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

APRIL 2011

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

1. Northern Hemisphere

The 500-hPa circulation during April featured above average heights across the southern United States, Europe, and eastern Siberia, and below average heights extending from Canada to Greenland, over western Russia, and across the western North Pacific (Fig. E9). This pattern projected strongly onto several main teleconnection patterns, including the positive phase (+2.6) of the North Atlantic Oscillation (NAO), and the negative phases (-1.9 and -1.8, respectively) of the West Pacific and Pacific/ North American (PNA) patterns.

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 La Niña.

The main surface temperature signals during April included well above average temperatures across the south-central U.S. and eastern Siberia, and below average temperatures in the northwestern U.S. and western Canada (Fig. E1). The main precipitation signals included above average totals in the northwestern, Midwestern and northeastern U.S., and below-average totals along the U.S. Gulf Coast, much of Europe, and northeastern China (Fig. E3).

a. North Pacific and North America

In the lower latitudes, the 200-hPa circulation during April 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). These conditions are 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 April featured a broad trough across the entire Pacific Basin, along with deep trough across Canada. These conditions were accompanied by above average heights across the eastern Pacific and the southern U.S. This overall pattern was associated with enhanced jet stream winds and increased storminess from the eastern Pacific to the northeastern U.S. (Fig. T21). It was also associated with an enhanced low-level frontal boundary that extended across the Midwestern U.S. This combination of conditions led to well above average precipitation across the central U.S., with the Great Lakes, Midwest, and Ohio Valley regions each recording totals in the upper 90^th percentile of occurrences (Fig. E5). Many locations recorded precipitation surpluses exceeding 150 mm, which was generally more 200% of normal. As a result, by early May there was significant flooding of the Mississippi River.

The extensive upper-level trough across Canada also contributed to significantly cooler (Fig. E1) and wetter than normal (Fig. E3) conditions in the northwestern quadrant of the U.S., where many locations recorded temperatures in the lowest 10^th percentile of occurrences and precipitation totals in the upper 90^th percentile of occurrences.

Conversely, the U.S. Gulf Coast continued to experience exceptionally dry conditions in association with above average 500-hPa heights across the region. April marks the 7^th consecutive month with area-averaged totals in the lowest 10^th percentile of occurrences for this region.

b. North Atlantic and Europe

The 500-hPa circulation during April featured a large-amplitude trough centered just west of Greenland, and above average heights across the western North Atlantic and Europe (Fig. E9). This pattern projected strongly onto the positive phase (+2.6) of the NAO.

This circulation was associated with enhanced southwesterly winds across Scandinavia, and with a strong poleward transport of heat into the high latitudes of Eurasia. The ridge over Europe resulted in exceptionally warm and dry conditions, with many areas recording temperatures in the upper 90^th percentile of occurrences and precipitation totals in the lowest 30^th percentile of occurrences. For northern Europe, this marks the second straight month with area-averaged totals in the lowest 10^th percentile of occurrences (Fig. E4).

c. Asia

For the past three 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 mild air into the high latitudes of Eurasia (Fig. E9). These conditions led to an early snow-melt, and to a continuation of well above average temperatures during April across most of Siberia.

Also during April, precipitation was well below average in northeastern China. This dryness continued from March, in response to upper-level convergence (Fig. T23) focused in the area between an upstream amplified upper-level ridge and a downstream amplified trough (Fig. E9). Much of the region was also situated in enhanced sinking motion within an enhanced left entrance region of the East Asian jet stream (Fig. T21).

2. Southern Hemisphere

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

The 500-hPa circulation during April featured above average heights throughout the polar region, along with below average heights over southern South America and southern Africa (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 circulation anomalies are consistent with La Niña.

The South African rainy season lasts from October to April. During April, rainfall for the region as a whole was above average, with area-averaged totals in the upper 80^th percentile of occurrences (Fig. E4). Rainfall during the 2010-11 rainy season was above average in five of the seven months, and below average in two months (October and February).  The South African rainy season is often above average during La Niña.