Extratropical Highlights –November
2019
1. Northern Hemisphere
The 500-hPa circulation during November
featured above-average heights over western North America, Greenland, and western
Russia, and below-average heights over eastern North America and Europe (Fig. E9). The
main land-surface temperature signals included above-average temperatures in
Alaska and eastern Europe, and below-average
temperatures in the eastern U.S. and central Russia (Fig. E1). The main precipitation signals
included above-average totals in southwestern U.S. and central Europe, and
below-average totals in the northwestern and Gulf Coast regions of the U.S. (Fig. E3).
a. North America
The 500-hPa circulation during November
featured an amplified ridge-trough pattern across North America, with a strong
ridge in the west and an amplified Hudson Bay trough in the east (Fig. E9). This
pattern contributed to above-average surface temperatures in Alaska, and to
below-average temperatures in the eastern U.S. and southeastern Canada (Fig. E1). It also contributed to below-average precipitation within and
downstream of the mean ridge axis (Fig. E3), with the northwestern U.S. recording totals in the lowest 5th
percentile of occurrences (Fig. E5).
b. Europe
The 500-hPa circulation during November
featured an amplified wave pattern with below-average heights over Europe and
above-average heights over western Russia. This pattern was associated with
exceptionally warm surface temperature in eastern
Europe (Fig. E1), and with above-average
precipitation across central and southeastern Europe (Fig. E3). For the Southern Europe region as a whole, the
area-averaged precipitation total was in the upper 95th percentile
of occurrences (Fig. E4).
c. Atlantic hurricane
season
The 2019 Atlantic hurricane season
produced 18 named storms, with six becoming hurricanes and three of those
becoming major hurricanes. The 2019 Accumulated Cyclone Energy (ACE) value was
about 135% of the 1981-2010 median. Based on this activity, NOAA classifies the
season as being above normal. This marks the fourth consecutive above-normal
season. The only other period in the record (1950-present) that produced four
consecutive above-normal seasons was 1998-2001. An average season has 12 named
storms, six hurricanes, and three major hurricanes.
The above-normal activity is
consistent with the ongoing high-activity era for Atlantic hurricanes, which
began in 1995 in association with a transition to the warm phase of the
Atlantic Multi-Decadal Oscillation (AMO).
Conditions that favored more, stronger, and longer-lasting storms this
year included a stronger west African monsoon system (Fig. E4),
warmer Atlantic waters, and weak vertical wind shear across the western Atlantic
Ocean and Gulf of Mexico.
2. Southern Hemisphere
The 500-hPa height field during November
featured above-average heights over the polar region and in the area west of southern
Australia, and below-average heights to the south of eastern Australia, and
over the three central ocean basins (Fig.
E15). In Australia, the anomalous ridge-trough
pattern contributed to drier than average conditions (Fig. E1) across much of the continent, with many areas recording totals in
the lowest 10th percentile of occurrences (Fig. E3).
The South African monsoon season
runs from October to April. During November 2019, much of this area recorded below-average
precipitation (Fig. E3),
and area-averaged totals were in the lowest 30th percentile of
occurrences (Fig. E4).
The region recorded even stronger deficits in October.
The Antarctic ozone hole typically
develops during August and reaches peak size in September. The ozone hole then
gradually decreases during October and November, and dissipates on average in
early December (Fig. S8 top). During 2019, the ozone hole
disappeared in early November, which is far sooner than average. Overall, the
spatial extent and duration of the 2019 ozone hole were significantly below
average. This evolution reflected highly anomalous stratospheric conditions
that began in early September in association with a stratospheric warming event,
which led to a sharp reduction in the size of the polar vortex (Fig. S8 middle). These conditions resulted in a complete
disappearance of PSC’s (Fig. S8 bottom) and a dramatic lessening of the ozone
hole.