Extratropical Highlights –September
2020
1. Northern Hemisphere
The 500-hPa circulation during September
featured above-average heights over the high latitudes of Siberia, the
northwestern U.S., the North Atlantic, and western Europe, and below-average
heights over the North Pacific, eastern Canada, the high latitudes of the North
Atlantic, and the mid-latitudes of Eurasia (Fig. E9). For the Atlantic, the pattern
projected strongly onto the positive phase of the North Atlantic Oscillation
(NAO, +1.1 std. dev.) (Fig. E7, Table E1). The atmospheric circulation anomalies also had
strong projection onto the East Atlantic (EA, +1.9 std. dev.) and West Pacific
(WP, -2.4 std. dev.) patterns (Fig. E7, Table E1).
The main land-surface temperature signals
during September included above-average temperatures in the western U.S. and
much of Eurasia, and near-average temperatures across central and eastern North
America (Fig. E1).
The main precipitation signals included
above-average totals in the southeastern and mid-Atlantic regions of the U.S.,
and southern China, and below-average totals in the northeastern, north-central
and western U.S. (Fig. E3).
a. United States
The 500-hPa circulation during September
featured an amplified ridge in the western U.S., and a deep trough in the central
U.S. (Figs. E9,
E11). This anomalous wave pattern was associated
with above-average surface temperatures in the western U.S. and mostly near-average
temperatures in the central U.S. (Fig. E1). Well below-average precipitation occurred in large
portions of the western U.S., which was situated between the mean ridge and trough
axes (Fig. E3).
Area-average totals have been below the 10th percentile of
occurrences in the Southern California region for the past three months (Fig. E5). In
the Southeast region, rainfall totals have been above the 80th
percentile of occurrences for the past two months.
According to the U.S. Drought
Monitor, severe-to-exceptional drought continued in the southwestern U.S. One
of the hardest-hit regions is the 4-corner states of Utah, Colorado, Arizona,
and New Mexico. Another major drought area extended from the southern Washington
to the northern California.
b. North Atlantic/ Europe
The 500-hPa height pattern featured
above-average heights over the mid-latitudes of the North Atlantic and Europe, and
below-average heights over the high latitudes (Fig. E9). This pattern reflected a positive
phase of the NAO (+1.1 std. dev.) (Fig. E7, Table E1).
The overall circulation contributed
to a continuation of exceptionally warm surface temperatures across Europe (Fig. E1), with
most of the continent again recording departures in the upper 90th
percentile of occurrences. Below-average precipitation in southern Africa has recorded
for the last five months (Fig. E4).
c. West African
monsoon
The west African monsoon extends
from June through September, with a peak during July-September. During this September,
the monsoon rains were enhanced (Fig. E3, Fig. T24) with area-average rainfall totals above
the 90th percentile of occurrences (see Sahel region, Fig. E4).
This region has recorded above-average precipitation for the last seven months.
2. Southern Hemisphere
The mean 500-hPa circulation during
September featured above-average heights over the mid-latitudes of both the
Pacific and Atlantic Oceans, and in a region from the northern Australia to the
Antarctic edge of the southeastern Atlantic Ocean, and below-average heights at
the high latitudes of both the Pacific and Indian Oceans (Fig. E15). The main surface temperature
signals included well above-average temperatures in Australia and much of South
America, and well below-average SSTs in the southern Indian Ocean (Fig. E1). The main precipitation signals included
below-average totals across large portions of South America, and near-average
totals in southern Africa and Australia (Fig.
E3)
The Antarctic ozone hole typically
develops during August and reaches its peak size in September. The ozone hole
then gradually decreases during October and November, and dissipates in early
December (Fig. S8
top). During this September, the size of the ozone hole was mostly above
average at 20 to 25 million square kilometers. This increased size was
associated with a slightly larger than average size of the SH polar vortex (Fig. S8 middle),
and with an anomalously large area of Polar stratospheric cloud (Fig. S8 bottom).
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