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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 May 2026

- In the tropical Pacific, positive sea surface temperature (SST) anomalies were observed on a basin-wide scale, with particularly strong positive anomalies extending from 150°E to the dateline (Fig. 4). The NINO.3 index was +1.2°C (Fig. 5). Positive SST anomalies also prevailed in the South China Sea and in the western North Pacific except for limited areas. In the tropical Indian Ocean, positive SST anomalies were dominant, except in the western part, where negative anomalies were observed.
- In the tropics, convective activity was strongly enhanced along the Intertropical Convergence Zone in the North Pacific and was also enhanced from the central to the eastern Indian Ocean. Meanwhile, it was suppressed around the Caribbean Sea, from the Atlantic to Africa, and around the Philippines (Fig. 6).
- In the tropical upper troposphere, pairs of anticyclonic anomalies straddling the equator were observed from the central to the eastern Pacific and over the western Indian Ocean (Fig. 8).
- In the tropical lower troposphere, pairs of cyclonic anomalies straddling the equator were observed from the western to the central Pacific and over the Indian Ocean (Fig. 9).
- Sea level pressure anomalies were generally negative across the tropics, except over the western Indian Ocean, and were accompanied by significantly negative anomalies over the Pacific. The Southern Oscillation Index was -1.5 (Fig. 5).
- In the Northern Hemisphere, the westerlies were located near their normal position and were stronger than normal over North America and from Europe to West Asia. From East Asia to the North Pacific, the subtropical jet stream was weaker than normal at its climatological position, with particularly notable weakening from eastern China to eastern Japan. In contrast, the polar front jet stream was significantly stronger than normal along around 50°N.
- In the sea-level pressure (SLP) field, negative anomalies were widely observed north of 50°N, except over the North Atlantic and around Baffin Island, while positive anomalies prevailed in the mid-latitudes. This pattern indicates a negative phase of the summer Arctic Oscillation mode.
- Significantly above-normal temperatures were observed across Japan, except in Okinawa/Amami (Fig. 1). The monthly anomaly of the average surface temperature over Japan was +1.37°C, ranking as the second highest for April since 1898. Precipitation amounts were above normal on the Sea of Japan sides of northern and eastern Japan and in Okinawa/Amami, while they were below normal in western Japan and on the Pacific side of eastern Japan. Sunshine durations were above normal except in Okinawa/Amami, with significantly above-normal values on the Sea of Japan sides of eastern and western Japan.

Climate in Japan (Fig. 1):

- Monthly mean temperatures were significantly above normal in northern/eastern/western Japan, due to warm-air coverage and warm-air inflow.
- The monthly anomaly of the average surface temperature over Japan was +1.37°C (2nd warmest for May since 1898). On a longer time scale, the average surface temperatures have risen at a rate of about 1.77°C per century in May.
- Monthly precipitation amounts were below normal in western Japan and on the Pacific side of eastern Japan. Monthly sunshine durations were significantly above normal on the Sea of Japan side of eastern/western Japan, and above normal in northern Japan and on the Pacific side of eastern/western Japan. The regions were frequently covered by high-pressure systems mainly in the second ten days of the month.
- Monthly precipitation amounts were above normal on the Sea of Japan side of northern/eastern Japan due to the influence of low-pressure systems and fronts mainly in the first and third ten days of the month.
- Monthly precipitation amounts were above normal in Okinawa/Amami due to the influence of stationary fronts and moist air inflow.

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 +0.46°C (2nd warmest for May since 1891) (preliminary value) (Fig. 2). On a longer time scale, global average surface temperatures have risen at a rate of about 0.78°C per century in May (preliminary value).
- Extreme climate events were as follows (Fig. 3).
  - Monthly mean temperatures were extremely high from Japan to the Korean Peninsula, in Mexico, in and around Colombia and in and around southeastern Australia.
  - Monthly mean temperatures were extremely low in and around Turkey.
  - Monthly precipitation amounts were extremely high from eastern Mongolia to North China, in and around Turkey, in southeastern Canada and in and around southeastern Australia.
  - Monthly precipitation amounts were extremely low in eastern Europe and in and around Colombia.

Oceanographic Conditions:

- In the equatorial Pacific, positive SST anomalies were observed on a global scale, and remarkably positive SST anomalies were observed between 90°W and 120°W (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 +1.2℃ (Fig. 5).
- In the North Pacific, positive SST anomalies were widely observed except for negative anomalies from the western parts between 15°N and 30°N with the western to central parts of the mid-latitude and the eastern parts of the subtropics displaying remarkably positive anomalies.
- In the South Pacific, positive SST anomalies were widely observed except for negative anomalies from the eastern parts between 10°S and 20°S with many areas displaying remarkably positive anomalies.
- In the Indian Ocean, positive SST anomalies were widely observed except for negative anomalies from the eastern parts along 20°S and the western parts with many areas displaying remarkably positive anomalies.
- In the North Atlantic, around 30°N, remarkably positive SST anomalies were observed in the western part. Between 40°N and 60°N, remarkably negative SST anomalies were observed in the western to central parts, while positive anomalies were observed in the eastern part.
- In the South Atlantic, positive SST anomalies were widely observed except for remarkably negative anomalies from the western parts between 40°S and 60°S and the eastern parts of the subtropics with the western parts of the subtropics displaying remarkably positive anomalies.

Tropics:

- Convective activity was suppressed from the Atlantic to Africa and around the Philippines. It was enhanced from the central to the eastern Indian Ocean, and extremely enhanced from the western to the eastern part of the tropical North Pacific (Fig. 6).
- The global distribution of convective activity anomalies exhibited a wave-number-1 pattern. The active phase of equatorial intraseasonal oscillation stagnated over the Indian Ocean in the first half of the month, then it migrated eastward toward the eastern Pacific in the latter half of the month (Fig. 7).
- In the upper troposphere, paired anticyclonic circulation anomalies prevailed from the western to the central Indian Ocean and from the central to the eastern Pacific. In the Northern Hemisphere, cyclonic circulation anomalies were seen from the eastern Indian Ocean to the South China Sea (Fig. 8).
- In the lower troposphere, paired cyclonic circulation anomalies were seen over the Indian ocean and from the western to the central Pacific, accompanied by dominant westerly wind anomalies along the equator. Meanwhile, pronounced anticyclonic circulation anomalies were seen in the subtropical regions of the Northern Hemisphere in both the Pacific and Atlantic (Fig. 9).
- In the sea level pressure field, positive anomalies were seen in the tropical regions of the Indian Ocean and the subtropical regions of the North Pacific. Negative anomalies were seen over the tropical Pacific, with particularly pronounced positive anomalies from the central to the eastern Pacific. The Southern Oscillation Index value was −1.5 (Fig. 5).

Extratropics:

- In the 500‑hPa height field (Fig. 10), the polar vortex was centered in the Arctic Ocean, accompanied by negative anomalies from Central Siberia to Alaska and positive anomalies over northwestern Siberia and around the Baffin Island. In the Northern Hemisphere mid-latitudes, positive anomalies were seen except around the Black Sea and eastern Canada, with the significant positive anomalies from eastern Asia to the Pacific.
- The westerly jet stream was weaker than normal over East Asia. In the North Pacific, the polar front jet stream shifted southward compared to the climatological normal and merged with the subtropical jet stream, where the westerlies were stronger than normal (Fig. 11).
- In the sea level pressure field (Fig. 12), north of 50°N, negative anomalies were observed in a wide area except around the North Atlantic and Baffin Island. In the mid-latitudes, remarkably negative anomalies were observed around the Caspian Sea, and positive anomalies were observed except over China and western North America, with the significant positive anomalies around the North Pacific and the Mediterranean Sea.
- 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 over East Asia and western Europe, while remarkably negative anomalies were observed around Turkey.

Zonal mean:

- In the zonal-mean zonal wind in the troposphere, the subtropical jet stream in the Northern Hemisphere was located near its normal position, while the westerly jet stream was intensified between 45°N and 55°N.
- The zonal mean temperatures in the troposphere were above normal except for the north of 75°N, with the significantly above normal over the tropics.

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 (May 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 (May 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 (May 2026) 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 (May 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 (May 2026) 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