Extratropical Highlights: August 2021

 

1. Northern Hemisphere

The 500-hPa circulation during August featured above-average heights over the northeastern North Pacific Ocean, eastern Canada, the north North Atlantic Ocean, eastern Europe, and eastern Siberia, and below-average heights over the north-central North Pacific Ocean and Beaufort Sea (Fig. E9). The main land-surface temperature signals included above-average temperatures in east-central North America and eastern Europe (Fig. E1). The main precipitation signals included above-average totals in the southeast and midwest states of the U.S. and eastern Europe (Fig. E3).

 

a. North America

Much of the area featured near-normal 500-hPa circulation anomalies with the exception of above-average heights over eastern Canada and the New England region of the U.S. (Fig. E9). This pattern contributed to above-average surface temperatures in the area with percentiles exceeding the 90th percent (Fig. E1).

Also during August, above-average precipitation was recorded in the southeast and midwest regions of the U.S. reaching the 90th percentile of occurrences (Figs. E3 and E5). While precipitation anomalies for the western half of the U.S. appear broadly near-normal (Fig. E3), other metrics indicate below-average rainfall with departures from normal at 25% for the region and reaching the 30th percentile of occurrences for the Pacific Northwest (Figs. E5 and E6), including the Drought Monitor which indicates prevailing Severe-Exceptional drought conditions for the west coast states.

 

b. Europe and Asia

The 500-hPa height pattern featured above-average heights across eastern Europe and eastern Siberia, and slight below-average heights over central Europe (Fig. E9). This pattern contributed to above-average surface temperatures across eastern Europe, with the largest departures (+4.0 to +5.0C) observed between the Caspian Sea and Ural Mountains (Fig. E1). Much of the eastern half of Asia also recorded above-average surface temperatures. Above-average rainfall was observed in eastern Europe and broadly along the southeastern-coastal boundaries of Asia (Fig. E3).

 

c. West African monsoon

The west African monsoon extends from June through September, with a peak during July-September. During August 2021, the west African monsoon system was enhanced (Fig. E3, Fig. T23, Fig. T24) with area-average rainfall totals above the 100^th percentile of occurrences (see Sahel region, Fig. E4). In July, this region also recorded totals above the 100^th percentile of occurrences.

 

 

2. Southern Hemisphere

The 500-hPa height field during August broadly featured near-normal heights with the exception of above-average heights over Wilkes Land and the Ross Sea of Antarctica, and below-average heights over the Drake Passage (Fig. E15). Near-normal precipitation anomalies were observed in Australia and South Africa, and below-average precipitation anomalies were observed in SE South America with dry conditions reaching less than the 10th percentile of occurrences (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 August 2021, the size of the ozone hole approached 19 million square kilometers, which is above its 2010-2019 average size of 14 million square kilometers. Associated with the ozone hole during August 2021, the size of the Southern Hemisphere polar vortex and the areal extent of polar stratospheric clouds were slightly- and above-average, respectively (Fig. S8).