Extratropical Highlights –September
1. Northern Hemisphere
The 500-hPa circulation during September
featured above-average heights over the high latitudes of the North Pacific, eastern
North America, and Europe, and below-average heights over western Canada and
the high latitudes of the North Atlantic (Fig.
E9). For the Atlantic, the pattern projected
strongly onto the positive phase of the North Atlantic Oscillation (NAO, +1.8
std. dev.) (Fig. E7, Table E1). A
strong positive NAO pattern has been present for almost all of 2018.
The main land-surface temperature signals
during September included above-average temperatures in both the western and eastern
United States and Europe, and below-average temperatures in western Canada (Fig. E1). The
main precipitation signals included above-average totals across the central U.S.
and eastern Canada, and below-average totals in the northwestern and
southeastern U.S and in much of Europe (Fig.
a. United States
The 500-hPa circulation during September
featured an amplified ridge in the eastern U.S. and a deep trough in the west (Figs. E9, E11). This anomalous wave pattern was associated
with well above-average surface temperatures in the eastern half of the U.S.,
with many areas recording departures of 3+ºC and in the upper 90th
percentile of occurrences (Fig. E1). Well above-average precipitation occurred
across the central U.S., which was situated between the mean trough and ridge
axes (Fig. E3).
Area-average totals exceeded the 90th percentile of occurrences in
the Great Plains, Midwest, Ohio Valley, and Great Lakes regions (Fig. E5). In
the Midwest region, rainfall totals have been above the 90th
percentile of occurrences for the past two months.
According to the U.S. Drought
Monitor, severe-to-exceptional drought continued in the southwestern and
northwestern U.S. One of the hardest-hit regions is the 4-corner states of
Utah, Colorado, Arizona, and New Mexico. Another major drought area includes
the states of Oregon and Washington, with extreme
drought evident across central Oregon and moderate or severe drought present
b. North Atlantic/ Europe
The 500-hPa height pattern featured
above-average heights over the central North Atlantic and Europe, and below-average
heights over the high latitudes of the North Atlantic (Fig. E9). This pattern reflected a continued
strong positive phase of both the NAO (+1.8 std. dev.) (Fig. E7, Table
E1). A positive NAO pattern has generally
prevailed since January 2018.
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.
It also contributed to a continuation of below-average precipitation in
northern Europe, an area which has recorded deficits for the last five months (Fig. E4).
c. West African
African monsoon extends from June through September, with a peak during
July-September. During September 2018, 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 four months.
2. Southern Hemisphere
The mean 500-hPa circulation during
September featured above-average heights over the central portions of the three
ocean basins, and below-average heights at high latitudes (Fig. E15). The most significant surface
temperature anomalies were present in southern Sough America, where departures
were above the 90th percentile of occurrences (Fig. E1). Much of this area also recorded
above-average precipitation (Fig. E3). These conditions were associated with an
amplified trough situated over the high latitudes of the South Pacific and
located just upstream of continental South America (Fig. E15).
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 the latter half of September 2018, the size of the ozone hole was above
average at 23 to 24 million square kilometers. This increased size was
associated with larger than average size of the SH polar vortex (Fig. S8, middle), and with an anomalously large area of Polar stratospheric cloud (Fig.