Highlights –December 2021
The 500-hPa circulation during December featured
above-average heights over the North Pacific Ocean, eastern U.S., Greenland,
and Scandinavian seas and below-average heights over western Canada, eastern
Siberia, and northern Europe (Fig. E9). The main
land-surface temperature signals during December included above-average
temperatures across most of the U.S., eastern Canada, and central and eastern
Asia, and below-average temperatures in western Canada and across parts of
northern Eurasia (Fig. E1). The main
precipitation signals included above-average totals across the west coast of
the U.S., western Alaska, and southern Europe, and below-average totals in the
Alaskan Panhandle, southeast U.S., and Scandinavia (Fig. E3).
a. North America
The 500-hPa circulation over North America in December
featured above-average heights over the central and eastern U.S. and
below-average heights over the Pacific Northwest and central and western Canada
(Fig. E9). Alaska featured above-average heights in the
west and below-average heights in the east (Fig. E9).
This anomalous height pattern reflected a strong negative PNA pattern (Fig. E7). The pattern was associated with anomalous
southerly flow into western Canada and northwesterly flow across the central
and eastern U.S. (Fig. E10). The result was below-average surface
temperatures (below the 30th percentile of occurrences) in western
Canada and above-average surface temperatures (above the 90th percentile
of occurrences) across much of the central and eastern U.S. (Fig. E1). This pattern brought above-average
precipitation totals to the western half of Alaska, the Mountain West of the
U.S. and U.S. West Coast and below-average totals to the south-central and
southeast U.S. (Fig. E3).
According to the U.S. Drought Monitor, moderate-to-exceptional drought
persisted across much of the central and western U.S.
b. Europe and Asia
The 500-hPa height pattern featured large positive height
anomalies over Greenland and moderate height anomalies over central Siberia and
Asia and moderate below-average height anomalies from the Beaufort Sea to
eastern Siberia and over central Europe (Fig.
pattern contributed to below-average temperatures across much of northern
Russia and above-average temperatures across Europe and Asia (Fig. E1). Much of Eurasia experienced near-normal
precipitation totals with the exception of the area from southern Europe to
western Russia where precipitation totals exceeded the 70th (and in
some areas the 90th) percentiles and below-average precipitation in
Scandinavia exceeding the 10th percentile of occurrences (Fig. E3).
The 500-hPa height pattern during December featured a ring
of above-average height anomalies over the Southern Ocean and below-average
height anomalies inside the ring. The
areas of greatest departure from normal were the Falkland Islands and the
region between the Ross Sea and Antarctic Peninsula (Fig. E15). Moderately above-average temperatures were
observed for coastal Australia, Madagascar, and southern South America and
below-average temperatures for South Africa and some areas of coastal
Antarctica (Fig. E1). Rainfall totals exceeding the 70th
and 90th percentiles were observed in South Africa, central Africa,
and Brazil (Fig. E3).
Regions around Zambia in Africa, Uruguay in South America, and the interior of
Australia experienced below-average rainfall totals reaching the 10th
percentile of occurrences (Fig. E3). Uruguay
and surrounding regions have experienced persistent below-average rainfall for
much of 2021 (see SE South America on Fig.
The South African monsoon season runs from October to April.
This area has recorded well above-average precipitation during December
following a month of well below-average precipitation totals in November 2021 (Fig. E4).