Extratropical Highlights ľNovember
1. Northern Hemisphere
The 500-hPa circulation during November
featured above-average heights over Alaska, western Canada, Greenland and
Scandinavia. Below-average heights were present from central North America to western Europe, and over central Russia (Fig. E9). This
overall anomaly pattern projected strongly onto the positive phases of the East
Atlantic/ Western Russia (+1.5 std. dev.) and Scandinavia (+1.9 std. dev.)
teleconnection patterns (Fig. E7, Table E1). Notably, November was the first month since
March, and only the second month since January, without a strong positive phase
of the North Atlantic Oscillation (NAO).
The main land-surface temperature signals
during November included above-average temperatures in Alaska, Europe and northeastern
Asia, and below-average temperatures in the eastern half of North America (Fig. E1). The
main precipitation signals included above-average totals across the eastern U.S.,
and below-average totals in northern Europe (Fig. E3).
a. United States
The 500-hPa circulation during November
featured an amplified wave pattern across North America, with above-average
heights over Alaska and below-average heights over the central U.S. and Canada
This pattern brought a continuation of exceptionally warm surface temperatures
to Alaska, and below-average temperatures to central North America (Fig. E1). In
Alaska, many areas again recorded departures of 3+║C and in the upper 70th
percentile of occurrences. In contrast, temperatures across central North
America were generally 1-2║C below average and in the lowest 30th
percentile of occurrences. The amplified trough also contributed to well
above-average precipitation across the eastern U.S. (Fig. E3). The most significant surpluses (in
the upper 90th percentile of occurrences) were observed in the Ohio
Valley, Northeast, Mid-Atlantic, Southeast, and Gulf Coast regions (Fig. E5).
According to the U.S. Drought
Monitor, severe-to-exceptional drought continued in the four-corner states of
Utah, Colorado, Arizona, and New Mexico. Farther north in Oregon, extreme
drought persisted across the central part of the state, and severe drought
b. North Atlantic/ Europe
The 500-hPa height pattern featured
above-average heights over Scandinavia and the high latitudes of the North
Atlantic, and below-average heights over the central North Atlantic (Fig. E9). This
pattern projected strongly onto the positive phases of the East Atlantic/
Western Russia (+1.5 std. dev.) and Scandinavia (+1.9 std. dev.) teleconnection
patterns (Fig. E7,
The overall circulation contributed to a continuation of exceptionally warm
surface temperatures across Europe and Scandinavia (Fig. E1), with most areas again recording departures in the upper 70th
percentile of occurrences. It also contributed to a continuation of
below-average precipitation in northern Europe, an area which has experienced rainfall
deficits for the last seven months (Fig.
2. Southern Hemisphere
The mean 500-hPa circulation during
November featured above-average heights over the central Indian Ocean and in
the area south of New Zealand, and below-average heights over the high
latitudes of the eastern South Pacific and in the area south of Australia (Fig. E15). The
most significant precipitation anomalies during November reflected a
continuation of above-average precipitation in central South America, and well
below-average totals in the South African monsoon region (Fig. E3).
The South African monsoon season
runs from October to April. So far in 2018, this area recorded well
below-average precipitation during October and November, with many locations
during November recording totals in the lowest 10th percentile of
occurrences (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 November 2018, the size of the ozone hole was exceptionally large (near 18
million square kilometers) at the start of the month, and had completely
dissipated by the end of the month. Overall for 2018, the ozone hole was larger
than average during mid-September through mid-November, in association with an
above-average amount of polar stratospheric cloud (PSC, Fig. S8 bottom) and an expanded polar vortex (Fig. S8 middle).