Extratropical Highlights – March 2012

 

1. Northern Hemisphere

The 500-hPa circulation during March featured a persistent zonal wave-3 pattern of height anomalies (Figs. E9, E11). Regional aspects of this pattern included above-average heights over the central North Pacific, eastern North America, and Europe, and below-average heights over western North America, western Russia, and eastern Siberia.

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

The main land-surface temperature signals during March included well above-average temperatures across the eastern half of North America and all of Europe, and below-average temperatures in Alaska and western Russia (Fig. E1). The main precipitation signals (Figs. E3, E6) included above-average totals in the U.S. Pacific Northwest and portions of the southern Plains states, and below-average totals along the U.S. eastern seaboard and across Europe.

 

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 mean 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, along with a westward-shift and amplification of the jet exit region (Fig. T21). During March, the East Asian jet core was focused over the western Pacific, and the jet exit region began well west of the date line (Fig. T21). The jet exit region was also enhanced between 150^oE-180, as indicated by anomalous southeasterly winds immediately south of the jet axis.

Over North America, the mean 500-hPa circulation during March featured a deep trough in the West and a ridge in the East (Fig. E9). This pattern is opposite to the climatological mean, which features a ridge in the West and a trough in the East. These conditions were associated with exceptionally warm surface temperature across the eastern half of North America, with most areas recording departures in excess of +5^oC and well above the 90^th percentile of occurrences (Fig. E1). They were also associated with an enhanced storm track across the Pacific Northwest (Fig. E13), which contributed to well above-average precipitation in that region (Figs. E3, E5).

In contrast, below-average precipitation was recorded along the U.S. eastern seaboard, with the mid-Atlantic and Northeast regions receiving below-average totals for a fifth straight month (Fig. E5). These ongoing precipitation deficits have led to severe or exceptional drought across the southern half of Georgia and portions of northern and western Florida. Exceptional drought conditions also persisted in western Texas.

 

b. North Atlantic and Eurasia

The 500-hPa circulation during March featured a strong ridge across Europe and a deep trough over western Russia (Figs. E9, E11). This pattern contributed to exceptionally warm (Fig. E1) and dry (Fig. E3) conditions across Europe, with many areas recording temperature departures above the 90^th percentile of occurrences and precipitation departures in the lowest 10^th percentile of occurrences. For the northern half of Europe, area-averaged precipitation totals were the lowest in the record dating back to 1979 (Fig. E4).

 

2. Southern Hemisphere

In the extratropics, the mean 500-hPa circulation during March was remarkably near-normal, with the exception of large positive height anomalies south of New Zealand (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, along with an enhanced ridge over the Indian Ocean in association with enhanced convection across Indonesia (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 March, rainfall was near average for the region as a whole (Figs. E3, E5). For the 2011-12 rainy season, precipitation has been near average in every month but February. Seasonal rainfall in this region is often above average during La Niña.