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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 December 2025

- In the tropical Pacific, remarkably positive sea surface temperature (SST) anomalies were observed in the western region, while negative anomalies were observed along the equator from the central to eastern regions, indicating a La Niña-like pattern. The NINO.3 index was -0.6°C. Significantly positive SST anomalies were also observed in the eastern tropical Indian Ocean, the mid-latitude North Pacific and the subtropical North Atlantic.
- In the tropics, extremely enhanced convective activity was observed over Africa and the western Pacific, whereas suppressed convective activity was seen over the Indian Ocean and from the central to the eastern Pacific.
- In the tropical upper troposphere, cyclonic circulation anomalies were dominant globally except over western Africa and the western Pacific. Cyclonic circulation anomalies were significant in the subtropical central North Pacific.
- In the tropical lower troposphere, circulation anomalies were generally small, except over the central Pacific, where paired anticyclonic circulation anomalies were observed. In contrast, significant cyclonic circulation anomalies were present over the subtropical central North Pacific. Sea level pressure anomalies were negative from the Indian Ocean to the western Pacific, and positive from the central to eastern equatorial Pacific. Although this anomaly pattern was present, the Southern Oscillation Index (SOI) was -0.2 because the low-pressure anomaly at Tahiti was influenced by mid-latitude disturbances.
- In the Northern Hemisphere, the zonal-mean subtropical jet shifted southward relative to its climatological position and the polar-front jet was stronger than normal between 60°N and 70°N. The core of the subtropical jet was remarkably weaker than normal due to the significant blocking event over the North Pacific. Over East Asia, the confluence between the subtropical jet and the polar-front jet was weak.
- In the 500-hPa height field, negative anomalies were observed over central Siberia and northern Canada, where the polar vortex had split. In addition, significantly positive anomalies were seen over northern Europe and around the Bering Sea, where a blocking high developed markedly. The negative Eurasia (EU) pattern prevailed over Asia, and as a result, the southward displacement of the polar-front jet remained between 40°N and 45°N over the Far East.
- In the sea level pressure field, accompanied by the blocking event, a high pressure system was observed around the Aleutian Sea where the climatological Aleutian Low is normally located. As a result, the Aleutian Low split into two small low-pressure systems over the Sea of Okhotsk and the Gulf of Alaska. In addition, significantly negative anomalies were dominant over mid-latitude Eurasia. These features indicated a weaker-than-normal winter monsoon over East Asia.
- Above-normal temperatures were observed nationwide in Japan. The monthly anomaly of the average surface temperature over Japan was +0.99°C. Precipitation amounts were above normal in northern Japan, while below normal in western Japan. Sunshine duration were below normal in northern Japan, while above normal in eastern and western Japan, and were significantly above normal on the Pacific side of western Japan.

Climate in Japan (Fig. 1):

- Monthly mean temperatures were above normal nationwide due to being less affected by cold air.
- The monthly anomaly of the average surface temperature over Japan was +0.99°C (8th warmest for December since 1898). On a longer time scale, the average surface temperatures have risen at a rate of about 0.99°C per century in December.
- As northern Japan was frequently affected by low-pressure systems, the region experienced above normal monthly precipitation amounts and below normal sunshine durations.
- As eastern and western Japan were frequently covered by high-pressure systems, monthly precipitation amounts were below normal in western Japan, while monthly sunshine durations were above normal in eastern and western Japan, especially on the Pacific side of western Japan where they were extremely above normal.

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 +0.41°C (5th warmest for December since 1891) (preliminary value) (Fig. 2). On a longer time scale, global average surface temperatures have risen at a rate of about 0.81°C per century in December (preliminary value).
- Extreme climate events were as follows (Fig. 3).
  - Monthly mean temperatures were extremely high in Eastern Siberia, from Central China to South China, in the eastern part of Western Africa, in Mexico, from Colombia to Peru and from Uruguay to central Chile.
  - Monthly precipitation amounts were extremely high from Uzbekistan to Turkmenistan, in southwestern Canada and from Paraguay to northern Argentina.
  - Monthly precipitation amounts were extremely low in central Europe.

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). The monthly mean SST anomaly averaged over the NINO.3 region was -0.7°C and the SST deviation from the latest sliding 30-year mean over the region was -0.6°C (Fig. 5).
- In the North Pacific, remarkably positive SST anomalies were observed in a wide area of the tropics to mid-latitudes, while negative SST anomalies were observed around the Sea of Okhotsk and the east-southeast of Japan.
- In the South Pacific, remarkably positive SST anomalies were observed in the western part and in the eastern part of the mid-latitudes, while remarkably negative SST anomalies were observed in the central subtropics.
- 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 northwestern part of the tropics.
- In the North Atlantic, remarkably positive SST anomalies were observed except in the mid-latitudes.
- In the South Atlantic, remarkably positive SST anomalies were seen in the eastern part of the mid-latitudes.

Tropics:

- Convective activity was markedly enhanced over western to central Africa and enhanced over the western Pacific, and suppressed over the Indian Ocean and the central to eastern Pacific (Fig. 6).
- The eastward propagation in the active phase of equatorial intraseasonal oscillation was unclear (Fig. 7).
- In the upper troposphere over the tropics, anticyclonic circulation anomalies were seen over western Africa and the western Pacific in the Northern Hemisphere, while cyclonic circulation anomalies prevailed over almost the entire globe. In particular, pronounced cyclonic circulation anomalies were seen over the subtropical central North Pacific (Fig. 8).
- In the lower troposphere, cyclonic circulation anomalies were seen over the subtropical central North Pacific (Fig. 9).
- In the sea level pressure field, positive anomalies were seen over the tropical eastern North Atlantic and the equatorial central to eastern Pacific, while negative anomalies prevailed over the other wide regions. The Southern Oscillation Index value was -0.2 (Fig. 5).

Extratropics:

- In the 500-hPa height field (Fig. 10), negative anomalies were seen over Central Siberia and northern Canada, where the split polar vortices were located, while significant positive anomalies were seen over around northern Europe and around the Bering Sea, where a blocking high developed. In the Northern Hemisphere mid-latitudes, positive anomalies were seen except over eastern North America and western Europe, with the significant positive anomalies over western North America.
- The subtropical jet over the North Pacific was significantly weak near its axis associated with the blocking high. The confluence with the subtropical jet and the polar-front jet was weak over East Asia (Fig. 11).
- In the sea level pressure field (Fig. 12), significant positive anomalies were seen over around the Bering Sea associated with the blocking high, and the Aleutian Low was split over the Sea of Okhotsk and the Gulf of Alaska. Elsewhere, except around northern Europe where positive anomalies were seen, negative anomalies were widespread, with significant negative anomalies over the midlatitude Eurasia, indicating the weak Siberian High.
- In the 850-hPa temperature field (Fig. 13), significant negative anomalies were seen from Alaska to Canada, as well as negative anomalies were seen from Central Siberia to Primorsky Krai and over the eastern North Atlantic. In the midlatitudes, temperatures were mostly above normal, with significant positive anomalies over eastern Eurasia, north of 40°N over the Pacific, and western North America.

Zonal mean:

- In the zonal mean zonal wind in the troposphere, the subtropical jet in the Northern Hemisphere shifted southward compared to the climatological normal, while the polar-front jet was intensified between 60°N and 70°N.
- The zonal mean temperatures in the troposphere were above normal, except over around 70°N, 20°N, 50°S, and the polar regions of the Southern Hemisphere. The zonal mean temperatures in the Northern Hemisphere stratosphere were below normal south of 60°N and above normal north of 60°N.

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 (December 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 (December 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 (December 2025) The black lines show wind speed at intervals of 15 m/s. The brown lines show its normal at intervals of 30 m/s. The base period for the normal is 1991-2020.
Fig.12 Monthly mean sea level pressure and anomaly in the Northern Hemisphere (December 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 (December 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