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 90^th 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 100^th 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 30^th 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).