Extratropical Highlights–October 2024

 

1. Northern Hemisphere

During the month of October, the 500-hPa height pattern featured anomalous ridging over most of North America, the central Pacific Ocean, and Europe, and anomalous troughing over the Norwegian Sea (Fig. E9). The main land-surface temperature signals include above average temperatures for North America and Eurasia (Fig. E1). The main precipitation signals include below average rainfall in North America and slightly above average rainfall in Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during October featured strong above average height anomalies for much of North America and weak below average height anomalies off the coast of British Columbia and Nunavut, Canada (Fig. E9). The anomalous ridging contributed to widespread above average temperatures for Canada and the U.S. Many areas recorded large departures from normal for the month of October and reached the highest 90th percentile of occurrences (Fig. E1). Rainfall totals were below average with many areas reaching the lowest 10th percentile of occurrences (Fig. E3). The region bordered by Texas, Oklahoma, Arkansas, and Louisiana recorded departures from normal that exceeded 150mm and reached the lowest (<10th) percentile of occurrences, along with the Great Plains, Midwest, Ohio Valley, and Southeast (Figs. E5, E6). Other areas recorded below average rainfall in the lowest 20th percentile of occurrences such as Southern California and the Southwest (Fig. E5).

 

b. Eurasia

Across much of Europe, the 500-hPa height pattern featured above average height anomalies during the month of October, weak below average heights were recorded over central Siberia, and moderate above average heights were recorded over Manchuria, China (Fig. E9). Temperatures were above average across most of Eurasia with many areas in Europe reaching the highest 90th percentile of occurrences (Fig. E1). Below average temperatures were recorded for the coastal regions near the Barents and Kara Seas (Fig. E1). Rainfall across Eurasia was largely near normal. Southern Europe and central Ukraine recorded above average rainfall during October, and the region north of the eastern Mediterranean Sea recorded below average rainfall with many areas recording totals in the lowest 10th percentile of occurrences (Figs. E3, E4).

 

c. West African monsoon

The west African monsoon season extends from June through October, with a peak during July-September. During 2024, the west African monsoon system has been overall enhanced, starting in June with many areas exceeding the highest 90th percentile of occurrences, and also in the months following June (Fig. E4). Rainfall totals remained above normal through the month of October across the Sahel (Figs. E3, E4).

 

2. Southern Hemisphere

The 500-hPa height field during October featured a wavetrain pattern with above average height anomalies alternating with below average height anomalies in the middle and polar latitudes (Fig. E15). The main temperature signals were above average anomalies for South America and Australia with many of those regions reaching the highest 90th percentile of occurrences, and moderate above average anomalies for most of Africa (Fig. E1). Below average rainfall was recorded for most of South America, with a narrow region along the Andes Mountain Range that recorded above average rainfall (Figs. E3, E4). Near normal rainfall was recorded for most of Australia with the exception of the region near the Great Dividing Range where below average rainfall was recorded (Figs. E3, E4).

The South African monsoon season runs from October to April. During October 2024, this area recorded above average rainfall, with many locations recording totals in the highest 70th percentile of occurrences (Figs. E3, E4).

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 October 2024, the size of the ozone hole was just over 10 million square kilometers, which is near normal for the 2014-2023 period. Polar stratospheric clouds (PSCs) were near normal, while the vortex area maintained slightly below average conditions (Fig. S8).