Extratropical Highlights –September 2024

 

1. Northern Hemisphere

The 500-hPa circulation during September featured above average heights across most of North America and central Russia, and weakly below average height anomalies over the Alaska Panhandle and Europe (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 Europe and below average totals in western portions of North America (Fig. E3).

 

a. North America

The 500-hPa circulation over North America featured predominantly above average height anomalies. The largest departures from average in the height field were located over the Hudson Bay and Cumberland Peninsula (Fig. E9). Centered over the Alaska Panhandle was a weak tilt toward below average heights, while near normal heights were recorded for the southern tier of the U.S. (Fig. E9). This pattern contributed to the moderate and strongly above average temperature anomalies across most of Canada and the U.S., with many areas reaching the highest 90th percentile of occurrences (Fig. E1). The main precipitation signals include above average rainfall totals across the Ohio Valley, Southeast, and Gulf Coast regions in the U.S., with many areas recording rainfall at the highest 90th percentile and higher (Figs. E3, E5, E6). Across the Northeast, Great Lakes, Great Plains, Southwest, Southern California, and Inter-Mountain West, below average rainfall was recorded with the Great Plains receiving rainfall in the lowest 10th percentile of occurrences (Figs. E3, E5, 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 central Russia and moderately above average heights over the Korean Peninsula (Fig. E9). Moderately below average heights were observed over the Stanovoy Mountain Range in eastern Russia, and weakly below average heights near the Bay of Biscay and central Europe (Fig. E9). Temperatures were above normal for a large portion of Scandinavia, eastern Europe and western Russia, as well as across southern China, where many of these areas reached the highest 90th percentile of occurrences (Fig. E1). Precipitation totals were above normal across Northern and Southern Europe, central and eastern Russia, and southern Asia, with many of these regions reaching the highest 90th (or greater) percentile of occurrences (Fig. E3, E4). Western Russia recorded below average rainfall with a large area recording totals in the lowest 10th percentile of occurrences (Fig. E3).

 

c. West African monsoon

The west African monsoon extends from June through September, with a peak during July-September. Since June, the Sahel region has recorded rainfall in the highest 90th (and greater) percentile each month (Figs. E3, E4). During September 2024, the west African monsoon system was enhanced with a continuation of strongly above average rainfall for the region (Figs. E3, E4)

 

2. Southern Hemisphere and Ozone Hole

The 500-hPa height field during September featured a wavetrain pattern of alternating above and below average height anomalies (Fig. E15). Two maxima were recorded. One maxima was recorded in the South Atlantic Ocean and the other maxima in the South Pacific Ocean (Fig. E15). The strongest minima in heights was located over the Drake Passage (Fig. E15). Temperatures across South America, Africa, and Australia were near-to-above normal, with some areas reaching the highest 90th percentile of occurrences (Fig. E1). Precipitation totals were well below average for much of South America, parts of central Africa (south of the Sahel region), and southern Australia, where many of these areas recorded rainfall in 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 2024, the size of the ozone hole approached 20 million square kilometers, which is similar to the 2014-2023 average size of the ozone hole (Fig. S8). Associated with the ozone hole during September was a slightly below average polar vortex area and a slightly above average polar stratospheric cloud coverage area, although this area appears to be trending downward rapidly as October begins (Fig. S8).