Extratropical
Highlights –October 2021
1.
Northern Hemisphere
The 500-hPa circulation during October featured
above-average heights across the central North Pacific Ocean, the northeastern
quadrant of North America, and Europe and below-average heights over the Bering
Strait, Gulf of Alaska, and North Atlantic Ocean (Fig. E9). The main land-surface
temperature signals included above-average temperatures for most of North
America, eastern Europe, and Russia (Fig. E1). The
main precipitation signals included above-average totals in eastern and western
regions of the Canadian and U.S. border, Great Britain, parts of Scandinavia,
and eastern Russia, and below-average totals in the Alaskan Panhandle, Hudson
Bay, and parts of Europe (Fig. E3).
a. North America
The 500-hPa circulation during October featured an anomalous
wave pattern extending from the central North Pacific to eastern Canada, with
near-normal heights over much of the coterminous U.S. (Fig. E9). This pattern reflected
amplified ridges over the central North Pacific and eastern Canada and an
amplified trough from Alaska to the Pacific Northwest. These conditions
contributed to above-average surface temperatures in Alaska, northern and
eastern Canada and the eastern U.S. (Fig.
E1). The amplified trough lead to higher
departures from normal precipitation in the Pacific Northwest and Northern
California (Fig. E6). Above-average precipitation in the Great
Lakes region was also observed and below-average precipitation in the Alaskan
Panhandle (Fig. E3).
b. Europe and Russia
The 500-hPa circulation during October featured
above-average heights across southern Europe eastward to western Russia, and
then again in eastern Russia (Fig. E9). This pattern was associated with enhanced
sea-level pressure anomaly (Fig. E8) however above-average upper-level wind anomalies
were not evident (Fig. E10). The
anomalous above-average height pattern contributed to the onshore flow of
relatively mild marine air and contributed to well above-average surface
temperatures across much of North Asia, with departures reaching the 70th and exceeding
the 90th percentile for some areas (Fig. E1).
b. West African monsoon
The west African monsoon season
extends from June through October, with a peak during July-September. During
2021, the west African monsoon system was enhanced
from July-October (see Sahel region, Fig.
E4), with area-average rainfall totals at or
above the 100th percentile of occurrences in July, August, and
September (Fig. E3).
2.
Southern Hemisphere
The 500-hPa height field during October featured an
anomalous wave pattern across the South Pacific Ocean and a tilt toward
above-average heights over the South Atlantic and Indian Oceans (Fig. E15).
Above-average heights were observed over much of Antarctica. In Australia, above-average temperatures were
observed for the north and eastern portions (Fig. E1) and near-normal precipitation
conditions (Fig. E3).
The South African monsoon season runs from October to April.
During October 2021, this area recorded below-average precipitation, with many
locations recording totals in the lowest 30th percentile of
occurrences (Fig. E3).
The Antarctic ozone hole typically develops during August
and reaches peak size in September. The ozone hole then gradually decreases
during October and November, and dissipates on average in early December (Fig. S8 top).
By the end of October 2021, the size of the ozone hole was about 20 million
square kilometers, which is well above the 2010-2019 average size of 11.5
million square kilometers.
Polar stratospheric clouds (PSCs) were near zero and
near-normal (Fig. S8
bottom) while the vortex area was well-above average (Fig. S8 middle). This
highly anomalous ozone hole and vortex is not associated with a sudden
stratospheric warming (Fig. S4, see September thru October). The 50-hPa height anomalies show well-below
average heights over Antarctica (Fig. S1).