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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 April 2026

- In the tropical Pacific, positive sea surface temperature (SST) anomalies were observed on a global scale, and remarkably positive SST anomalies were observed from 150°E to the dateline (Fig. 4). The NINO.3 index was +0.7℃ (Fig. 5). In the tropical Indian Ocean and the mid-latitude Pacific, positive SST anomalies were widely observed with many areas displaying remarkably positive anomalies.
- In the tropics, convective activity was strongly enhanced over the central Pacific and from South America to western Africa. Meanwhile, it was suppressed from the eastern Indian Ocean through Indonesia to the western Pacific, with particularly strong suppression around the Maritime Continent (Fig. 6).
- In the tropical upper troposphere, a pair of cyclonic anomalies straddling the equator was observed over the Maritime Continent (Fig. 8). In the tropical lower troposphere, a pair of cyclonic anomalies straddling the equator was observed from the western to the central Pacific, accompanied by dominant westerly anomalies along the equator (Fig. 9).　Sea level pressure anomalies were negative over the tropical Pacific while positive around the Maritime Continent. The Southern Oscillation Index was −0.6 (Fig. 5).
- In the Northern Hemisphere, the westerlies were split from the Western Hemisphere to Europe and were significantly weaker than normal over southern Europe. Over the Far East, the polar front jet stream was stronger than normal between 45°N and 50°N. It merged with the subtropical jet east of Japan. As a result, the westerlies were stronger than normal (Fig. 11).
- In the 500 hPa height field, the polar vortex split, with its centers located over northeastern Canada and central Siberia where negative anomalies were observed, while positive anomalies were observed over northern Europe and eastern Siberia. In the mid-latitudes, positive anomalies were generally observed, except over western Russia and West Asia. Significant anomalies were found over the North Pacific, from North America to the western North Atlantic, and over western Europe (Fig. 10).
- In the sea level pressure (SLP) field, over Eurasia, negative anomalies were dominant except over Europe, where positive anomalies were observed (Fig. 12). Significantly negative anomalies were observed over central China.
- Significantly above-normal temperatures were observed throughout Japan (Fig. 1). The monthly anomaly of the average surface temperature over Japan was +1.89°C, ranking as the third highest for April since 1898. Precipitation amounts were significantly above normal on the Pacific side of western Japan and sunshine durations were significantly below normal over western Japan.

Climate in Japan (Fig. 1):

- Monthly mean temperatures were significantly above normal nationwide, due to warm-air coverage and warm-air inflow.
- The monthly anomaly of the average surface temperature over Japan was +1.89°C (3rd warmest for April since 1898). On a longer time scale, the average surface temperatures have risen at a rate of about 1.46°C per century in April.
- Monthly precipitation amounts were significantly above normal on the Pacific side of western Japan, and above normal on the Pacific side of eastern Japan and on the Sea of Japan side of western Japan. Monthly sunshine durations were significantly below normal in western Japan, and below normal on the Pacific side of eastern Japan, due to the frequent influence of low-pressure systems and fronts.
- Monthly precipitation amounts were below normal on the Pacific side of northern Japan, and monthly sunshine durations were above normal in northern Japan and Okinawa/Amami, as the regions were frequently covered by high-pressure systems mainly in the second half 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 +0.45°C (4th warmest for April since 1891) (preliminary value) (Fig. 2). On a longer time scale, global average surface temperatures have risen at a rate of about 0.82°C per century in April (preliminary value).
- Extreme climate events were as follows (Fig. 3).
  - Monthly mean temperatures were extremely high from Japan to North China, from South China to Sumatra Island, in southern Europe and from Mexico to Colombia.
  - Monthly precipitation amounts were extremely high in and around Pakistan, in Western Russia, in and around Turkey and in southeastern Canada.
  - Monthly precipitation amounts were extremely low in central Indonesia and from central to southern Europe.

Oceanographic Conditions:

- In the equatorial Pacific, positive SST anomalies were observed on a global scale, and remarkably positive SST anomalies were observed from 150°E to the dateline (Fig. 4). 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 +0.7℃ (Fig. 5).
- In the North Pacific, positive SST anomalies were widely observed except for negative anomalies between 15°N and 30°N with the western to central parts of the mid-latitude and the central to eastern parts of the subtropics displaying remarkably positive anomalies.
- In the South Pacific, positive SST anomalies were widely observed except for negative anomalies between 10°S and 20°S, around 40°S, with many areas displaying remarkably positive anomalies.
- In the Indian Ocean, positive SST anomalies were widely observed except for negative anomalies from the central to the western parts along 20°S, with many areas displaying remarkably positive anomalies.
- In the North Atlantic, around 30°N, remarkably positive SST anomalies were observed in the western part, while remarkably negative anomalies were observed in the eastern part. Between 40° and 60°N, remarkably negative SST anomalies were observed in the western part, while positive anomalies were observed in the eastern part.
- In the South Atlantic, remarkably positive SST anomalies were seen in a wide area.

Tropics:

- Convective activity was extremely enhanced over the central Pacific and from South America to thewestern Africa. It was suppressed from the eastern Indian Ocean to the western Pacific, with strong suppression around the Maritime Continent (Fig. 6).
- The global distribution of convective activity anomalies exhibited a wave-number-2 pattern. One active phase of the equatorial intraseasonal oscillation migrated from the Indian Ocean at the beginning of the month to the Indian Ocean at the end of the month (Fig. 7).
- In the upper troposphere, paired cyclonic circulation anomalies prevailed around the Maritime Continent. In the Northern Hemisphere, a wavy anomaly pattern was seen along the subtropical jet, with anticyclonic circulation anomalies over the eastern Africa and the Bay of Bengal (Fig. 8).
- In the lower troposphere, paired cyclonic circulation anomalies were seen from the western to the central Pacific, accompanied by dominant westerly wind anomalies. Meanwhile, paired anticyclonic circulation anomalies were seen from the eastern Indian Ocean to the Maritime Continent. In particular, pronounced cyclonic circulation anomalies were seen over the tropical central North Pacific. (Fig. 9).
- In the sea level pressure field, positive anomalies were seen from the eastern Maritime Continent to the northern coast of Australia, and negative anomalies were seen over the tropical Pacific. The Southern Oscillation Index value was −0.6 (Fig. 5).

Extratropics:

- In the 500‑hPa height field (Fig. 10), the polar vortex was split into two centers, located over northeastern Canada and central Siberia, both associated with negative anomalies. Positive anomalies were observed over northern Europe and eastern Siberia. In the mid-latitudes, positive anomalies generally prevailed, except over western Russia and West Asia, where negative anomalies were present. Remarkably positive anomalies were observed over the Pacific, from North America to the western Atlantic and western Europe.
- Westerlies were split from the Western Hemisphere to Europe and were significantly weaker than normal over southern Europe. In the Far East, the polar front jet stream was stronger than normal and merged with the subtropical jet stream east of Japan, where the westerlies were stronger than normal (Fig. 11).
- In the sea level pressure field (Fig. 12), negative anomalies were dominant over Eurasia except for Europe. Over the North Pacific, negative anomalies were observed in the Aleutians, while positive anomalies extended from the mid‑latitudes over the western North Pacific to the Gulf of Alaska. As a result, the storm track was shifted northward relative to its normal position. Positive anomalies were also observed from eastern North America to the western North Atlantic.
- In the 850‑hPa temperature field (Fig. 13), the anomaly pattern closely resembled that of the 500‑hPa height field. Remarkably positive anomalies were observed in the mid‑latitudes, except over western Russia and the western North Atlantic.

Zonal mean:

- In the zonal‑mean zonal wind in the troposphere, the subtropical jet stream in the Northern Hemisphere was displaced southward compared to the climatological normal. The polar-front jet stream was significantly stronger than normal between 50°N and 60°N, while the westerlies were significantly weaker than normal between 30°N and 40°N.
- Zonal‑mean temperatures in the troposphere were significantly above normal in the mid‑latitudes, while negative temperature anomalies were observed between 50°N and 65°N. In the tropics, temperature anomalies have been above normal for two consecutive months, with increasing magnitude.

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 (April 2026) 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 (April 2026) 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 (April 2026) 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 (April 2026) 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 (April 2026) 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