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 Mexico.

 

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. E3).

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 monsoon

The west 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 30^th 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 90^th 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).