Extratropical Highlights ľAugust
1. Northern Hemisphere
The 500-hPa circulation during August
featured above-average heights over the Gulf of Alaska, the central North
Atlantic and eastern Europe, and below-average heights
at high latitudes (Fig. E9). This overall pattern projected strongly onto
the positive phases of the North Atlantic Oscillation (NAO, +2.4) and the East
Atlantic teleconnection pattern (EA, +1.8) (Fig. E7, Table E1).
The main land-surface temperature signals
during August included above-average temperatures in the western and
northeastern United States, Europe, and eastern Asia (Fig. E1). The main precipitation signals
included above-average totals in the eastern half of the U.S. and eastern China,
and below-average totals in the northwestern U.S, southwestern Canada, northern
Europe, and western Russia (Fig. E3).
a. United States
The 500-hPa circulation during August
featured above-average heights over the northwestern and northeastern U.S. (Figs. E9, E11). This pattern was associated with above-average
surface temperatures in both regions (Fig.
along with below-average precipitation in the Northwest (Fig. E3). The eastern half of the U.S. recorded
above-average precipitation during August. The most significant surpluses were
observed in the Midwest, where area-averaged totals exceeded the 90th
percentile of occurrences (Fig. E5).
Regarding drought in the western
U.S., ongoing anomalously warm and dry conditions have led to a continuation of
extreme or exceptional drought in the Southwest, and to an expansion of moderate-to-severe
drought in the Northwest. Drought also developed/ intensified in the northern
Plains states, while areas of severe and extreme drought expanded in Missouri.
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 (+2.4 std. dev.) and the East Atlantic
(+1.8) teleconnection pattern (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
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 four months (Fig. E4).
Much of China recorded well
above-average surface temperatures during August, with departures generally in
the upper 90th percentile of occurrences (Fig. E1). Area-averaged rainfall totals
also exceeded the 90th percentile of occurrences for the country as
a whole (Fig. E4), with the most significant
surpluses observed in the east. These conditions were associated with an
amplified monsoon ridge (Fig. T22), and with five tropical systems
that impacted eastern China.
d. West African
African monsoon extends from June through September, with a peak during
July-September. During August 2018, the west African
monsoon system was 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 during June-August.
2. Southern Hemisphere
The mean 500-hPa circulation during
August featured an anomalous zonal wave-3 pattern. Above-average heights were
observed over the central portions of the three ocean basins (Fig. E15). Below-average
heights were observed south of Australia, over the high latitudes of the South
Pacific, and in the area south of Africa. Above-average heights were also
observed across Antarctica.
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).
During August 2018, the ozone hole was nearly absent for most of the month, but
then rapidly intensified and reached 15 million square kilometers by the end of
the month. This size is roughly the average for the 2007-2016 period.