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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 September 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, remarkably positive and negative SST anomalies were observed in the eastern region and in the Arabian Sea, respectively, indicating a negative Indian Ocean Dipole like pattern. Remarkably positive SST anomalies were also observed north of 20°N in the western North Pacific. In the East China Sea and around western Japan, these anomalies exceeded three times the climatological standard deviation.
- In the tropic, the large scale pattern of upper level divergence showed a wave number 1 and convective activities were remarkably more active than normal from the eastern Indian Ocean to the Maritime Continent while they were remarkably less active than normal around the date line and from Africa to the western Indian Ocean in the equator. Total activities of the Asian Monsoon were more active than normal besides a intra-seasonal oscillation such as BSISO was unclear. Paired anticyclonic circulation anomalies and paired cyclonic circulation anomalies were observed across both hemispheres in the upper and lower troposphere over the Indian Ocean, respectively. In contrast, reversed circulation anomalies were observed from the western to central Pacific. These anomalies indicate that the Walker circulation was stronger than normal in both the Pacific and Indian Oceans.
- In the Northern Hemisphere, wind speeds associated with the subtropical jet were generally stronger than normal. From East Asia to the North Pacific, the core of the subtropical jet shifted northward relative to its climatological position and exhibited a wavy pattern, which contributed to dominant anticyclonic anomalies over East Asia. In the subtropical region over the western North Pacific, pronounced anticyclonic anomalies were also observed.
- In the 500-hPa height fields, the polar vortex was located north of Eastern Siberia and was stronger than normal, while positive height anomalies were observed from Northern Europe to Western Siberia. A prominent wave-like anomaly pattern dominated, resulting in positive anomalies over Northern Canada, the Mediterranean region, East Asia, and south of the Aleutian Islands, and negative anomalies over areas south of Alaska, the eastern United States, west of England, and from the Caspian Sea to Mongolia. Sea level pressure anomalies were broadly negative across Eurasia, with particularly pronounced negative anomalies over Southern Siberia, while significantly positive anomalies were observed south of Japan.
- In September, Japan experienced significantly above-normal temperature throughout the country, with Okinawa/Amami recording the highest temperature anomalies for September since 1946. The monthly anomaly of the average surface temperature over Japan was +2.49°C which was the 3rd warmest for September since 1898. Precipitation was above normal in northern Japan and on the Sea of Japan side of eastern Japan, while below normal on the Pacific side of eastern Japan and in Okinawa/Amami, where it was significantly below normal. Sunshine duration ratios were above normal across most of Japan, except for the Sea of Japan side of western Japan. Significantly above-normal sunshine durations were observed on the Pacific side of northern Japan and in Okinawa/Amami.

Climate in Japan (Fig. 1):

- Monthly mean temperatures were significantly above normal nationwide, with Okinawa/Amami recording the highest temperature anomalies for September 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 over eastern Asia, and also due to warm-air inflow.
- The monthly anomaly of the average surface temperature over Japan was +2.49°C (3rd warmest for September since 1898) . On a longer time scale, the average surface temperatures have risen at a rate of about 1.49°C per century in September.
- Monthly sunshine durations were significantly above normal on the Pacific side of northern Japan, and above normal on the Sea of Japan side of northern Japan, eastern Japan and the Pacific side of western Japan, due to the regions were frequently covered by high-pressure systems. Monthly precipitation amounts were below normal on the Pacific side of eastern Japan. Meanwhile, monthly precipitation amounts were above normal on northern Japan and the Sea of Japan side of eastern Japan due to the influence of stationary fronts and low-pressure systems.
- In Okinawa/Amami, monthly precipitation amounts were significantly below normal and monthly sunshine durations were significantly above normal, because the region was well covered by the North Pacific Subtropical High.

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 +0.50°C (3rd 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 0.74°C per century in September (preliminary value).
- Extreme climate events were as follows (Fig. 3).
  - Monthly mean temperatures were extremely high from eastern to southern East Asia, in southern Central Siberia, in and around northern Europe and from central to western Canada.
  - Monthly precipitation amounts were extremely high in and around Central Siberia, in and around Kalimantan, around the Caspian Sea, in central Europe and from Bolivia to northern Argentina.
  - Monthly precipitation amounts were extremely low in and around Western Russia and in and around southern Canada.

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 the mid-latitudes.
- In the South Pacific, remarkably positive SST anomalies were observed from the western part to the eastern part of the mid-latitudes.
- In the Indian Ocean, remarkably positive SST anomalies were observed from the central to eastern parts, and remarkably negative SST anomalies were observed in the western part.
- In the North Atlantic, remarkably positive SST anomalies were observed from the tropics to the western part of the mid-latitudes, and remarkably negative SST anomalies were observed from the central to eastern parts of 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 enhanced from the tropical eastern Indian Ocean to the Maritime Continent, and suppressed over the equatorial central Indian Ocean, the equatorial western and central Pacific, and the subtropical western and central North Pacific (Fig. 6). The overall activity over the Asian monsoon convection was stronger than normal.
- Eastward propagation of the active phase of equatorial intraseasonal oscillation was unclear (Fig. 7).
- In the upper troposphere, anticyclonic circulation anomalies straddling the equator were seen over the tropical Indian Ocean, and cyclonic circulation anomalies straddling the equator were seen from the Maritime Continent to the tropical central Pacific. In the Northern Hemisphere, a wavy anomaly pattern was seen along the subtropical jet, with anticyclonic circulation anomalies over the eastern Middle East, East Asia and the central North Pacific (Fig. 8).
- In the lower troposphere, cyclonic circulation anomalies straddling the equator were seen over the tropical Indian Ocean, and anticyclonic circulation anomalies straddling the equator were seen over the western and central Pacific (Fig. 9).
- In the sea level pressure field, positive anomalies were seen over the tropical Pacific, with significant positive anomalies over the south of Japan. The Southern Oscillation Index value was 0.0 (Fig. 5).

Extratropics:

- In the 500-hPa height field (Fig. 10), the polar vortex was stronger than normal to the north of Eastern Siberia, and positive anomalies were seen from northern Europe to Western Siberia. A hemispheric-scale wavy anomaly pattern was dominant with positive anomalies over northern Canada, around the Mediterranean Sea, over East Asia, and to the south of the Aleutian Islands, and negative anomalies to the south of Alaska, over the eastern USA, to the west of the UK, and from the Caspian Sea to Mongolia.
- The subtropical jet stream was stronger than normal from East Asia to the central North Pacific and shifted northward from its normal position (Fig. 11). The polar-front jet stream was clear from Central Siberia to Canada. The westerly jet stream over the North Atlantic was stronger than normal.
- In the sea level pressure field (Fig. 12), positive anomalies were seen from Europe to Western Siberia and to the south of Japan, and negative anomalies were seen to the west of the UK and over a wide area from East Asia to Alaska. The subtropical high over the North Atlantic was stronger than normal.
- Temperatures at 850-hPa were above normal over northern Europe, northern Canada, and from East Asia to the midlatitude central North Pacific, and below normal over northern Central Siberia and to the west of the UK (Fig. 13).

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 subtropical jet stream in the Southern Hemisphere shifted southward.
- The zonal mean temperatures were above normal over a wide area in the troposphere, and the Southern Hemisphere high-latitudes in the stratosphere.

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 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 (September 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 (September 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 (September 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 (September 2025) 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