Extratropical Highlights –September 2016

 

1. Northern Hemisphere

The 500-hPa circulation during September featured above-average heights over eastern North America, northern Europe and central Russia, and below-average heights over the high latitudes of the North Atlantic (Fig. E9).

The main land-surface temperature signals during September included well above-average temperatures in eastern North America, Europe, and central Russia (Fig. E1). The main precipitation signals included above-average totals in the north-central and southeastern U.S., and below-average totals along the U.S. Gulf Coast, the northeastern U.S., and northern Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during September featured a broad trough over western North America and above-average heights over eastern North America (Fig. E9). This pattern was associated with well above-average surface temperature across the eastern half of North America, with many areas recording departures exceeding the 90^th percentile of occurrences (Fig. E1).

Precipitation during September was above average in both the north-central and southeastern U.S., and below-average along the U.S. Gulf Coast and in the northeastern U.S. (Fig. E3). Area-averaged rainfall totals in the northeastern U.S. have been below-average for the past seven months (Fig. E5).

According to the U.S. Drought Monitor, exceptional or extreme drought continued during September across central and southern California, and was also observed in northern Georgia, northern Alabama, western New York, and large portions of eastern New England. Severe drought was evident in eastern Oregon, western Nevada, and central/ northern Mississippi.

 

b. Europe/ western Russia

The 500-hPa circulation during September featured above-average heights over Europe and central Russia, and below-average heights over the high latitudes of the North Atlantic (Fig. E9). This overall pattern was associated with a continuation of above-average surface temperatures across Europe and central Russia, with departures in many areas exceeding the 90^th percentile of occurrences (Fig. E1). Northern Europe also recorded below-average precipitation during September, with area-average totals near the lowest 10^th percentile of occurrences (Fig. E4).

 

 

c. Northern Africa

The West African monsoon season, which lasts from June-September, was above average this year (Fig. E3). Area-averaged totals were above average during each month from June-September 2016, with the June and July totals nearing the 100^th percentile of occurrences and the September totals nearing the 90^th percentile of occurrences (Fig. E4).

 

2. Southern Hemisphere

The mean 500-hPa circulation during September featured below-average heights over most of Antarctica. It also featured an anomalous zonal wave-3 pattern in the middle latitudes, with above-average heights over the central ocean basins and below-average heights over the high latitudes of the South Pacific and southern Australia (Fig. E15).  The anomalous trough over southern Australia contributed to well below-average surface temperatures in the southwest (Fig. E1) and to above-average precipitation in the east (Fig. E3). Northeastern Australia has recorded well above-average precipitation during the last four months (Fig. E4).

The Antarctic ozone hole typically develops during August and reaches peak size in late September. It then typically decreases in size during October and November, and dissipates in early December (Fig. S8). During September 2016, the size of the ozone hole was below the average of the 2006-2015 period. This decreased size was associated with anomalously low areal extents of both the SH polar vortex and polar stratospheric clouds (Fig. S8).