Extratropical Highlights - December 2025

 

1. Northern Hemisphere

The main features of the 500-hPa circulation during December included an amplified wavetrain pattern across North America and Europe with the strongest positive anomalies located over the Bering Sea in the North Pacific Ocean (Fig. E9). The main temperature signals include above average anomalies across most of the United States (U.S.), Europe, and Asia (Fig. E1). The main precipitation signals include above average anomalies for central portions of North America and below average anomalies in central Europe (Fig. E3).

 

a. North America

The 500-hPa circulation pattern over North America during December featured a meridionally oriented dipole pattern with a strong minima in height anomalies located over Nunavut, Canada and a moderately strong maxima in height anomalies located over the Southwest portion of the U.S. (Fig. E9). The anomalous ridge ushered in warmer than average temperatures across the western half of the U.S. with most of the area recording temperatures in the highest 90th percentile of occurrences (Fig. E1). The anomalous trough brought cooler than average temperatures to Alaska and the western half of Canada where some regions along the Alaska Range and Coast Range of Canada recorded temperatures in the lowest 10th percentile of occurrences (Fig. E1). Drier than average conditions were recorded in the Southern Interior and South Coast of Alaska, and in the Central and Southern Plains, and Midwest valleys of the U.S., where many regions recorded precipitation in the lowest 10th percentile of occurrences (Fig. E3). Above average precipitation was recorded along the Rocky Mountains with many areas in the central Rockies receiving precipitation in the highest 90th percentile (Fig. E3).

 

b. Eurasia

The 500-hPa circulation across Eurasia featured moderately strong anomalous troughing over Spain, moderately strong anomalous ridging over Scandinavia and central Europe, and moderately strong anomalous troughing over Russia (Fig. E9). Temperature anomalies were largely above normal across Europe and Scandinavia with several areas in eastern Europe recording values in the highest 90th percentile of occurrences (Fig. E1). Conversely, below average temperatures were recorded in the lower 30th percentile of occurrences across Russia (Fig. E1). Drier than average conditions were recorded across central Europe with most of the region recording precipitation is the lowest 10th percentile of occurrences (Fig. E3).

 

2. Southern Hemisphere

The 500-hPa height circulation pattern during December resembled a weak annular mode with anomalous troughing encircling moderately strong ridging over Antarctica (Fig. E15). Starting in September, the ozone hole area was significantly smaller than average and smaller than the range of ozone holes during 2015-2024 (Fig. S6). The ozone hole typically reaches a minimum by the end of December, as the polar vortex area decreases in size in response to a seasonally warming stratosphere. By the end of November, the ozone hole had reached its minimum size for the season, signaling a significantly earlier than normal reduction (Fig. S6). The vortex area and the polar stratospheric cloud coverage also decreased earlier than the average over the previous 10 years, but still within the expected range (Fig. S6).

The main land-surface temperature signals include warmer than average temperatures across much of the Sahel in Africa where many areas recorded temperatures in the highest 90th percentile of occurrences (Fig. E1). Below average precipitation was recorded across Argentina and the Brazilian Highlands where drought conditions have persisted (Figs. E3, E4). The South African monsoon season runs from October to April. Precipitation totals in South Africa were above normal for the second consecutive month and in the highest 90th percentile of occurrences (Figs. E3, E4). Precipitation totals were above normal across Peru and Chile in South America and above normal in the northern region of Australia and the Gulf of Carpentaria (Fig. E3). Precipitation totals were below normal across southwestern Australia and Angola, Africa (Fig. E3).