami.

Climate in Japan (Fig. 1):

- Monthly mean temperatures were significantly above normal in northern/ eastern/western Japan, and above normal in Okinawa/Amami. This was due to the regions being covered by warm air associated with the subtropical jet stream, which was located frequently further north than usual over eastern Asia, and also due to warm-air inflow.
- The monthly anomaly of the average surface temperature over Japan was +1.84°C (2nd warmest for August since 1898) . On a longer time scale, the average surface temperatures have risen at a rate of about 1.33°C per century in August.
- Monthly precipitation amounts were significantly above normal on the Sea of Japan side of eastern Japan and above normal on the Sea of Japan side of northern/western Japan, because the regions were affected by fronts in the first half of August.
- Monthly sunshine durations were above normal on the Pacific side of northern/eastern/western Japan and on the Sea of Japan side of eastern/western Japan, and monthly precipitation amounts were below normal on the Pacific side of eastern Japan, because the regions were frequently covered by the North Pacific Subtropical High (NPSH).
- In Okinawa/Amami, monthly precipitation amounts were significantly below normal and monthly sunshine durations were significantly above normal, because the region was particularly covered by the NPSH.

World Climate:

- The monthly anomaly of the global average surface temperature (i.e., the combined average of the near-surface air temperature over land and the SST) was +0.43°C (3rd warmest for August since 1891) (preliminary value) (Fig. 2). On a longer time scale, global average surface temperatures have risen at a rate of about 0.76°C per century in August (preliminary value).
- Extreme climate events were as follows (Fig. 3).
  - Monthly mean temperatures were extremely high from Japan to central China, around the Barents-Kara seas and western Europe.
  - Monthly precipitation amounts were extremely high in northern Argentina and in and around southeastern Australia.
  - Monthly precipitation amounts were extremely low from central to western Europe.

Oceanographic Conditions:

- In the equatorial Pacific, negative SST anomalies were observed over the central part, and remarkably positive SST anomalies were observed in the western part (Fig. 4). The monthly mean SST anomaly averaged over the NINO.3 region was –0.2°C and the SST deviation from the latest sliding 30-year mean over the region was –0.3°C (Fig. 5).
- In the North Pacific, remarkably positive SST anomalies were observed in the western part of the tropics and in the mid-latitudes.
- In the South Pacific, remarkably positive SST anomalies were observed from the western part to the eastern part of the mid-latitudes.
- In the Indian Ocean, remarkably positive SST anomalies were observed in the eastern part, and remarkably negative SST anomalies were observed in the northwestern part.
- In the North Atlantic, remarkably positive SST anomalies were observed to the southeast of North America and to the west of Europe, and remarkably negative SST anomalies were observed to the south of Greenland.
- In the subtropical South Atlantic, remarkably positive SST anomalies were seen in the western part, and negative SST anomalies were seen in the eastern part.

Tropics:

- Convective activity was enhanced from eastern Africa to India and around the Maritime Continent, and suppressed over the equatorial Indian Ocean, the Indochina Peninsula, the subtropical western North Pacific, and the equatorial western Pacific (Fig. 6). The overall activity over the Asian monsoon convection was weaker than normal in early August, and stronger than normal in middle to late of the month.
- The active phase of equatorial intraseasonal oscillation propagated eastward from South America to the western Pacific through the Indian Ocean (Fig. 7).
- In the upper troposphere, cyclonic circulation anomalies straddling the equator were seen over the western Pacific. Zonally elongated anti-cyclonic circulation anomalies were seen over mid-latitude Eurasia, corresponding to the intensified northward extension of the Tibetan high (Fig. 8).
- In the lower troposphere, cyclonic circulation anomalies straddling the equator were seen over the eastern Indian Ocean. Anti-cyclonic circulation anomalies were seen over the subtropical western North Pacific, corresponding to the intensified southwestward extension of the North Pacific subtropical high (Fig. 9).
- In the sea level pressure field, positive anomalies were seen over the tropical Pacific. Positive anomalies were clearly seen to the south of Japan. The Southern Oscillation Index value was +0.6 (Fig. 5).

Extratropics:

- In the 500-hPa height field (Fig. 10), the polar vortex was stronger than normal to the north of Eastern Siberia, the significant positive anomalies were seen around the Kara Sea. Over Eurasia, negative anomalies were seen in the latitudinal bands of 45-60°N, and significant positive anomalies were seen south of 45°N.
- The subtropical jet stream over Eurasia shifted slightly northward and was stronger than normal (Fig. 11). The polar-front jet stream flowed along the Arctic Ocean coast, with no southward meandering in the Far East.
- In the sea level pressure field (Fig. 12), significant positive anomalies were seen over northern Siberia and northeastern North America, and negative anomalies were seen in the latitudinal bands of 45-60°N over Eurasia and around the Sea of Okhotsk. The extension of the North Pacific subtropical high toward south of Japan was stronger than normal, covering eastern and western Japan.
- Temperatures at 850-hPa were below normal from north of Eastern Siberia to the Baffin Bay, and above normal around the Kara Sea in the high-latitudes. Above-normal temperatures were seen in the mid-latitudes except the latitudinal bands of 50°N over Eurasia and the latitudinal bands of 40°N from North America to the Atlantic Ocean (Fig. 13).

Zonal mean:

- In the zonal mean zonal wind in the troposphere, the subtropical jet stream shifted poleward and equatorward in the Northern and Southern Hemisphere, respectively.
- The zonal mean temperatures in the troposphere were mostly above normal, particularly in the mid-latitudes of both hemispheres and in the polar regions of the Southern Hemisphere.

Supplemental information

- Climate Anomaly Table over Japan
- Extratropics in the Southern Hemisphere
- Snow in the Northern Hemisphere
- Arctic sea ice (link to the National Snow and Ice Data Center)

Fig.1 Monthly climate anomaly/ratio over Japan (August 2025) Top: temperature anomalies (degree C) Middle: precipitation ratio (%) Bottom: sunshine duration ratio (%) The base period for the normal is 1991-2020.

Fig.10 Monthly mean 500-hPa height and anomaly in the Northern Hemisphere (August 2025) The contours show 500-hPa height at intervals of 60 m. The shading indicates its anomalies. The base period for the normal is 1991-2020.	Fig.11 Monthly mean 200-hPa wind speed and vectors in the Northern Hemisphere (August 2025) The black lines show wind speed at intervals of 10 m/s. The brown lines show its normal at intervals of 20 m/s. The base period for the normal is 1991-2020.
Fig.12 Monthly mean sea level pressure and anomaly in the Northern Hemisphere (August 2025) The contours show sea level pressure at intervals of 4 hPa. The shading indicates its anomalies. The base period for the normal is 1991-2020.	Fig.13 Monthly mean 850-hPa temperature and anomaly in the Northern Hemisphere (August 2025) The contours show 850-hPa temperature at intervals of 3 degree C. The shading indicates its anomalies. The base period for the normal is 1991-2020.

Back Number

The descriptions from May-2011 to April-2021 issue are based on the former climatological normal (1981-2010 average).
In the descriptions until April-2011 issue, 1979-2004 average is used as climatological normal unless otherwise stated.
The descriptions until January-2014 issue are based on the JRA-25/JCDAS datasets.
The descriptions from February-2014 to April-2023 issue are based on the JRA-55 reanalysis.

Figures and Tables

Notice: Products based on JRA-3Q were updated to those with improved quality in terms of tropical cyclone analysis. OLR-related products from January 1991 are based on NOAA CPC Blended OLR (CBO).

Notice: Figures of 'Atmospheric Circulation', 'Time Cross Section', and 'Indices' have been revised with improved quality data regarding tropical cyclone analysis. (18 June 2024)

Notice: Depending on the availability of NOAA CPC Blended OLR (CBO) data, updates may be delayed or figures may be filled with gray indicating data missing.

• Explanation on the Products for Climate System Monitoring
• 5-day Mean Figures
• 10-day Mean Figures
• Monthly Mean Figures
• 3-month Mean Figures
• Time Cross Sections
• Indices
• Oceanic Figures and Tables