Extratropical Highlights –December 2021

 

1. Northern Hemisphere

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 30^th percentile of occurrences) in western Canada and above-average surface temperatures (above the 90^th 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. E9).  This 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 70^th (and in some areas the 90^th) percentiles and below-average precipitation in Scandinavia exceeding the 10^th percentile of occurrences (Fig. E3).

 

2. Southern Hemisphere

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 70^th and 90^th 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. E4).

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).