Extratropical Highlights –October
2018
1. Northern Hemisphere
The 500-hPa circulation during October
featured above-average heights over Alaska, western Canada, the southeastern
U.S., the eastern North Atlantic, Europe, and eastern Siberia. Below-average
heights were observed over the central North Pacific, eastern Canada, and the
high latitudes of the North Atlantic (Fig.
E9). For the Atlantic basin, the anomaly pattern
projected strongly onto the positive phase of the North Atlantic Oscillation
(NAO, +1.5 std. dev.) (Fig. E7, Table E1). A
strong positive NAO pattern has been present for nearly all of 2018.
The main land-surface temperature signals
during October included above-average temperatures in Alaska, the southeastern
U.S., Europe and northern Asia, and below-average temperatures in much of
Canada (Fig. E1).
The main precipitation signals included above-average totals across the central
U.S. and southern Europe, and below-average totals in northern Europe (Fig. E3).
a. United States
The 500-hPa circulation during October
featured an amplified wave pattern across the central North Pacific and North
America (Fig. E9).
This pattern was associated with anomalous southerly flow and exceptionally
warm surface temperature in Alaska (Fig.
E1). Many areas in Alaska recorded departures of
4+ºC, which were in the upper 90th percentile of occurrences. Downstream,
below-average surface temperatures were observed across much of Canada, with
the largest anomalies observed between the mean upper-level ridge and
downstream trough axis.
Another aspect of the 500-hPa
circulation was anomalous southwesterly flow over the south-central U.S. in the
region between the mean ridge and trough axes. These conditions contributed to
above-average precipitation the central U.S. (Fig. E3), with totals exceeding the 70th percentile from Texas
to North Dakota. In the Great Plains region, area-averaged rainfall
totals were above the 90th percentile of occurrences for the second
consecutive month (Fig. E5). Area-averaged totals also exceeded the 90th
percentile of occurrences in the drought-stricken Inter-Mountain region, and in
large portions of the Desert Southwest.
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 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 moderate or severe
drought persisted elsewhere.
b. North Atlantic/ Europe
The 500-hPa height pattern featured
above-average heights over the eastern North Atlantic and Europe, and below-average
heights over the high latitudes of the North Atlantic (Fig. E9). This pattern reflected an
ongoing strong positive phase of the NAO (+1.5 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 and
western Russia (Fig. E1), with
large areas 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 rainfall deficits
for the last six months (Fig. E4).
2. Southern Hemisphere
The mean 500-hPa circulation during
October featured an anomalous zonal wave-3 pattern,
with above-average heights in the areas poleward of all three continents and
below-average heights over the high latitudes of the central ocean basins. (Fig. E15). The
most significant surface temperature anomalies were present in eastern
Australia, where many locations recorded departures exceeding 2ºC and in the
upper 90th percentile of occurrences (Fig. E1). The most significant
precipitation anomalies during October were observed in central South America,
where totals were above the 90th 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 October 2018, the size of the ozone hole was well above average at 20
million square kilometers. This increased size was associated with a larger
than average size of the SH polar vortex (Fig.
S8 middle), and with an anomalously large area of Polar
stratospheric cloud (PSC) (Fig. S8
bottom).