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

MAY 2011

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

1. Northern Hemisphere

The 500-hPa circulation during May featured above average heights over the central North Pacific Ocean, from Newfoundland to central Europe, and across Siberia, and below average heights over the high latitudes of the North Pacific and North Atlantic, the western U.S., and the Mediterranean Sea (Fig. E9). At 200-hPa, enhanced troughs in both hemispheres over the tropical and subtropical central/eastern Pacific were consistent with La Niña (Fig. T22).

The main surface temperature signals during May included well above average temperatures in western Europe and Siberia, and below average temperatures in the western and central U.S., western Russia, and western Canada (Fig. E1). The main precipitation signals included above average totals across the northern tier of the United States, Great Britain, and Scandinavia, and below-average totals along the U.S. Gulf Coast, southern Alaska, and much of Europe (Fig. E3).

a. North Pacific and North America

In the lower latitudes, the 200-hPa circulation during May featured amplified mid-Pacific troughs in both hemispheres (Fig. T22). This pattern is linked to the ongoing suppressed convection over the central Pacific in association with La Niña (Fig. T25).

In the extratropics, the mean 500-hPa circulation during May featured a strong high-latitude trough that extended from Mongolia to the western United States. This pattern was associated with increased storminess and above average precipitation across the northern tier of the United States, and with below average precipitation across the Gulf Coast (Figs. E3, E6). It was also associated with below average temperatures across the western U.S. (Fig. E1). In the U.S. Midwest, the above average precipitation exacerbated flooding that began early in the month in response to exceptionally heavy rainfall in April.

Conversely, the U.S. Gulf Coast continued to experience exceptionally dry conditions during May. This marks the 8^th consecutive month of well below average precipitation for this region (Fig. E5). The southeastern and mid-Atlantic regions of the U.S. also recorded well below average precipitation during May, with area-averaged totals in both regions falling into the lowest 10^th percentile of occurrences.

b. North Atlantic and Eurasia

The 500-hPa circulation during May featured a zonally-elongated pattern of height anomalies, with positive anomalies in the middle latitudes and negative anomalies at both high and low latitudes (Fig. E9). This pattern was associated with enhanced southwesterly winds across Scandinavia, and with a strong poleward transport of heat into the high latitudes of Eurasia. It was also associated with warmer (Fig. E1) and wetter (Fig. E3) than average conditions across Great Britain and Scandinavia. Farther south, the ridge over southern 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 10^th percentile of occurrences.

2. Southern Hemisphere

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

In the subtropics, the upper-level (200-hPa) circulation during May reflected an amplified trough across the central South Pacific and a marked westward retraction of the mean South Pacific jet stream to west of the date line (Figs. T21, T22). These conditions are consistent with La Niña.

The 500-hPa circulation featured above average heights over the central South Pacific and in the areas south of both Australia and South Africa, and below average heights over the high latitudes of the eastern South Pacific, eastern Australia, and southern Africa (Fig. E15).  This pattern was associated with below average precipitation across eastern Australia, in response to large-scale descending motion upstream of the mean trough axis. It was also associated with well above-average precipitation in southern Africa, where monthly totals were generally above the 70^th percentile of occurrences (Figs. E3, E4).