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 2007

       1. Northern Hemisphere

The 500-hPa height pattern during August featured positive anomalies over the eastern United States , the central North Atlantic, western Russia , Greenland, and Alaska , and negative anomalies over western Canada and central Siberia (Figs. E9, E11).  At 200-hPa, the streamfunction field indicated an extensive area of positive anomalies (reflecting anticyclonic circulation anomalies) spanning the U.S. , the North Atlantic, and most of Europe and northwestern Africa (Fig. T22). A strong inter-hemispheric symmetry of this pattern is also evident, suggesting links to the distribution of anomalous tropical convection. In particular, this circulation pattern likely is consistent with 1) the continuation of conditions that have been in place since the current active Atlantic hurricane era began in 1995 ( Bell and Chelliah, 2006, J. Climate. 19, 590-612), and 2) the ongoing La Niña-related pattern of anomalous tropical convection. The eastern portion of this streamfunction anomaly pattern has been a mainstay of the August-October circulation since 1995. These anomalies are partly related to the combination of an enhanced West African monsoon system and suppressed convection over the Amazon Basin . This combination was again pronounced during August 2007 (Fig. T24).

The main surface temperature departures during August reflected warmer than average conditions across the United States , Alaska , western Russia , and central China , and cooler than average conditions in western Canada (Fig. E1). The main precipitation anomalies included above average totals in the mid-western U.S., and below average totals in the southeastern and Ohio Valley regions of the U.S., and in the area north of the Caspian Sea (Figs. E3, E6).  

a. North America

The 500-hPa circulation pattern during August featured above-average heights over the eastern U.S. and much of Alaska , and below-average heights across the western half of Canada (Fig. E9). The surface temperature anomalies were very consistent with this pattern. The most significant anomalous warmth (+1°-3°C above average) was recorded in the southwestern and southeastern states, and in western Alaska, were temperatures were generally in the upper 90^th percentile of occurrences.

In the continental U.S. , the main precipitation departures reflected above-average totals in the Midwest, and below-average totals in the southeastern and Ohio Valley regions (Fig. E3, E6). Long-term precipitation deficits have resulted in severe drought in both regions.  

b. Europe/ western Russia

The 500-hPa circulation pattern during August featured an anomalous wave pattern extending from the central North Atlantic Ocean to central Russia (Fig. E9). This pattern featured strong ridges over the central North Atlantic and central Russia , and a deep trough over Europe . The persistent ridge in western Russia contributed to exceptionally warm and dry conditions throughout the region.  

2. Southern Hemisphere

      The 500-hPa height anomalies during August exhibited a pronounced zonal wave-1 pattern, with above-average heights from Australia to South America, and below-average heights across the southeastern South Atlantic and southern Indian Oceans (Fig. E15). Below-average heights were also observed over the high latitudes of the central South Pacific Ocean, and over southern South America . In the subtropics, negative (anticyclonic) streamfunction anomalies at 200-hPa extended from southeastern Brazil to southern Africa, and from Australia to the eastern South Pacific (Fig. T22). Over the South Pacific Ocean , the anticyclonic anomalies were centered along the axis of the wintertime South Pacific jet stream, and reflected a pronounced weakening and westward retraction of the jet core (Fig. T21). This jet structure is consistent with La Niña conditions.

In southern Australia , the anomalous anticyclonic circulation was associated with exceptionally warm and dry conditions. In portions of the southeast, temperatures were in the highest 90^th percentile of occurrences and rainfall totals were in the lowest 10^th percentile of occurrences.

Extratropical South America was influenced by a strong upper-level trough during August, with a substantial southeastward transport of cold air evident upstream of the trough axis. These conditions contributed to well below-average temperatures across the southern half of the continent, with many areas recording values in the lowest 10^th percentile of occurrences. This pattern reflects a continuation of the exceptionally cold temperatures observed during July.