Highlights –September 2021
The 500-hPa circulation during September featured
above-average heights over the central and eastern portions of the North
Pacific Ocean, and also extending from the Arctic Archipelago, to the North
Atlantic Ocean, Scandinavia, and Siberia, and below-average heights over the the Gulf of Alaska, Chukchi Sea, central Asia, and
Greenland (Fig. E9).
The main land-surface temperature signals included above-average temperatures
in much of North America, Europe, and Asia (Fig. E1). The main precipitation signals
included above-average totals across southern and eastern Asia, the Gulf States
of the U.S., and below-average totals in the central U.S. and Europe (Fig. E3).
a. North America
The 500-hPa circulation during September resembled an
anomalous wave pattern extending from the central North Pacific Ocean to the
Arctic Archipelago (Fig. E9). This enhanced ridging over the western half of
the U.S. and portions of Canada contributed to above-average surface
temperatures for much of the region with the U.S. Central Plains exceeding the
90th percentile (Fig. E1).
Below-average rainfall was observed for the Central Plains,
North-Atlantic States, and South Slope of Alaska (Fig. E3), with rainfall in the lowest
10th percentile for these regions.
Above-average rainfall was observed along the Alaska Panhandle,
Northwest U.S., Mid-Atlantic states, and the central
Gulf states (Fig. E3).
b. West African monsoon
The west African monsoon extends
from June through September, with a peak during July-September. During September
2021, the west African monsoon system was enhanced (Figs. E3 and E4). For the
July-September 2021 period as a whole, the monsoon was enhanced, with
area-average rainfall totals near the 100th percentile of
occurrences (see Sahel region, Fig. E4).
The 500-hPa height field during September featured
above-average heights over the South Pacific Ocean, the South Atlantic Ocean,
and the Indian Ocean, and below-average heights over the Southern Ocean (Fig. E15).
Conditions for Australia were near-normal for both temperature and
precipitation with the exception of Northeast Australia which observed above
average rainfall (Figs. E1, E3, E4). In South America and Southern Africa,
above-average temperatures were observed (Fig.
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). By
the end of September 2021, the size of the ozone hole was nearly 25 million
square kilometers, which is well above the 2010-2019 average size of 18.5
million square kilometers. This significantly enhanced size of the ozone hole
reflected a markedly enhanced size of the polar vortex (35 million square
kilometers compared to the average of 32 million), along with near-average
polar stratospheric clouds (PSCs) (Fig. S8). On average, the PSC area covers about 13
million square kilometers by the end of September. These anomalous
stratospheric conditions were associated with near-normal stratospheric zonal
mean temperatures (Fig. S4) and with well below-average heights throughout
the polar stratosphere at levels above 100-hPa (Fig. S1).