Extratropical Highlights –August 2019

 

1. Northern Hemisphere

The 500-hPa circulation during August featured above-average heights over western Alaska, northeastern Canada, southeastern Europe, and eastern Siberia, and below-average heights over central Canada, the eastern North Atlantic, and the northern North Pacific Ocean (Fig. E9). The main land-surface temperature signals included above-average temperatures in the western, southern, and eastern regions of the U.S., central Europe, and most of the eastern half of Asia (Fig. E1). The main precipitation signals included above-average totals in the central and northern Plains states of the U.S. and below-average totals in the southwestern U.S. and Texas (Fig. E3).

 

a. North America

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

Also during August, above-average precipitation was recorded in the central and northern Plains states of the U.S. (Fig. E3), with area-averaged totals in the Great Plains reaching the 90^th percentile of occurrences (Fig. E5). Elsewhere, rainfall was below-average in the southwestern U.S. and Texas, with many locations recording totals in the lowest 30^th percentile of occurrences (Fig. E3).

 

b. Europe and Asia

The 500-hPa height pattern featured above-average heights across south-eastern Europe and the eastern half of Siberia, and below-average heights over the eastern North Atlantic Ocean (Fig. E9). This pattern contributed to above-average surface temperatures across most of Europe, with the largest departures (+3.0 to +4.0°C) observed in eastern Europe (Fig. E1). Much of the eastern half of Asia also recorded above-average surface temperatures. The most significant departures were recorded in China, where much of the country experienced temperatures in the upper 90^th percentile of occurrences.

 

c. West African monsoon

The west African monsoon extends from June through September, with a peak during July-September. During August 2019, the west African monsoon system was enhanced (Fig. E3, Fig. T23, Fig. T24) with area-average rainfall totals above the 90^th percentile of occurrences (see Sahel region, Fig. E4). In July, this region also recorded totals above the 90^th percentile of occurrences.

 

 

2. Southern Hemisphere

The 500-hPa height field during August featured above-average heights over the high latitudes of the eastern South Pacific, across the central South Atlantic Ocean, and in the area south of Australia, and below-average heights over the central South Pacific and central Indian Ocean (Fig. E15). In southeastern Australia, the above-average heights contributed to anomalously dry conditions (Fig. E3), with many locations recording rainfall totals in the lower 10^th percentile of occurrences.

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 August 2019, the size of the ozone hole approached 10 million square kilometers, which is below its 2008-2017 average size of 14 million square kilometers. Associated with the ozone hole during August 2019, the size of the Southern Hemisphere polar vortex and the areal extent of polar stratospheric clouds were both below-to near-average (Fig. S8).