Extratropical Highlights – April 2023

 

1. Northern Hemisphere

The 500-hPa circulation features during April resembled a wintertime negative North Atlantic Oscillation (NAO) teleconnection pattern with anomalous above-average heights over Greenland and below-average heights over the North Atlantic Ocean (Fig. E9).  Moderate above-average height anomalies were observed over eastern Russia, the North Pacific Ocean, Scandinavia, and parts of Europe and the Mediterranean.  Moderate below-average heights were observed over Alaska (Fig. E9).  The main land-surface temperature signals during April included above-average temperatures for Europe, eastern Russia, and parts of Canada and the U.S., and below-average temperatures for Alaska, central Canada, and central Asia (Fig. E1).  The main precipitation signals during April were above-average rainfall totals in part of the U.S., parts of Europe and east Russia, and below-average totals in the Mediterranean and central U.S. (Fig. E3).

 

a. North America

The height pattern across much of North America was near-normal for the month of April.  Anomalous below-average heights were observed over Alaska, and above-average heights were observed flanking both U.S. coasts (Fig. E9).  Across Alaska, central Canada, and off the coast of the Pacific Northwest, below-average temperatures were observed with departures from normal reaching the 30th percentile of occurrences (Fig. E1).  Mildly above-average temperatures were observed along both U.S. coasts and Southwest region (Fig. E1).  Precipitation anomalies were also near-normal for much of North America.  Exceptions included above-average rainfall north of the Great Lakes region, the Mid-Atlantic Seaboard, and the Gulf Coast states (Figs. E3, E5). Anomalous below-average precipitation was recorded in the central U.S. and West Coast (Figs. E3, E5).

 

b. Eurasia

Across Europe, above-average heights were observed, followed by below-average heights in central Russia (Fig. E9).  This pattern contributed to the above-average temperatures recorded across eastern and western Europe with some western European countries recording temperatures in the 90th percentile of occurrences (Fig. E1).  Below-average temperatures were recorded across Scandinavia and central Asia (Fig. E1).  Precipitation patterns across Eurasia were predominantly near-normal with the exception of below-average rainfall across Spain, and above-average rainfall in neighboring regions to the east (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 Antarctica that also extended toward the Indian Ocean and South Pacific Ocean (Fig. E15).  Moderate below-average heights were observed along the Southern Ocean, specifically south of South America, Africa, and Australia (Fig. E15).  The main land-surface signals included above-average temperatures across much of South America and below-average temperatures in western Australia (Fig. E1).  The departures from normal for these regions were in the 90th and 30th percentiles, respectively (Fig. E1).   The main precipitation signals were below-average rainfall across much of South America, above-average rainfall in central Africa, and near-normal rainfall across much of Australia (Fig. E3).  The South African monsoon season runs from October to April. Observed April rainfall totals were recorded in the lowest 30th percentile of occurrences bringing a rather dry end to the monsoon season for the region (Figs. E3, E4).