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Seasonal Highlights on the Climate System

'Seasonal Highlights on the Climate System' has been issued since March 2007 as a seasonal bulletin focusing on the seasonal highlights of the monitoring results. For monthly information, please refer to 'Monthly Highlights on the Climate System.'

Notice: Products have been upgraded from PDF to HTML format starting from the issue of spring 2025 for improved accessibility.

Highlights in Summer (June 2026 - August 2026)

- In the tropics, extremely positive sea surface temperature (SST) anomalies were observed in the central and eastern Pacific, while SST anomalies were near normal in the western Pacific (Fig. S3). El Niño conditions intensified rapidly, and the Niño-3 index reached +3.4°C, the highest value on record for August. Significantly positive SST anomalies prevailed in the western and central North Pacific, except along 30°N, where zonally elongated negative anomalies were observed. Positive SST anomalies were widespread in the tropical Indian Ocean, with significant positive anomalies observed in many areas. In the Atlantic, significantly positive SST anomalies were also observed in the subtropics and around Europe.
- Convective activity was extremely enhanced along the equator east of 165°W in the Pacific, while it was extremely suppressed from South America to the Maritime Continent via the Atlantic Ocean, Africa, and the Indian Ocean (Fig. S4). Convective activity was generally suppressed over the northwestern Pacific, but was near normal along 20°N over the South China Sea.
- In the tropical upper troposphere (Fig. S5), pairs of anticyclonic and cyclonic anomalies straddling the equator were observed over the Pacific and across the region extending from South America to the western Indian Ocean via the Atlantic Ocean and Africa, respectively.
- In the tropical lower troposphere (Fig. S6), pairs of exceptionally strong cyclonic anomalies straddling the equator were observed over the Pacific, extending into the Northern Hemisphere mid-latitudes. Meanwhile, pairs of weak anticyclonic anomalies were observed from the Atlantic Ocean to the western Indian Ocean.
- In the Northern Hemisphere, the zonal-mean subtropical jet stream was generally weaker than normal and exhibited a pronounced split-flow pattern over the Pacific. Meanwhile, the polar front jet stream was stronger than normal around 80°N (Fig. S8).
- In the 500-hPa height field (Fig. S7), positive height anomalies prevailed over most of the extra tropics, except over the Arctic Ocean. Anomalies exceeding three standard deviations were observed over central Siberia, northeastern Canada, the Mediterranean region, and central Asia, while anomalies exceeding two standard deviations were observed around western Japan. Weak negative height anomalies were observed east of Japan.
- Sea-level pressure anomalies (Fig. S8) were significantly negative over the Arctic Ocean, indicating the positive phase of the Arctic Oscillation (AO). Significantly positive anomalies extended from central Siberia to the Aleutian Islands, while significantly negative anomalies were observed from southern China to the central Pacific in the subtropics. The westward extension of the Pacific High (subtropical high-pressure system) was weaker than normal south of Japan, while the Okhotsk High appeared with near-normal frequency.
- The characteristics of Japan's climate in summer were as follows (Fig. S1). Seasonal mean temperatures were significantly above normal in northern and western Japan, and the seasonal mean surface air temperature anomaly averaged over Japan was +1.02°C, the fifth highest on record for summer since 1898. Precipitation amounts were significantly above normal on the Pacific side of eastern Japan and above normal in Okinawa/Amami, while they were below normal on the Sea of Japan side of eastern and western Japan. Sunshine durations were significantly above normal on the Sea of Japan side of northern and western Japan, and above normal on the Pacific side of northern and western Japan and the Sea of Japan side of eastern Japan.

Supplemental information

- Climate Anomaly Table over Japan

Fig.S1 Seasonal climate anomaly/ratio over Japan (June 2026 - August 2026) Top: temperature anomalies (°C) Middle: precipitation ratio (%) Bottom: sunshine duration ratio (%) The base period for the normal is 1991-2020.

Fig.S7 Three-month mean 500-hPa height and anomaly in the Northern Hemisphere (June 2026 - August 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.S8 Three-month mean 200-hPa wind speed and vectors in the Northern Hemisphere (June 2026 - August 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.S9 Three-month mean sea level pressure and anomaly in the Northern Hemisphere (June 2026 - August 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.S10 Three-month mean 850-hPa temperature and anomaly in the Northern Hemisphere (June 2026 - August 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
• 3-month Mean Figures
• Time Cross Sections
• Indices
• Oceanic Figures and Tables