Extratropical Highlights –August 2020

 

1. Northern Hemisphere

The 500-hPa circulation during August featured above-average heights over the North Pole, the mid-latitudes of North Pacific, the northwestern U.S. and northern Canada, and below-average heights over the northern North Pacific, western Canada, the northern North Atlantic, and the Caspian Sea (Fig. E9). The main land-surface temperature signals included above-average temperatures in the western U.S., western Europe, and most of the high-latitudes of central Russia (Fig. E1). The main precipitation signals included above-average precipitation in the eastern and southeastern U.S., and below-average precipitation in the western and central U.S. and southern Florida (Fig. E3).

 

a. North America

The 500-hPa circulation during August featured a broad ridge cross the inter-mountain region of the western U.S. and the central U.S., and a broad trough over eastern Canada and the eastern U.S. (Fig. E9). This pattern contributed to above-average surface temperatures in the western and central U.S., and to near- or below-average temperatures in the eastern U.S. (Fig. E1).

Also during August, area-averaged total precipitation in the Southern California, Inter-Mountain, and Southwestern U.S. regions were less than the 10^th percentile of occurrences, while totals in the Ohio Valley and Mid-Atlantic regions were larger than the 90^th percentile of occurrences (Fig. E5).

 

b. Europe and Asia

The 500-hPa height pattern featured a wave pattern with above-average heights in western Europe and the eastern half of Siberia, and below-average heights over the Caspian Sea (Fig. E9). This circulation pattern contributed to above-average surface temperatures in central Russia, with the largest departures (+3.0 to +4.0°C) observed (Fig. E1). Much of the mid-latitudes of Asia and eastern Siberia recorded near-average surface temperatures. The significant departures were observed in the low-latitudes of Asia, where some of the region experienced temperatures in the 90^th percentile of occurrences.

 

c. West African monsoon

Climatologically, the west African monsoon extends from June through September, with a peak during July-September. During August, the west African monsoon system strengthened in the north and weakened in the south (Figs. E3, T23, T24) with area-average rainfall totals near the 100^th percentile of occurrences (see Sahel region, Fig. E4). In July, this region also recorded totals near the 100^th percentile of occurrences.

 

 

2. Southern Hemisphere

The 500-hPa height field during August featured above-average heights over the South Pole, and a wave-3 pattern in the mid- and high latitudes with above-average centered near 160^oE, 75^oW and 10^oE, and below-average centered near 135^oW, 30^oW and 75^oE  (Fig. E3).

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). By the end of this August, the size of the ozone hole was below its 2010-2019 average. Associated with the ozone hole during this August, the size of the Southern Hemisphere polar vortex and the areal extent of polar stratospheric clouds were both above- to near-average (Fig. S8).