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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 changed from PDF to HTML format starting from the issue of May 2025 for improved accessibility.

Highlights in September 2026

- In the tropical Pacific, sea surface temperature (SST) anomalies were significantly positive east of the dateline. El Niño conditions continuously intensified, and the NINO.3 index reached +4.0°C, the highest annual value on record (Fig. 8). In contrast, negative SST anomalies were widely distributed west of the dateline and over the Maritime Continent. In the North Pacific, SST anomalies were positive over the subtropical region, negative near 30°N, and significantly positive in the latitudinal band between 35°N and 50°N. Positive SST anomalies were widespread in the tropical Indian Ocean and the tropical Atlantic, with significantly positive anomalies observed in many areas.
- In the tropics, convective activity was significantly enhanced between the equator and the ITCZ over the central and eastern Pacific. In contrast, convective activity was suppressed from South America to the western Pacific through the Atlantic Ocean, Africa, the Indian Ocean, and the Maritime Continent, with particularly strong suppression extending from the Indian Ocean to the western Pacific (Fig. 6). Convective activity associated with the Asian summer monsoon was markedly suppressed, except over the Indochina Peninsula and the southern Mariana Islands, where convective activity was near normal.
- The circulation anomalies in the tropical troposphere exhibited a zonal wave-number-1 pattern associated with the convective activity anomalies. In the tropical upper troposphere, pairs of anticyclonic and cyclonic anomalies straddling the equator were observed over the Pacific and across a broad region extending from South America through the Atlantic and Africa to the Indian Ocean, respectively (Fig. 8). In the tropical lower troposphere, significant cyclonic and significant anticyclonic anomalies straddling the equator were observed over the Pacific and the Indian Ocean, respectively. In the sea-level pressure field, significantly positive anomalies prevailed over Australia, with record-breaking positive anomalies observed at Darwin (Fig. 9). Consistent with these anomalies, the Southern Oscillation Index (SOI) was −2.1, the lowest September value on record (Fig. 5).
- In the Northern Hemisphere, the polar-front jet was stronger than normal along the Arctic coast. Although the zonal-mean subtropical jet remained near its climatological latitude, positive wind anomalies were confined to the jet core, while negative anomalies prevailed around 30°N (Fig. 11).
- In the 500-hPa height field (Fig. 10), a stronger-than-normal polar vortex was located over the Arctic Ocean. Positive anomalies dominated much of the Northern Hemisphere, except over the Arctic and the northern North Atlantic, where significant negative anomalies prevailed. Particularly large positive anomalies were observed over western Siberia, western Europe, central North America, and the region south of 40°N, exceeding three standard deviations around China.
- In the sea-level pressure (SLP) field (Fig. 12), the positive phase of the Arctic Oscillation dominated. Negative anomalies prevailed over the high latitudes except over central Siberia, with significantly negative values observed across much of the Arctic. In contrast, significantly positive anomalies extended from central Siberia to East Asia and were also observed around the Mediterranean Sea. Negative anomalies were observed south of Japan, contributing to persistent rainy and cloudy conditions over southern Honshu.
- Monthly mean temperatures were above normal in western Japan and Okinawa/Amami, and near normal in northern and eastern Japan (Fig. 1). The monthly mean surface air temperature anomaly averaged over Japan was +0.49°C. Precipitation amounts were significantly above normal on the Pacific side of eastern Japan, breaking the September record since 1946, and above normal on the Sea of Japan side of eastern Japan and the Pacific side of western Japan, while below normal on the Sea of Japan side of northern Japan. Sunshine durations were significantly below normal on the Pacific side of eastern Japan, breaking the lowest September record since 1946, and were also significantly below normal on the Sea of Japan side of eastern Japan and the Pacific side of western Japan, and below normal on the Sea of Japan side of western Japan, while above normal on the Sea of Japan side of northern Japan.

Climate in Japan (Fig. 1):

- The monthly anomaly of the average surface temperature over Japan was +0.49°C. On a longer time scale, the average surface temperatures have risen at a rate of about 1.50°C per century in September.
- Monthly mean temperatures were above normal in western Japan and Okinawa/Amami, because the regions were covered by warm air in the second half of September.
- Monthly precipitation amounts were significantly above normal on the Pacific side of eastern Japan. Monthly sunshine durations were significantly below normal on the Sea of Japan side of eastern Japan and on the Pacific side of eastern/western Japan. On the Pacific side of eastern Japan, monthly precipitation amounts were the largest and monthly sunshine durations were the shortest on record for September since 1946. These regions were affected by Typhoon KROVANH (T2624)/Typhoon DUJUAN (T2625)/Typhoon SURIGAE (T2626), moist air and stationary fronts.

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 +0.79°C (the warmest for September 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 September (preliminary value).
- Extreme climate events were as follows (Fig. 3).
  - Monthly mean temperatures were extremely high in Southeast Asia, in and around southern India, from Western Siberia to Western Russia, from the eastern part of Central Asia to the southwestern Middle East, from western Europe to the western Mediterranean region, in the southwestern part of Western Africa, from the eastern USA to the northern part of South America via Central America and in southern Australia.
  - Monthly precipitation amounts were extremely high from the northern to the southwestern USA.
  - Monthly precipitation amounts were extremely low from southern China to northeastern India, from eastern to western Europe, in the southeastern USA and in southeastern Australia.

Oceanographic Conditions:

- In the tropical Pacific, significantly positive SST anomalies were observed east of the dateline (Fig. 4). The monthly mean SST anomaly and the SST deviation from the latest sliding 30-year mean over the NINO.3 region were +4.0°C (Fig. 5). Significantly negative SST anomalies were observed from the Maritime Continent to the western tropical Pacific.
- In the North Pacific, significantly positive SST anomalies were observed between 35°N and 55°N, except off the coast of North America. Meanwhile, widespread negative SST anomalies were observed between 25°N and 35°N in the central to the eastern Pacific.
- In the South Pacific, positive SST anomalies were widely observed, except in the western and central subtropical region. Significant positive anomalies were observed in the eastern South Pacific and around New Zealand.
- In the Indian Ocean, positive SST anomalies prevailed over most areas, with widespread significant positive anomalies.
- In the North Atlantic, positive SST anomalies were generally observed over most areas and significantly positive SST anomalies were observed between 20°N and 30°N.
- In the South Atlantic, positive SST anomalies were observed except in the western part between 30°S and 60°S.

Tropics:

- Convective activity was extremely enhanced from the central to eastern Pacific between the equator and the ITCZ, While it was strongly suppressed from Indian Ocean to the western Pacific (Fig. 6). The global distribution of convective activity anomalies exhibited a wave-number-1 pattern. Convective activity related to the Asian monsoon was suppressed. The active phase of equatorial intraseasonal oscillation was unclear (Fig. 7).
- In the upper troposphere, paired anticyclonic circulation anomalies straddling the equator were observed from the central to the eastern Pacific. Meanwhile, paired cyclonic circulation anomalies straddling the equator prevailed from South America to Maritime Continent through the Indian Ocean (Fig. 8).
- In the lower troposphere, paired significant cyclonic circulation anomalies straddling the equator were seen around the Pacific. Meanwhile, paired significant anticyclonic circulation anomalies straddling the equator were prominently seen in the Indian Ocean (Fig. 9).
- In the sea level pressure field, pronounced positive anomalies were observed from the Indian Ocean to the western Pacific, while negative anomalies prevailed from the central to the eastern tropical Pacific. The Southern Oscillation Index value was −2.1 (Fig. 5).

Extratropics:

- In the 500‑hPa height field (Fig. 10), the polar vortex was centered over the Arctic Ocean, and positive anomalies were seen except for negative anomalies over the Arctic Ocean and the northern North Atlantic in the high latitudes, with significant positive anomalies over western Siberia, western Europe and central North America.
- The subtropical jet stream was stronger than normal around its normal position from Eurasia to the Pacific (Fig. 11).
- In the sea level pressure field (Fig. 12), significant negative anomalies were observed over the Arctic Ocean and the northern North Atlantic, and negative anomalies were observed over northwestern Canada and eastern Siberia, while significant positive anomalies were observed from Central Siberia to East Asia and around the Mediterranean Sea, and the positive phase of the Arctic Oscillation prevailed.
- In the 850‑hPa temperature field (Fig. 13), the anomaly pattern closely resembled that of the 500-hPa height field except for weaker positive temperature anomalies in the mid-latitudes and weak negative temperature anomalies over the eastern Mediterranean Sea.

Zonal mean:

- In the zonal-mean zonal wind in the troposphere, the subtropical jet stream in the Northern Hemisphere was stronger than normal around its normal position. The polar front jet stream was stronger than normal around 80°N.
- The zonal mean temperatures in the troposphere were above normal over a wide area in the Northern Hemisphere, with significantly above-normal temperatures between 50°N and 70°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 (September 2026) Top: temperature anomalies (°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 (September 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 (September 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 (September 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 (September 2026) The contours show 850-hPa temperature at intervals of 3 °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