Extratropical Highlights –October 2017
1. Northern Hemisphere
The 500-hPa circulation during October
featured above-average heights over the Gulf of Alaska, eastern Canada, and western Europe, and below-average heights over northeastern
Europe and western Russia (Fig. E9). At 200-hPa, the circulation featured an
amplified ridge extending from the Middle East to northeastern China (Fig. T22).
The 200-hPa circulation also featured amplified mid-Pacific troughs in the
subtropics of both hemispheres, a characteristic feature of La Niña.
The main land-surface temperature signals
during October included above-average temperatures in western Alaska, eastern
Canada, and Europe (Fig. E1). The main precipitation signals included above-average
totals in the northeastern and mid-western U.S., northeastern Europe, and large
portions of China, and below-average totals in central and southern Europe (Fig. E3).
The Atlantic hurricane season
remained active during the first half of October, with Major Hurricane Ophelia
making landfall as an extratropical storm in Ireland with maximum sustained
winds above 100 mph. Ophelia is the most eastward Atlantic major hurricane on
record. By the end of October, the 2017 Atlantic hurricane season had produced
16 named storms, of which 10 were hurricanes and six were major hurricanes. The
season produced the most hurricanes since 2012 and the most major hurricanes
since 2005. The seasonal ACE was 245% of the 1981-2010 median, tying several
other seasons as the second largest ACE in the 1950-2017 record.
a. North Pacific and North America
The 500-hPa circulation during October
featured above-average heights over the Gulf of Alaska and eastern Canada. This
anomaly pattern contributed to a well-defined trough over central North America.
(Fig. E9). This overall
pattern contributed to anomalously warm conditions in Alaska, the northeastern
U.S., and eastern Canada (Fig. E1). It contributed to above-average precipitation
in the mid-western and eastern U.S. (Fig.
with the Great Lakes region recording totals in the upper 90th
percentile of occurrences (Fig. E5).
circulation featured above-average heights over western
Europe and below-average heights over northeastern Europe and western Russia (Fig. E9). This
pattern was associated with exceptionally warm (Fig. E1) and dry conditions (Fig. E3) in southern and western
Europe, with southern Europe recording rainfall totals in the lowest 10th
percentile of occurrences (Fig. E4). The
pattern was also associated with above-average precipitation in northeastern
Europe and western Russia.
c. Southeastern Asia/
circulation featured an amplified ridge extending from the Middle East to
northeastern China (Fig. T22). Over China, this pattern was
associated with a northward shift of the East Asian jet stream, and with
anomalous upper-level divergence (Fig. T21). At 850-hPa, the amplified ridge
was associated with an extensive onshore flow of marine air into China (Fig. T20).
These conditions contributed to well above-average precipitation in large
portions of the country, with country-wide totals above the 90th
percentile of occurrences (Fig. E4).
2. Southern Hemisphere
The mean 500-hPa circulation during
October featured above-average heights in the area southeast of Australia, and
below-average heights over southern South America (Fig. E15). At 200-hPa
the circulation featured an amplified trough in the subtropics that extended
across the central and eastern South Pacific Ocean. This pattern is
typical of La Niña, and was associated with a reduced strength of the
mid-latitude jet stream across most of the Pacific basin.
The main surface temperature signal
during October was well above-average temperatures in eastern Australia (Fig. E1).
Rainfall in northeastern Australia was well above average, with area-averaged
totals above the 90th percentile of occurrences (Fig. E4).
This marks the first month for above-average precipitation in this region since
The Antarctic ozone hole typically
develops rapidly during August and reaches peak size in September. The ozone
hole then gradually decreases during October and November, and dissipates in
early December (Fig. S8). During October 2017, the ozone hole size was smaller than the 2007-2016 mean, but still
averaged about 12 million square kilometers. This below-average size was
associated with a reduced size of the SH polar vortex and with a sharply
reduced amount of polar stratospheric cloud (Fig. S4).