Highlights – December 2022
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.
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).
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.
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).