Extratropical Highlights: September 2025

 

1. Northern Hemisphere

The 500-hPa circulation during September featured a wavetrain pattern with above average heights across most of North America, Scandinavia, Europe and western Russia, and below average heights over central Russia (Fig. E9). The main land-surface temperature signals include above average temperatures for most of the Northern Hemisphere (Fig. E1). The main precipitation signals include drier than average conditions across eastern North America and western Russia, and wetter than average conditions across central and eastern Russia (Fig. E3).

 

a. North America

The 500-hPa circulation over North America featured predominantly above average height anomalies with the maxima located over central Canada (Fig. E9). Below average heights were observed over Alaska's North Slope (Fig. E9). Across most of the U.S. and Canada, temperatures were above normal with many areas across Canada recording anomalies in the highest 90th percentile of occurrences (Fig. E1). The northern coast of Alaska recorded below average temperatures while most of the state recorded near normal temperatures, along with the U.S. Eastern Seaboard (Fig. E1). Wetter than average conditions were recorded along Alaska's North Slope, the U.S. Southwest, Southern California, and Inter-Mountain West, while drier than average conditions were recorded across most of Canada, the U.S. Great Lakes, Midwest, Ohio Valley and Southeast, where several areas reached the lowest 10th percentile of occurrences (Figs. E3, E5).

 

b. Europe and Asia

Predominantly above average height anomalies across Europe and Russia were observed during September, along with below average height anomalies over north-central Russia (Fig. E9). Above average temperatures were observed across most of Europe and Asia, with many areas reaching the highest 90th percentile of occurrences (Fig. E1). Below average temperatures were recorded within the northern most regions of Russia where many areas recorded temperatures in the lowest 10th percentile of occurrences, while the rest of Russia recorded near normal temperatures (Fig. E1). Drier than average conditions were recorded for Spain, western France, and eastern Russia where many regions recorded precipitation in the lowest 10th percentile of occurrences (Fig. E3). Wetter than average conditions were recorded for most of Russia where several areas observed precipitation anomalies in the highest 90th percentile of occurrences (Fig. E3).

 

c. West African monsoon

The west African monsoon extends from June through September, with a peak during July-September. After a normal August rainfall, September rainfall in the Sahel reached the 80th percentile of occurrences (Figs. E3, E4). With the exception of August, the 2025 west African monsoon season was generally enhanced.

 

2. Southern Hemisphere and Ozone Hole

The 500-hPa height field during September featured a wavetrain pattern of alternating above and below average height anomalies across Antarctica and adjacent oceans (Fig. E15). A strong maxima was recorded over Antarctica and a strong minima was recorded over the Pacific and Indian Oceans, respectively (Fig. E15). Temperatures across South America, Africa, and Australia were above normal with many areas in southern South America and eastern Africa reaching the highest 90th percentile of occurrences (Fig. E1). South America faces extensive drought conditions in northern regions of the country, and especially for Brazil, as precipitation totals continue to be in the lowest 10th percentile of occurrences (Figs. E3, E4).

The Antarctic ozone hole typically develops during August and reaches peak size in September. The ozone hole then gradually decreases during October and November, and dissipates on average in early December. By the end of September 2025, the size of the ozone hole approached 25 million square kilometers, which is below the 2015-2024 mean size but within the expected range (Fig. S6). Associated with the ozone hole during September was a below average polar vortex area and a below-to-near normal polar stratospheric cloud coverage area, although the area appears to be trending downward rapidly as October begins (Fig. S6).