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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 October 2025

- 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, pronounced positive and negative SST anomalies were observed in the eastern and western regions, respectively, meeting the criteria for the development of a negative Indian Ocean Dipole. Positive SST anomalies were observed in the subtropical to midlatitude regions of the western North Pacific. In the East China Sea and around western Japan, these anomalies exceeded three times the climatological standard deviation.
- Enhanced convective activity was observed from the eastern Indian Ocean to the Maritime Continent, whereas suppressed convection extended from the central Pacific to Africa, passing through South America and the Atlantic. Active convection was also observed in the vicinity of the Caribbean Sea. The amplitude of the Madden-Julian Oscillation (MJO) increased significantly over the Indian Ocean in mid-October, and its phase propagated eastward to the Maritime Continent in late October.
- In the upper troposphere, paired anticyclonic circulation anomalies prevailed in both the tropical Northern and Southern Hemispheres, with the exception of the western Pacific. In particular, pronounced anticyclonic circulation anomalies dominated the western Indian Ocean.
- In the lower troposphere, paired cyclonic circulation anomalies were prominently observed over the Indian Ocean, accompanied by dominant westerly wind anomalies. Meanwhile, paired anticyclonic circulation anomalies were prominently observed from the western to the central Pacific, accompanied by easterly wind anomalies. Sea level pressure anomalies were negative over the Indian Ocean and positive over the Pacific, respectively, with the Southern Oscillation Index (SOI) indicating a value of +1.2.
- In the Northern Hemisphere, the subtropical jet was stronger and shifted northward relative to its climatological position over East Asia, whereas it was weaker than normal and displaced southward over the Atlantic. The polar-front jet was stronger than normal around 70°N.
- In the 500-hPa height field, the polar vortex was centered in the eastern part of Eastern Siberia, accompanied by significantly negative anomalies around the Kamchatka Peninsula and significantly positive anomalies in northeastern Canada and Central Siberia, where a ridge was dominant. In the mid-latitude Northern Hemisphere, positive anomalies were seen over a wide area, with the significant anomalies over southern East Asia.
- In the sea level pressure field, significantly positive anomalies were observed from Western Siberia to northern Japan, associated with a markedly developing Siberian High that brought significantly negative anomalies in the 850 hPa temperature from Mongolia to the vicinity of the Kamchatka Peninsula.
- Western Japan and Okinawa/Amami experienced significantly above-normal temperature, with Okinawa/Amami recording the highest temperature anomalies for October since 1946. The monthly anomaly of the average surface temperature over Japan was +1.35°C which was the 2nd warmest for October since 1898. Precipitation was above normal on the Sea of Japan side of eastern Japan and in Okinawa/Amami. Sunshine duration ratios were significantly below normal on the Pacific side of eastern Japan, while they were significantly above normal in Okinawa/Amami.

Climate in Japan (Fig. 1):

- Monthly mean temperatures were significantly above normal in western Japan and Okinawa/Amami, with Okinawa/Amami recording the highest temperature anomalies for October since 1946. 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 from North China to the Sea of Japan.
- The monthly anomaly of the average surface temperature over Japan was +1.35°C (2nd warmest for October since 1898) . On a longer time scale, the average surface temperatures have risen at a rate of about 1.53°C per century in October.
- Monthly sunshine durations were significantly below normal on the Pacific side of eastern Japan, and below normal on the Sea of Japan side of northern/eastern/western Japan and on the Pacific side of northern Japan. Monthly precipitation amounts were above normal on the Sea of Japan side of eastern Japan. These were due to northern/eastern/western Japan being frequently influenced by low-pressure systems and stationary fronts in the second and third ten days of the month.
- In Okinawa/Amami, monthly sunshine durations were significantly above normal, because the region was well covered by high-pressure systems in the first and second ten days of the month, while monthly precipitation amounts were above normal due to a tropical cyclone, wet air inflow, and a stationary front in the last ten days of the month.

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 +0.40°C (3rd warmest for October since 1891) (preliminary value) (Fig. 2). On a longer time scale, global average surface temperatures have risen at a rate of about 0.73°C per century in October (preliminary value).
- Extreme climate events were as follows (Fig. 3).
  - Monthly mean temperatures were extremely high from western Japan to the Philippines via South China, in central Southeast Asia, in eastern Canada and in and around northeastern Australia.
  - Monthly mean temperatures were extremely low from Mongolia to eastern Kazakhstan.
  - Monthly precipitation amounts were extremely high from Tsushima Island, Japan to North China, in and around western India, in and around southeastern Europe and in and around northern Germany.
  - Monthly precipitation amounts were extremely low in Eastern Siberia and around the western Mediterranean Sea.

Oceanographic Conditions:

- In the equatorial Pacific, negative SST anomalies were observed from the central to eastern parts, and remarkably positive SST anomalies were observed in the western part (Fig. 4). Both the monthly mean SST anomaly averaged over the NINO.3 region and the SST deviation from the latest sliding 30-year mean over the region were -0.5°C (Fig. 5).
- In the North Pacific, remarkably positive SST anomalies were observed in the western part of the tropics and in a wide area of the mid-latitudes, while remarkably negative SST anomalies were observed in the northern and central parts of the Sea of Okhotsk, to the southeast of Japan and to the southwest of Mexico.
- In the South Pacific, remarkably positive SST anomalies were observed in the western part and in the eastern part of the mid-latitudes.
- In the Indian Ocean, remarkably positive SST anomalies were observed in the southeastern part of the tropics, and remarkably negative SST anomalies were observed in the western part of the tropics.
- In the North Atlantic, remarkably positive SST anomalies were observed in the western part of the tropics and in the eastern part of the mid-latitudes.
- In the South Atlantic, remarkably positive SST anomalies were seen in the mid-latitudes, and remarkably negative SST anomalies were seen in the western part of the subtropics.

Tropics:

- Convective activity was significantly enhanced in 10°S-20°S and 10°N-20°N, extending from the eastern Indian Ocean to the Maritime Continent, where wave number 1 of upper-level large-scale divergence prevailed (Fig. 6). Active convection was also observed in the vicinity of the Caribbean Sea. The amplitude of the Madden-Julian Oscillation (MJO) increased significantly over the Indian Ocean in mid-October, and its phase propagated eastward to the Maritime Continent in late October (Fig. 7).
- In the upper troposphere, paired anticyclonic circulation anomalies prevailed in both the Northern and Southern Hemispheres, except in the region from the eastern Maritime Continent to the central Pacific, where paired cyclonic circulation anomalies were prominently observed. In particular, pronounced anticyclonic circulation anomalies dominated the western Indian Ocean (Fig. 8).
- In the lower troposphere, paired cyclonic circulation anomalies were prominently observed over the Indian Ocean, accompanied by dominant westerly wind anomalies. Additionally, cyclonic circulation anomalies also prevailed around the South China Sea. Meanwhile, paired anticyclonic circulation anomalies were prominently observed from the western to the central Pacific, accompanied by easterly wind anomalies (Fig. 9).
- Sea level pressure anomalies were negative over the Indian Ocean and positive over the Pacific, respectively, with the Southern Oscillation Index (SOI) indicating a value of +1.2 (Fig. 5).

Extratropics:

- In the 500-hPa height field (Fig. 10), the polar vortex was centered in the eastern part of Eastern Siberia, accompanied by significantly negative anomalies around the Kamchatka Peninsula and significantly positive anomalies in northeastern Canada and Central Siberia, where a ridge was dominant. In the mid-latitude Northern Hemisphere, negative anomalies were seen in the western Atlantic, eastern Europe, and Mongolia. In contrast, positive anomalies were seen over a wide area, with significant anomalies over southern East Asia.
- The subtropical jet stream was stronger and shifted northward compared to the climatological normal in the Eastern Hemisphere, with this feature being particularly pronounced over East Asia. In contrast, in the Western Hemisphere, the jet stream was weaker than normal and displaced southward (Fig. 11).
- In the sea level pressure field (Fig. 12), significantly positive anomalies were seen from Western Siberia to northern Japan, and record-breaking positive anomalies exceeding three times the standard deviation of the interannual variability were seen from Central Siberia to northeastern China. In contrast, negative anomalies were observed from the Bering Sea to the Norwegian Sea, with the significant anomalies over northern Canada.
- In the 850-hPa temperature field (Fig. 13), significantly positive anomalies were seen in the Arctic Ocean and eastern Canada, while widespread negative anomalies were seen from Mongolia to the vicinity of the Kamchatka Peninsula. In addition, Central Siberia and the area near the Sea of Okhotsk exhibited particularly pronounced negative anomalies.

Zonal mean:

- In the zonal mean zonal wind in the troposphere, the jet stream in both hemispheres was stronger than normal around its normal position. The polar-front jet stream in the Northern Hemisphere was stronger than normal around 70°N.
- The zonal mean temperatures in the troposphere were above normal except around 50° latitudes in both hemispheres.
- The zonal mean temperatures in the stratosphere were below normal, except north of 70°N in the Northern 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 (October 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 (October 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 (October 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 (October 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 (October 2025) The contours show 850-hPa temperature at intervals of 4 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