Extratropical Highlights: October 2025

 

1. Northern Hemisphere

During the month of October, the 500-hPa height pattern featured anomalous ridging over most of North America and a wavetrain pattern of anomalous heights across Eurasia (Fig. E9). The main land-surface temperature signals include widespread above average temperatures for North America and Europe, and below average temperatures across central Eurasia (Fig. E1). The main precipitation signals include below average rainfall in western Europe and above average rainfall in southern Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during October featured widespread above average height anomalies across most of North America with a maxima located over Quebec (Fig. E9). Near normal height anomalies were recorded over Alaska, the West Coast, and the U.S. Southeast (Fig. E9). The anomalous ridging contributed to widespread above average temperatures for central North America where many regions recorded temperatures in the highest 90th percentile of occurrences (Fig. E1). Rainfall totals were below average in the Southeast and above average across the western-third of the U.S., central Canada, and western Alaska with some areas reaching the highest 90th percentile of occurrences (Fig. E3). Notable observations include the U.S. Gulf Coast received significantly less than normal rainfall while the Inter-Mountain West recorded rainfall in the highest 80th percentile of occurrences (Figs. E3, E5, E6).

 

b. Eurasia

Across Europe a wavetrain of height anomalies were recorded during the month of October. Above average heights were recorded over Spain and the United Kingdom, followed by below average heights over Eastern Europe, above average heights over central Russia, and below average heights east of the Kolyma Range in Russia (Fig. E9). Temperatures were broadly above normal across Eurasia with the exception of central Eurasia and Russia's Kamchatka Peninsula (Fig. E1). Scandinavia and Spain recorded temperatures in the highest 90th percentile of occurrences (Fig. E1). Drier than average conditions were observed across western Europe and central Russia while wetter than average conditions were observed across eastern and southern Europe (Figs. E3, E5).

 

b. West African monsoon

The west African monsoon season extends from June through October, with a peak during July-September. During 2025, the west African monsoon system has been overall enhanced for most of the season with the exception of August when rainfall totals were near normal (Fig. E4).

 

2. Southern Hemisphere

The 500-hPa height field during October resembled an annular mode with above average heights recorded over the South Pole, surrounded by a near perfect circle of below average heights (Fig. E15). The main temperature signals include above average anomalies for Australia and southern South America (Fig. E1). Rainfall totals were below average across central Africa and continue to be below average across Brazil (Figs. E3, E4). The South African monsoon season runs from October to April. During October 2025, this area recorded below average rainfall, with many locations recording totals in the lowest 30th 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 (Fig. S6). By the end of October 2025, the size of the ozone hole was approximately 6 million square kilometers and well below the average size, as well as below the total range of ozone hole sizes during the last decade (Fig. S6). The Southern Hemisphere vortex area and Polar stratospheric clouds (PSCs) were also below their respective averages during October (Fig. S6).