Extratropical Highlights –September 2017
1. Northern Hemisphere
The 500-hPa circulation during
September featured above-average heights over north-central and eastern Canada
and western Russia, and below-average heights over the high latitudes of the eastern
North Atlantic (Fig. E9). At 200-hPa, the circulation featured amplified
ridges over the western North Atlantic and also over China (Fig. T22).
The main land-surface temperature signals
during September included above-average temperatures in northwestern and
eastern Canada, Scandinavia, the Middle East and China (Fig. E1). Surface temperatures were below
average in portions of central Russia.
The main precipitation signals
included above-average totals in the Plain states and southeastern U.S. and in
portions of eastern Europe, and below-average totals
in the mid-western U.S., eastern Canada, and southwestern Europe (Fig. E3).
The Atlantic hurricane season
remained extremely active during September, with five hurricanes of which four
became major hurricanes (MH). Two of the MH’s were cat.-5 storms (Irma and
Maria), one was a cat. 4 storm (Jose), and one was a cat. 3 storm (Lee). Irma
devastated the northern Caribbean Islands before making landfall in western
Florida. Marie devastated Puerto Rico. Hurricane Katia made landfall in eastern
a. North America
The 500-hPa circulation during
September featured above-average heights across north-central and eastern
Canada and the north-central U.S., along with troughs over both the western and
eastern U.S. (Fig. E9).
This pattern contributed to anomalously warm conditions in northwestern and
eastern Canada (Fig. E1). It also contributed to above-average
precipitation in the U.S. Plains states, and to below-average precipitation in
the mid-western U.S. and eastern Canada (Fig.
At 200-hPa, the circulation
featured an amplified ridge over the western North Atlantic (Fig. T22), which created exceptionally
weak vertical wind shear in the western portion of the Atlantic hurricane
basin. This pattern, combined with warm AMO conditions and exceptionally
warm SSTs across the tropical Atlantic and Caribbean Sea (Fig. T18),
resulted in extremely strong Atlantic hurricane activity during the month with
Cat.-5 hurricanes striking both the northern Caribbean Islands and Puerto Rico.
c. West African
African monsoon season extends from June through September, with a peak during
July-September. During September 2017, the west
African monsoon system had below-average precipitation (Fig. E3) with area-average totals near
the 30th percentile of occurrences (see Sahel region, Fig.
E4). This region had previously recorded well
above-average precipitation during May-August. An enhanced west
African monsoon, along with above-average SSTs across the tropical Atlantic and
Caribbean Sea (Fig. T18), typify the warm phase of the
Atlantic Multi-Decadal Oscillation (AMO). These conditions are also typical
features of an active Atlantic hurricane season.
2. Southern Hemisphere
The mean 500-hPa circulation during
September featured an anomalous zonal wave-3 pattern, with above-average heights
over the three central ocean basins, and below-average heights near New Zealand
and over much of Antarctica (Fig. E15). This pattern was associated with a
continuation of exceptionally warm surface temperatures (Fig. E1) in southeastern South America, where departures were again in the
upper 90th percentile of occurrences (Fig. E5).
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). The 2017 ozone hole had reached nearly 20
million square kilometers by early September, but then shrunk to 13- 15 million
square kilometers by mid-month. This well below-average size was associated
with a reduced SH polar vortex, a sharp reduction in the amount of polar
stratospheric cloud, and above-average temperatures in the polar stratosphere near
the 10 hPa level (Fig. S4).