Extratropical Highlights – April 2012

 

1. Northern Hemisphere

The 500-hPa circulation during April featured a persistent anomalous wave pattern from North America to central Russia (Figs. E9, E11). Regional aspects of this pattern included above-average heights over central North America, the central North Atlantic, and western/ central Russia, and below-average heights over Europe.

In the subtropics, the 200-hPa circulation featured cyclonic streamfunction anomalies in both hemispheres near the Date Line (Fig. T22). This pattern is linked to La Niña, and reflects enhanced mid-Pacific troughs in both hemispheres flanking the suppressed convection over the central equatorial Pacific (Fig. T25).

The main land-surface temperature signals during April included well above-average temperatures across the central United States and western/ central Russia (Fig. E1). The main precipitation signals (Figs. E3, E6) included above-average totals in the U.S. Pacific Northwest, western Canada, and much of Europe, and below-average totals in the eastern U.S. and southwestern Russia.

 

a. North Pacific and North America

The circulation over the North Pacific continued to exhibit a La Niña influence.  La Niña is associated with deep tropical convection focused over Indonesia and the eastern Indian Ocean, along with a disappearance of tropical convection from the central equatorial Pacific (Fig. T25). This westward retraction in the area of deep convection acts to amplify the mid-Pacific troughs at 200-hPa in both hemispheres (Fig. T22), which in the NH results in a westward retraction the East Asian jet stream. During April, the East Asian jet core remained focused near Japan, and the jet exit region was shifted to well west of the date line (Fig. T21).

Over North America, the mean 500-hPa circulation during April featured an amplified wave pattern, with troughs along the west and east coasts, a strong ridge over the center of the continent (Fig. E9). These conditions were associated with exceptionally warm surface temperatures across the central U.S., with many areas recording departures in excess of +3^oC and above the 90^th percentile of occurrences (Fig. E1). They were also associated with an enhanced storm track (Fig. E13) and above-average precipitation across the Pacific Northwest (Figs. E3, E5), and with above-average precipitation in western Canada.

Below-average precipitation was recorded across the eastern one-third of the U.S. in the area downstream of the mean ridge axis. Monthly totals in the Great Lakes region were in the lowest 10^th percentile of occurrences, and the mid-Atlantic and Northeast regions received below-average totals for the sixth straight month (Fig. E5). By the end of April, extreme or exceptional drought was present across Georgia, southeastern Alabama, and much of northern Florida. Severe drought conditions developed across the Delmarva Peninsula and portions of New England. Exceptional drought conditions persisted in western Texas and southeastern New Mexico.

 

b. Europe and Russia

The 500-hPa circulation during April featured a deep trough across Europe and a strong ridge over western/ central Russia (Figs. E9, E11). This pattern contributed to exceptionally wet conditions across Europe (Fig. E3), with area-averaged totals in both Northern and Southern Europe near the 90^th percentile of occurrences (Fig. E4). The persistent upper-level ridge was associated with exceptionally warm surface temperature across western/ central Russia, with many areas recording departures exceeding +5^oC and in the upper 90^th percentile of occurrences.

 

2. Southern Hemisphere

In the extratropics, the mean 500-hPa circulation during April featured above-average heights in the middle latitudes and below-average heights across the high latitudes of the South Pacific Ocean (Fig. E15). At 200-hPa, the subtropical circulation featured an enhanced mid-Pacific trough in response to the suppressed convection over the central equatorial Pacific (Figs. T22, T25).  A similar anomaly pattern was also evident in the Northern Hemisphere, and is consistent with La Niña.

The South African rainy season lasts from October to April. During April, rainfall was below average for the region as a whole, with many areas recording totals in the lowest 30^th and 10^th percentile of occurrences (Figs. E3, E4). For the 2011-12 rainy season, precipitation was near average during October-January and March, and below average during February and April (Fig. E4). Seasonal rainfall in this region is often above average during La Niña.