– 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
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.
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 150oE-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 +5oC and
well above the 90th 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 90th percentile of occurrences and precipitation
departures in the lowest 10th 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
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.