Extratropical Highlights – April 2024

 

1. Northern Hemisphere

The 500-hPa circulation during April featured near climatological heights over most of North America, and moderate to strongly above average heights over Greenland, Southern Europe, and East Asia (Fig. E9). Weakly below average heights were observed over the North Atlantic Ocean and strongly below average heights were observed over the Laptev Sea north of Russia (Fig. E9). The main land-surface temperature signals during April include above average anomalies across North America, Europe, and Asia, and below average anomalies along coastal Scandinavia (Fig. E1). The main precipitation signals during April include above average rainfall in central North America, Scandinavia and eastern Siberia (Fig. E3).

 

a. North America

The height pattern across much of North America was near normal for the month of April. Across southern Alaska and the Southeast U.S, height anomalies were weakly positive (Fig. E9). Temperatures were predominantly above average with much of the region reaching the highest 70th and 90th percentile of occurrences (Fig. E1). Above average rainfall was recorded across the Great Plains, Midwest, and Great Lakes with many areas reaching the highest 70th percentile of rainfall occurrences, or higher (Figs. E3, E5, E6). Rainfall totals were below average for the Pacific Northwest and Alaska Panhandle (Figs. E3, E4).

 

b. Eurasia

The East Atlantic (EA) pattern was strongly amplified during April with teleconnection indices reaching at least 3 standard deviations from normal (Fig. E7). The typical features of the EA pattern are recognizable in the observed temperature and precipitation across Europe for the month of April. The positive phase of the EA pattern likely contributed to the above average temperatures observed across all of Europe, where departures from normal were in the highest 90th percentile of occurrences for most of the region (Fig. E1). The EA pattern also likely contributed to the above average rainfall in Northern Europe and the below average rainfall in Southern Europe, where departures from average were, respectively, in the highest 90th and lowest 10th percentile of occurrences (Figs. E3, E4). Temperatures were also above average in central Russia and across much of Asia with many areas reaching at least the highest 70th percentile of occurrences and the highest 90th percentile of occurrences across Asia (Fig. E1). Precipitation totals were above average in east Asia and below average in the vicinity of the Sea Okhotsk (Fig. E3).

 

2. Southern Hemisphere

The 500-hPa height pattern for the month of April featured an anomalous maxima of above average heights centered over each ocean basin and an anomalous minima in heights over most of Antarctica and to the south of Madagascar (Fig. E15). Temperatures were above average in the observed areas of record across South America and most of Africa, where temperatures were recorded in the highest 70th and 90th percentile of occurrences in these regions (Fig. E1). Temperatures were anomalously below average for the interior regions of Australia and broadly reached the lowest 30th percentile of occurrences (Fig. E1). Across most of the northern half of South America, precipitation totals were below average for April and reached the lowest 10th percentile of occurrences along the border of Columbia and Brazil (Fig. E3). Rainfall totals were above average for southeast South America and below average for southern Chile (Figs. E3, E4). Precipitation totals were elevated for the Sahel region of Africa and Southern Africa (Figs. E3, E4). The South African monsoon season runs from October to April. After drier than average conditions were recorded for March, the rainfall totals in April were observed in the highest 90th percentile of occurrences (Figs. E3, E4).