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

AUGUST 2010

1. Northern Hemisphere

The 500-hPa circulation during August featured above-average heights over Greenland, western Russia, eastern Asia, and the Gulf of Alaska, and below average heights over western Europe and central Siberia (Fig. E9). Over the subtropical Atlantic basin, the 200-hPa streamfunction pattern continued to show a pronounced inter-hemispheric symmetry during August, with anticyclonic anomalies extending from the America’s to Africa in both hemispheres (Fig. T22). Conversely, an anomalous trough was evident across most of the tropical western North Pacific, with a similar anomaly pattern evident in the Southern Hemisphere. These signals are consistent with the combined influences of La Niña and an enhanced west African monsoon system (Figs. T23, T24).

The main surface temperature signals during August included warmer than average conditions across the eastern half of the United States, Mexico, western Russia, and most of China (Fig. E1). Monthly precipitation totals (Fig. E3) were above-average in northern Europe, and in the Inter-Mountain West and Gulf Coast regions of the U.S. (Figs. E5, E6). Monthly precipitation was below average in the mid-Atlantic, Northeast, and Pacific Northwest regions of the U.S., southwestern Europe and southwestern Russia.

a. North America

The mean 500-hPa circulation during August featured a broad and persistent ridge across the central and eastern United States (Fig. E9). This pattern was associated with a continuation from July of above average temperatures across the eastern half of the country, with August departures in many locations exceeding the 90^th percentile of occurrences (Fig. E1). The mid-Atlantic and northeastern states also experienced a continuation of well below average precipitation during August (Fig. E5). For both regions precipitation has been below average since April. During August, ongoing precipitation deficits in the mid-Atlantic region led to the development of moderate- to-severe drought. Elsewhere, precipitation was above average in the Midwest and Gulf Coast regions of the country, and in the Inter-Mountain West where totals have been above average for nine straight months.

b. North Atlantic

Over the subtropical Atlantic, anticyclonic streamfunction anomalies at 200-hPa extended from the America’s to Africa in both hemispheres (Fig. T22). This pronounced inter-hemispheric symmetry of the anomaly pattern was associated with an expanded area of upper-level easterly wind anomalies that extended across tropical northern Africa and the tropical North Atlantic (Fig. T21). It was also associated with an extensive area of low-level westerly wind anomalies across the tropical Atlantic (Fig. T20). These conditions lead to reduced vertical wind shear across the tropical Atlantic, and are typical of the high activity era for Atlantic hurricanes that began in 1995. This combination of conditions is consistent with an enhanced west African monsoon circulation, which was again present during August (Figs. T23, T24) and has been a prominent feature of the circulation since 1995.

c. Eurasia

The 500-hPa circulation during August featured a strong and persistent ridge centered over western Russia, along with an amplified trough over northern Europe (Fig. E9). This pattern affected temperature and precipitation patterns from Europe to Pakistan (Figs. E1, E3). The strong ridge led to exceptionally warm and dry conditions in western Russia, with many areas experiencing record heat. The amplified trough contributed to well above average precipitation in northern Europe, with many areas recording totals in the upper 90^th percentile of occurrences.

During part of the month the ridge in western Russia was exceptionally amplified and resembled a typical Omega-block. This blocking pattern was associated with a very strong downstream trough that extended southward from central Russia to Pakistan. This amplified trough interacted with a deep monsoonal flow of moisture coming into the Indian sub-continent, which led to extreme precipitation totals and record flooding in Pakistan.

 2. Southern Hemisphere

The 500-hPa circulation during August featured a zonally-symmetric pattern of height anomalies, with above average heights in the middle latitudes and generally below average heights over Antarctica (Fig. E15). In the subtropics, the upper-level streamfunction pattern reflected an anomalous trough across the western and central tropical South Pacific, and an anomalous ridge extending from the eastern subtropical South Pacific to southern Africa (Fig. T22). A similar anomaly pattern was evident in the Northern Hemisphere. These conditions have been present since June, and likely reflect the combined influences of La Niña and an enhanced west African monsoon system. One characteristic feature of La Niña was a marked westward retraction of the South Pacific jet core, as indicated by easterly wind anomalies centered along the jet axis east of the date line near 30°S (Fig. T21).