Big highs and lows across Asia in September

September brought unusually large swings in solar irradiance across Asia, with few regions experiencing a typical month and some areas around 30% above or below normal breaking records, according to analysis using the Solcast API. Japan and Borneo were notably cloudier, while parts of China, Korea, the Philippines and eastern Indonesia recorded much sunnier conditions. An active typhoon season contributed to the sharp contrasts, while the outlook for the final three months of 2026 remains brighter across much of Southeast Asia and cloudier in parts of eastern China and Japan as El Niño continues to strengthen.

Japan and Borneo experienced notably below-average irradiance in September with both close to 70% of normal. Japan suffered under a tropical storm and typhoons which each brought extensive cloud, while Borneo was affected by wildfire smoke followed by stronger cloud cover later in the month. In contrast, northern China, Korea, the Philippines and eastern Indonesia recorded substantially above-average irradiance. The Yunnan-Guizhou Plateau in southern China saw particularly strong positive anomalies, reaching up to 30% above normal, the greatest increase across Asia.

The historical ranking map below puts these differences into context by comparing September irradiance at each location with previous Septembers. Each quintile represents 20% of historical years, from the lowest fifth to the highest fifth. Unlike the anomaly map, which shows the size of the departure from average, this map shows how this September ranks compared with past years. Notably large portions of the map, such as in Japan and Borneo, or Korea and southern China saw the highest or lowest September on record.

September continued an already active 2026 typhoon season, with three typhoons, Dujuan, Surigae and Choi-wan. Extreme sea surface temperatures have contributed to the particularly active season. Dujuan and Surigae impacted the Japanese islands in close succession, consistent with the substantial irradiance reductions observed there. Less powerful tropical storms and depressions also brought extensive cloud and rainfall across parts of the region, particularly Vietnam, Thailand and Japan, further suppressing irradiance during affected periods.

The daily maps show how tropical storms and typhoons moved across the region, with multiple systems present simultaneously in the Pacific between 20 and 23 September.

Looking ahead to the final three months of 2026, weather agencies show a strong consensus for sunnier conditions across most of Southeast Asia. Farther north, eastern China and Japan are forecast to experience cloudier-than-average conditions, while the outlook farther west is more mixed. Thisregional divide comes as El Niño continues to strengthen, with measures already close to record levels.

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