Extratropical Highlights –September 2019

 

1. Northern Hemisphere

The 500-hPa circulation during September featured above-average heights over the eastern North Pacific Ocean, the eastern U.S., southern Europe, and the polar region, and below-average heights over the northwestern U.S. and central Russia (Fig. E9). The main land-surface temperature signals included above-average temperatures in Alaska, across the central and eastern U.S., in much of Europe, and in most of China (Fig. E1). The main precipitation signals included above-average totals across the northern tier of the U.S. and below-average totals in the eastern U.S. (Fig. E3).

 

a. North America

The 500-hPa circulation during September 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 the eastern U.S., and an amplified trough in the northwestern U.S.. These conditions contributed to above-average surface temperatures in Alaska, and across most of the U.S. east of the Rocky Mountains (Fig. E1).

The anomalous trough contributed to above-average precipitation in the northern tier of the U.S. (Fig. E3), with area-averaged totals exceeding the 90^th percentile of occurrences in both the Pacific Northwest and Great Plains regions (Fig. E5). Conversely, in the eastern U.S., the anomalous ridge contributed to well below-average rainfall, with area-averaged totals in the lowest 10^th percentile of occurrences in the Ohio Valley, Gulf Coast, and Mid-Atlantic regions (Fig. E5).

 

b. West African monsoon

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

 

 

2. Southern Hemisphere

The 500-hPa height field during September featured above-average heights over southern Australia, the high latitudes of the eastern South Pacific, and in the area south of Africa, and below-average heights over the central South Pacific and central Indian Ocean (Fig. E15). In southern Australia, the anomalous ridge contributed to warmer (Fig. E1) and drier (Fig. E3) than average conditions. In South America, the anomalous upstream ridge contributed to exceptionally dry conditions over much of the continent.

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 September 2019, the size of the ozone hole was only about 7.5 million square kilometers, which is well below the 2008-2017 average size of 20 million square kilometers. This significantly reduced size of the ozone hole reflected a markedly reduced size of the polar vortex (22 million square kilometers compared to the average of 32 million), along with a complete disappearance of polar stratospheric clouds (PSCs) (Fig. S8). On average, the PSC area covers about 15 million square kilometers by the end of September. These highly anomalous stratospheric conditions were associated with a pronounced stratospheric warming (Fig. S4) and with well above-average heights throughout the polar stratosphere at levels above 50-hPa (Fig. S1).