Extratropical Highlights –September 2023

 

1. Northern Hemisphere

The 500-hPa circulation during September featured predominantly above-average heights with the largest above-average height anomalies centered over North America and Europe, and moderately below-average height anomalies over eastern Russia and Alaska (Fig. E9). The main land-surface temperature signals include above-average temperatures for most of the Northern Hemisphere (Fig. E1). The main precipitation signals include above-average totals across parts of Asia and below-average totals in Europe and most of North America (Fig. E3).

 

a. North America

The 500-hPa circulation over North America featured strongly above-average height anomalies centered over the Hudson Bay, slightly above-average heights over the south-central U.S., near-normal heights over most of the continental U.S., and slightly below-average heights over Alaska (Fig. E9). This pattern contributed to the moderate and strongly above-average temperature anomalies recorded for much of Canada and the U.S, with most areas reaching the 90th percentile of occurrences, and near-normal to slightly below-average temperatures for Alaska (Fig. E1). The main precipitation signals include above-average rainfall totals for the north-central U.S. and the Rocky Mountain states and below-average rainfall totals across northern and eastern Canada and the eastern U.S. (Figs. E3, E6). The Alaska Panhandle also recorded below-average rainfall (Fig. E3).

 

b. Europe and Asia

The 500-hPa height pattern featured strongly above-average heights across Europe to central Russia, moderately above-average heights over east Mongolia, and slightly below-average heights over eastern Siberia (Fig. E9). Across most of Europe and Asia, temperatures were recorded in the highest 90th percentile of occurrences for the month of September (Fig. E1). Precipitation totals were below-average for Europe and reached the lowest 10th percentile of occurrences (Fig. E3). Precipitation totals were above-average across parts of Asia and reached the highest 90th percentile of occurrences (Fig. E3).

 

c. West African monsoon

The west African monsoon extends from June through September, with a peak during July-September. During September 2023, the west African monsoon system was near-normal to suppressed across the region (Fig. E3) with area-average rainfall totals at the 50^th percentile of occurrences (see Sahel region, Fig. E4).

 

2. Southern Hemisphere

The 500-hPa height field during September featured a tri-maxima, above-average height pattern over Antarctica, and across most of the South Pacific Ocean, and below-average heights above the surrounding Southern Ocean (Fig. E15). For much of Australia, central South America, and western Africa temperatures were above normal and reached the highest 90th percentile of occurrences while a small region in southern South America and southern Africa recorded below-average temperatures (Fig. E1). The main precipitation signals were either near-normal or below-normal for much of South America, Africa, and Australia with many areas reaching the lowest 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. By the end of September 2023, the size of the ozone hole approached nearly 21 million square kilometers, which is just slightly above the 2012-2021 average size of the ozone hole (Fig. S8). Associated with the ozone hole during September was a near-normal polar vortex area and near-normal polar stratospheric cloud coverage (Fig. S8).