Extratropical Highlights

  Table of Indices  (Table 3)

  Global Surface Temperature  E1

  Temperature Anomalies (Land Only)  E2

  Global Precipitation  E3

  Regional Precip Estimates (a)  E4

  Regional Precip Estimates (b)  E5

  U.S. Precipitation  E6

  Northern Hemisphere

  Southern Hemisphere

  Stratosphere

  Appendix 2: Additional Figures

Extratropical Highlights

MARCH 2011

Beginning with this month, all anomalies reflect departures from the 1981-2010 base period.

1. Northern Hemisphere

The 500-hPa circulation during March featured above average heights over the central North Pacific, Alaska, the southern United States, and northern Europe, and below average heights over the western North Pacific, the Gulf of Alaska, the southeastern North Atlantic, and the polar region (Fig. E9). At 200-hPa, the circulation featured enhanced troughs in both hemispheres over the tropical and subtropical central/eastern Pacific (Fig. T22).  This pattern is consistent with the ongoing mature phase of La Niña.

The main surface temperature signals during March included well above average temperatures across the south-central U.S., and eastern Siberia, and below average temperatures across western and central Canada, and portions of western China (Fig. E1). The main precipitation signals included above average totals in the western and eastern U.S., and southern Europe, and below-average totals along the U.S. Gulf Coast, northern Europe/ Scandinavia, and from northeastern China northeastward to southern Alaska (Fig. E3).

a. North Pacific and North America

In the lower latitudes, the 200-hPa circulation featured a 4-celled anomaly pattern of streamfunction anomalies characterized by amplified ridges over Australasia and amplified mid-Pacific troughs in both hemispheres (Fig. T22). This circulation is linked to the La Niña- related pattern of tropical convection, with enhanced subtropical ridges flanking the region of deep convection over Indonesia and the eastern Indian Ocean, and the amplified troughs flanking the region of suppressed convection over the central Pacific (Fig. T25).

In the extratropics, the mean 500-hPa circulation during March featured anomalous ridging over the central North Pacific, Alaska, and the southern U.S., along with deep troughs over both the western and eastern North Pacific (Fig. E9). In western North America, the trough-ridge couplet contributed a combination of well above average precipitation in the western U.S. (Fig. E3), and to well below average temperatures across western and central Canada (Fig. E1).

Temperature and precipitation patterns across the southern and eastern U.S. were strongly influenced by the extensive area of above average heights across the south. Prominent signals included exceptionally warm and dry conditions in the south-central U.S. and northern Mexico, along with continued well below average precipitation across the U.S. Gulf Coast (Fig. E5). Significant monthly rainfall deficits have prevailed across this region since October 2010. This signal is consistent with La Niña. Farther north, increased storminess and a series of major frontal passages contributed to above average precipitation extending from central Alabama northward to Maine (Fig. E6).

b. Europe

The 500-hPa circulation during March featured a 3-celled pattern of height anomalies, with above average heights across northern Europe flanked by negative anomalies to the north and south (Fig. E9). This pattern was associated with enhanced southwesterly winds across Scandinavia and northwestern Russia, and with increased storminess across southern Europe and northwestern Africa. These conditions mainly affected the precipitation patterns, which featured above average precipitation across southern Europe, and exceptionally dry conditions across northern Europe, southern Scandinavia, and northwestern Russia (Fig. E1). For northern Europe, area-averaged totals were the lowest for March dating back to 1979 (Fig. E5).

c. Asia

For the past two months, the 500-hPa circulation over northern Russia and Siberia has featured a trough in the west and a ridge in the east, accompanied by a deep southwesterly flow of marine air into the high latitudes of the Eurasian continent (Figs. E9, E10). These conditions led to an early snow-melt and well above average temperatures during March across most of Siberia.

Also during March, precipitation was well below average from eastern China northeastward to eastern Siberia. This dryness was associated with upper-level convergence (Fig. T23) that was focused in the area between the upstream amplified upper-level ridge and the downstream amplified trough (Fig. E9). Another contributing factor was a southeastward shift of the East Asian jet core, with much of the entire region also situated in enhanced sinking motion within the amplified left jet entrance region (Fig. T21).

2. Southern Hemisphere

All anomalies reflect departures from the 1981-2010 base period.

The 500-hPa circulation during March featured above average heights across the central South Pacific and over the southern Atlantic Ocean, and below average heights over the high latitudes of the eastern South Pacific (Fig. E15). In the subtropics, the upper-level (200-hPa) streamfunction pattern reflected an amplified trough across the central South Pacific, and an amplified ridge over Australia (Fig. T22). These overall circulation anomalies are consistent with La Niña.

The main precipitation signals during March reflected above average totals across most of Australia, but with well below average totals in the extreme southwest (Fig. E3). Much of central and western Australia also recorded significantly below average temperatures during the month, with large portions recording departures in the lowest 10^th percentile of occurrences (Fig. E1).

The South African rainy season lasts from October to April. During March, rainfall for the region as a whole was slightly above average. Totals were mainly above average in the western part of the monsoon region and below average in Mozambique (Fig. E4). A similar east-west dipole pattern was also evident during February. This pattern has been associated with a persistent anticyclonic circulation (Fig. T22) and upper-level divergence (Fig. T23) over the western part of the monsoon region, and with upper-level convergence and large-scale sinking motion farther east in the area immediately downstream of the mean ridge axis.