Extratropical Highlights: December 2023

 

1. Northern Hemisphere

The 500-hPa circulation during December favored an Arctic Oscillation (AO) teleconnection pattern that featured predominantly above-average height anomalies across North America and the Arctic (Fig. E9).  Moderately above-average heights were observed over southern Europe and Eurasia (Fig. E9).  Strongly below-average heights were observed over the Bering Strait and moderately below-average heights were observed over Scandinavia (Fig. E9).  The main land-surface temperature signals include above-average temperatures across most of North America and Eurasia and below-average temperatures in Scandinavia and central Russia (Fig. E1).  The main precipitation signals include above-average rainfall for the eastern coast of North America, United Kingdom, and regions of Europe and Russia (Fig. E3).

 

a. North America

The 500-hPa circulation over North America during December featured a large and strong ridge over most of Canada and the U.S., and a strong trough over Alaska (Fig. E9).  Temperature anomalies were strongly above normal for most of North America with a majority of the area reaching the highest 90th percentile of occurrences (Fig. E1).  Drier than average conditions were observed in western Alaska, and around the Hudson Bay, and wetter than average conditions were observed broadly along the Southeast and Atlantic coasts of the U.S. (Fig. E3).  Some areas across the U.S. experienced significant departures from average rainfall.  For example, the Great Lakes reached the lowest 20th percentile of occurrences, while the Great Plains, Gulf Coast, and Mid-Atlantic received rainfall in the highest 80th percentile (or higher) of occurrences (Figs. E5, E6).

 

b. Europe and Asia

The height pattern across Europe and Russia completed a wavetrain that originated in the North Pacific Ocean with above-average anomalies, below-average anomalies over the Bering Sea, above-average heights over the Kara Sea, below-average heights over Scandinavia, and above-average heights over the Mediterranean (Fig. E9).  The troughing over Scandinavia, along with the weakness in heights over Greenland, shares characteristics of an atypical North Atlantic Oscillation (NAO) teleconnection pattern.  This troughing coupled with the ridging over the North Atlantic Ocean and the Mediterranean contributes to the moderately strong positive NAO phase recorded for this December (Fig. E7).  The ridging pattern across southern Europe, the Middle East, and into China contributed to the above-average temperature anomalies observed in these regions.  Some areas, for example southern Europe, reached the highest 90th percentile of occurrences.  Some parts of Scandinavia and Russia recorded below-average temperatures with many areas reaching the lowest 30th percentile of occurrences, and some isolated areas reaching the lowest 10th percentile of occurrences, for example along the Great Khingan Range near Manchuria, China (Fig. E3). Northern Europe and India recorded above-average rainfall anomalies in the highest 90th percentile of occurrences (Figs. E3, E4).  In southern Europe, conditions were drier than average with rainfall in the lowest 30th percentile of occurrences (Figs. E3, E4).

 

2. Southern Hemisphere

The 500-hPa height pattern during December favored a positive Antarctic Oscillation (AAO) teleconnection pattern (Fig. E15) with positive height anomalies along the middle latitudes and below-average heights centered over the pole.  The ozone hole typically reaches a minimum by the end of December as the polar vortex decreases in response to a seasonally warming stratosphere (Fig. S8).  Notably in early December, the ozone hole began a rapid decline, along with a rapid breakdown of the polar vortex (Fig. S8).  These factors contributed to the ozone hole reaching just under the average size by the end of December.  The main land-surface temperature signals include above-average temperatures across South America with regions such as Bolivia, Paraguay, and southern Brazil reaching the highest 90th percentile of occurrences (Fig. E1). Much of Africa recorded near-normal temperatures during December, and Queensland and New South Wales observed above-average temperatures that reached the highest 70th percentile of occurrences (Fig. E1). Across Brazil and most of Australia, drier than average conditions were recorded with the majority of those areas reaching the lowest 10th percentile of occurrences, while the African Sahel region recorded near-normal rainfall (Figs. E3, E4). The South African monsoon season runs from October to April. Precipitation in Southern Africa was above-average for December following dry conditions in November (Figs. E3, E4).