Extratropical Highlights – October 2019

 

1. Northern Hemisphere

The 500-hPa circulation during October featured above-average heights across the high latitudes of the North Pacific, eastern North America, and southern Europe, and below-average heights over central North America, western Europe, and Scandinavia (Fig. E9). The main land-surface temperature signals included above-average temperatures in Alaska, the eastern U.S., Europe, and central Russia, and below-average temperatures in the western U.S. and southwestern Canada (Fig. E1). The main precipitation signals included above-average totals in western Alaska and the eastern U.S., and below-average totals in the northwestern U.S. and western Canada (Fig. E3).

 

a. North America

The 500-hPa circulation during October featured an anomalous wave pattern extending from the eastern North Pacific to the eastern U.S. (Fig. E9). This pattern reflected amplified ridges over the eastern North Pacific and eastern North America, and an amplified trough in the north-central U.S. These conditions contributed to above-average surface temperatures in Alaska and the eastern U.S., and to well below-average temperatures in much of the western U.S. (Fig. E1). The amplified trough also delineated areas of below-average precipitation in the northwestern U.S. and western Canada from areas of above-average precipitation in the central and eastern U.S. (Fig. E3).

 

b. Europe and Siberia

The 500-hPa circulation during October featured above-average heights across southern Europe eastward to the Caspian Sea, and below-average heights over Great Britain and Scandinavia (Fig. E9). This pattern was associated with enhanced lower-level (Fig. E8) and upper-level (Fig. E10) westerly winds extending from central and northern Europe to central Russia. This anomalous flow of relatively mild marine air contributed to well above-average surface temperatures in these areas, with departures exceeding the 90^th percentile of occurrences across much of Europe (Fig. E1).

 

c. West African monsoon

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

 

 

2. Southern Hemisphere

The 500-hPa height field during October featured above-average heights over southern Australia, the high latitudes of the central South Pacific, and Antarctica, and below-average heights in the area south of Africa (Fig. E15). In southeastern Australia, the anomalous ridge contributed to warmer (Fig. E1) and drier (Fig. E3) than average conditions, with most areas recording temperature departures in the upper 90^th percentile of occurrences (Fig. E1) and precipitation totals in the lowest 10^th percentile of occurrences (Fig. E3).

The South African monsoon season runs from October to April. During October 2019, this area recorded well below-average precipitation, with many locations recording totals in the lowest 10^th percentile of occurrences (Fig. E3). Area-averaged totals during October were in the lowest 1^st 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, top). By the end of October 2019, the size of the ozone hole was only about 3 million square kilometers, which is well below the 2008-2017 average size of 12.5 million square kilometers.

Overall, the spatial extent and duration of the 2019 ozone hole were significantly below average. During October, these conditions were associated with a markedly reduced size of the polar vortex (20 million square kilometers compared to the average of 29 million) (Figs. S8, middle), along with a complete absence of polar stratospheric clouds (PSCs) (Figs. S8 bottom, E15). These highly anomalous stratospheric conditions began in early-September in association with a pronounced stratospheric warming (Fig. S4) and the development of well above-average heights throughout the polar stratosphere at levels above 50-hPa (Fig. S1), which resulted in a complete disappearance of PSC’s and a dramatic lessening of the ozone hole.