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

SEPTEMBER 2007

       1. Northern Hemisphere  

                The 500-hPa height pattern during September reflected amplified meridional flow over much of the hemisphere with persistent positive height anomalies covering large portions of the middle and high latitudes (Figs. E9, E11). The positive height anomalies were particularly prominent over the Gulf of Alaska, eastern North America, the high latitudes of the North Atlantic, central Russia , and Mongolia , while small areas of negative height anomalies occupied the Bering Strait, western Canada , and southeastern Europe . The main surface temperature departures during September reflected warmer than average conditions across the eastern half of the U.S. , western Alaska , western Russia , and Mongolia (Fig. E1). The main precipitation anomalies included above average totals in eastern Canada and eastern Europe, and below average totals from the Tennessee Valley region of the U.S. to New York , and in portions of west-central Europe (Figs. E3, E6).

In the subtropics, an extensive area of cyclonic streamfunction anomalies at 200-hPa spanned the Pacific Ocean in both hemispheres, with the largest anomalies observed in the Southern (winter) Hemisphere (Fig. T22). This pattern is consistent with the La Niña-related suppression of convection across the eastern half of the equatorial Pacific Ocean (Fig. T25). Over the Atlantic Ocean and Africa, anticyclonic streamfunction anomalies during September reflected a 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), This anomaly pattern is partly related to the combination of an enhanced West African monsoon system and suppressed convection over the Amazon Basin. This combination was again evident during September 2007 (Fig. T24).

    a. North America  

            The 500-hPa circulation pattern during September featured persistent above-average heights over the eastern U.S. and the Gulf of Alaska, with below-average heights centered over western North America and just west of Alaska (Fig. E9). The associated anomalous meridional flow contributed to exceptionally warm and dry conditions over much of the eastern United States and to above-average temperatures in western Alaska (Fig. E1). This flow pattern also contributed to well below-average precipitation from eastern Tennessee northward to New York (Figs. E5, E6). Long-term precipitation deficits have resulted in drought conditions throughout the western U.S. and the entire southeastern quadrant of the country. In the Southeast, extreme to exceptional drought is now present in many states, including Mississippi, western Georgia, Tennessee, Kentucky, western South Carolina, North Carolina, and southern Virginia. Severe drought occupies the surrounding areas to the north from southern Illinois to New Jersey .

    b. Europe  

            The 500-hPa circulation pattern during September featured an anomalous wave pattern extending from the central North Atlantic Ocean to central Russia (Fig. E9). This pattern included strong ridges over the central North Atlantic and central Russia , and a deep trough over eastern Europe. This pattern resulted in a continuation from August of near-normal temperatures across Europe , following several months of anomalous warmth. The persistent ridge in western Russia contributed to exceptionally warm temperatures throughout the region, with the most significant departures (2°C-3°C) occurring near the Black Sea and Caspian Sea. In western Mongolia, a strong upper-level ridge also contributed to anomalously warm conditions, with temperatures ranging from 3°C-4°C above average throughout the region.

2. Southern Hemisphere

      The 500-hPa height anomalies during September exhibited a pronounced zonal wave-1 pattern similar to that observed in August. This pattern featured above-average heights from Australia to the western South Atlantic Ocean and below-average heights across the southeastern South Atlantic and southern Indian Oceans (Fig. E15). Below-average heights were also again observed over the high latitudes of the central South Pacific Ocean, and over southwestern South America . In the subtropics, positive (cyclonic) streamfunction anomalies at 200-hPa extended from Australia to the eastern South Pacific Ocean (Fig. T22). These anomalies were situated along the equatorward flank of the wintertime South Pacific jet stream, and reflected a pronounced weakening and westward retraction of the jet core consistent with La Niña (Fig. T21).

    In South America, warmer-than-average conditions prevailed from central Brazil to central Argentina (Fig. E1). Significantly below-average precipitation occurred in southeastern Brazil , while above-average totals were recorded in southern Argentina . For extratropical South America , the anomalously mild and wet conditions were associated with a strong upper-level trough-ridge couplet, with the mean upper-level trough situated just west of the continent.

    In southeastern Australia , rainfall totals were in the lowest 10^th percentile of occurrences during September, in association with broad descending motion upstream of the mean trough axis (Figs. E3, E15). This region also recorded substantial precipitation deficits during August, with totals in both months in the lowest 10^th percentile of occurrences.

    In southern Africa temperatures were 2°C-3°C above average during September, with many areas recording departures in the upper 90^th percentile of occurrences. This anomalous warmth was linked to an amplified upper-level ridge across the southern part of the continent (Fig. T22).

    During September the size of the Antarctic ozone hole was near the 1997-2006 mean, exceeding 23 million km² early in the month before decreasing to 20 million km² by the end of the month (Fig. S8a). The 2007 ozone hole developed rapidly in mid-August, quickly increasing to more than 22 million km² by the end of the month. This evolution is consistent with the large spatial extent of the polar stratospheric clouds (PSCs), which have been near the 1997-2006 mean since late May, and reached peak their peak spatial coverage in early August (Fig. S8c).