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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 Winter (December 2025 - February 2026)

- In the equatorial Pacific, notably positive SST anomalies were dominant in the western part, while negative SST anomalies were observed in the central and eastern parts (Fig. S3). This anomaly pattern resembled that typically seen during a La Niña event, although it did not meet the official criteria. Significantly positive anomalies were observed in the mid-latitudes of the North Pacific and in the subtropical North Atlantic. SST anomalies in the Indian Ocean were near normal to positive.
- Convective activity was enhanced from the Maritime Continent to the western Pacific and over equatorial Africa, while it was suppressed over the Indian Ocean and from the central to the eastern Pacific. Convective activity was also significantly enhanced in the subtropical western to central North Pacific.
- In the tropical upper troposphere (Fig. S5), a pair of weak anticyclonic circulation anomalies straddling the equator was observed from the eastern Indian Ocean to the western Pacific, while a pair of cyclonic circulation anomalies straddling the equator was observed from the central to the eastern Pacific. The cyclonic anomalies over the central Pacific extended northward to around 40°N.
- In the tropical lower troposphere (Fig. S6), a pair of cyclonic and anticyclonic circulation anomalies straddling the equator were observed over the central Pacific and from the eastern Indian Ocean to the western Pacific, respectively.
- In the Northern Hemisphere, the zonal-mean subtropical jet stream was slightly shifted northward compared to its climatological position and was significantly weaker than normal between 20°N and 30°N (Fig. S8). Over the Far East, the subtropical jet stream was shifted southward, while the polar-front jet stream was stronger than normal along 45°N, resulting in weak westerlies over Japan.
- In the 500-hPa height field (Fig. S7), the polar vortex split, and the main vortex was located over northern Canada. Significantly positive height anomalies were observed around Greenland and the Bering Sea, where blocking highs were frequently observed. Over the mid-latitudes of the Northern Hemisphere, negative height anomalies were observed from eastern North America to the Atlantic, while positive anomalies were observed in other regions, being especially significant over Asia and western North America. Negative height anomalies were also observed from western Siberia to Primorsky Krai.
- Sea level pressure anomalies were positive in the high latitudes and negative in the mid-latitudes except for the North Pacific, indicating a negative phase of the Arctic Oscillation (Fig. S9). In the Far East, both the Siberian High and the Aleutian Low were weaker than normal, resulting in a weak winter monsoon.
- The characteristics of Japan's climate in winter were as follows (Fig. S1). Above-normal temperatures were observed across Japan, and the seasonal anomaly of the average surface temperature was +0.90°C, the fifth-highest winter value since 1898. Precipitation amounts were significantly above normal on the Sea of Japan side of northern Japan and above normal on the Sea of Japan side of eastern Japan and near normal on the Pacific side of northern Japan, while they were below normal in other regions. Sunshine durations were above normal except in northern Japan, and were significantly above normal in western Japan and on the Sea of Japan side of eastern Japan. Snowfall amounts on the Sea of Japan sides of Japan were near normal, but maximum snow depths were greater than normal in western Japan and northern Tohoku.

Supplemental information

- Climate Anomaly Table over Japan

Fig.S1 Seasonal climate anomaly/ratio over Japan (December 2025 - February 2026) Top: temperature anomalies (degree 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 (December 2025 - February 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 (December 2025 - February 2026) The black lines show wind speed at intervals of 20 m/s. The brown lines show its normal at intervals of 40 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 (December 2025 - February 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 (December 2025 - February 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
• 3-month Mean Figures
• Time Cross Sections
• Indices
• Oceanic Figures and Tables