Extratropical Highlights – December 2022

 

1. Northern Hemisphere

The 500-hPa circulation during December favored a negative North Atlantic Oscillation (NAO) teleconnection pattern that featured anomalous above-average heights ('blocking') over Greenland and anomalous below-average heights over the North Atlantic Ocean (Fig. E9).  Above-average heights were also observed over the Bering Sea, across the North Pole, over the Kara Sea, and over the Mediterranean Sea (Fig. E9).  The main land-surface temperature signals during December include below-average temperatures in western and central North America, central Asia, parts of Scandinavia, and eastern Russia, and above-average temperatures in eastern and central North America, southern Europe, southern Asia, and northern Russia (Fig. E1).  The main precipitation signals include below-average rainfall across the Mediterranean region and Alaska Panhandle, and above-average rainfall in the Pacific Northwest and New England regions of the US., Spain, northern Europe, and northern and eastern Russia (Fig. E3).

 

a. North America

The 500-hPa circulation over North America during December featured a ridge over Alaska and North Pacific Ocean, a trough over central North America, and a ridge (associated with a negative NAO) over the eastern regions of North America (Fig. E9).  This pattern led to a significant cold air intrusion across central North America where temperatures reached the lowest 30th percentile of occurrences.  This cold snap was quickly followed by a warm spell which also led to above-average temperatures across the southern U.S. where temperatures reached the 70th percentile of occurrences (Fig. E7).  The anomalous ridge over Greenland also contributed to above-average temperatures in the New England area of the U.S. (Fig. E7).  Associated with the anomalous troughing pattern, above-average precipitation was observed for much of the U.S. West Coast, Intermountain West, and North Plains where rainfall was observed in the highest 70th percentile of occurrences for many areas and even higher for areas in the Southwest and Southeast (Figs. E3, E5, E6).  Drier than average conditions were observed across the Alaska Panhandle and U.S. Southwest (Figs. E3, E5).

 

b. Europe and Asia

The above-average height anomalies over the Mediterranean region contributed to above-average temperatures and below-average rainfall for the region (Figs. E1, E3, E9).  Above-average precipitation was observed in Spain, northern Europe, and eastern Russia (Fig. E3). Along the east Russia coast and across central Asia, below-average temperatures were observed reaching the lower 30th percentile of occurrences, or lower in some areas (Fig. E1).

 

2. Southern Hemisphere

The 500-hPa height pattern during December favored a positive Antarctic Oscillation (AAO) teleconnection pattern (Fig. E15).  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 November, the ozone hole begins an early decline, along with breakdown of the polar vortex, allowing the ozone hole to reach below-average size by the end of December (Fig. S8).  The main land-surface temperature signals include above-average temperatures across much of South America with some centrally located regions reaching the 90th percentile of occurrences (Fig. E1).  Below-average temperatures were observed in eastern Australia where many areas reached the lowest 10th percentile of occurrences (Fig. E1).  The South African monsoon season runs from October to April.  Precipitation in Southern Africa and the Sahel was near normal for December (Fig. E4).  Below-average rainfall was observed in northern and southern regions of South America, portions of southern Africa, and along the southwestern and eastern coasts of Australia, and above-average rainfall was observed in Indonesia and northern Australia (Figs. E3, E4).