Extratropical Highlights –September 2021

 

1. Northern Hemisphere

The 500-hPa circulation during September featured above-average heights over the central and eastern portions of the North Pacific Ocean, and also extending from the Arctic Archipelago, to the North Atlantic Ocean, Scandinavia, and Siberia, and below-average heights over the the Gulf of Alaska, Chukchi Sea, central Asia, and Greenland (Fig. E9). The main land-surface temperature signals included above-average temperatures in much of North America, Europe, and Asia (Fig. E1). The main precipitation signals included above-average totals across southern and eastern Asia, the Gulf States of the U.S., and below-average totals in the central U.S. and Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during September resembled an anomalous wave pattern extending from the central North Pacific Ocean to the Arctic Archipelago (Fig. E9). This enhanced ridging over the western half of the U.S. and portions of Canada contributed to above-average surface temperatures for much of the region with the U.S. Central Plains exceeding the 90th percentile (Fig. E1).  Below-average rainfall was observed for the Central Plains, North-Atlantic States, and South Slope of Alaska (Fig. E3), with rainfall in the lowest 10th percentile for these regions.  Above-average rainfall was observed along the Alaska Panhandle, Northwest U.S., Mid-Atlantic states, and the central Gulf states (Fig. E3).

 

b. West African monsoon

The west African monsoon extends from June through September, with a peak during July-September. During September 2021, the west African monsoon system was enhanced (Figs. E3 and E4). For the July-September 2021 period as a whole, the monsoon was enhanced, with area-average rainfall totals near the 100^th percentile of occurrences (see Sahel region, Fig. E4).

 

 

2. Southern Hemisphere

The 500-hPa height field during September featured above-average heights over the South Pacific Ocean, the South Atlantic Ocean, and the Indian Ocean, and below-average heights over the Southern Ocean (Fig. E15). Conditions for Australia were near-normal for both temperature and precipitation with the exception of Northeast Australia which observed above average rainfall (Figs. E1, E3, E4).  In South America and Southern Africa, above-average temperatures were observed (Fig. E1).

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 2021, the size of the ozone hole was nearly 25 million square kilometers, which is well above the 2010-2019 average size of 18.5 million square kilometers. This significantly enhanced size of the ozone hole reflected a markedly enhanced size of the polar vortex (35 million square kilometers compared to the average of 32 million), along with near-average polar stratospheric clouds (PSCs) (Fig. S8). On average, the PSC area covers about 13 million square kilometers by the end of September. These anomalous stratospheric conditions were associated with near-normal stratospheric zonal mean temperatures (Fig. S4) and with well below-average heights throughout the polar stratosphere at levels above 100-hPa (Fig. S1).