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 2010

1. Northern Hemisphere

The 500-hPa circulation during September featured above-average heights over Greenland, northern Russia, eastern Asia, and Alaska, and below average heights in the Gulf of Alaska, south-central Europe, and central Siberia (Fig. E9). Over the subtropical Atlantic basin, the 200-hPa streamfunction pattern continued to show a pronounced inter-hemispheric symmetry, 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, an enhanced west African monsoon system (Figs. T23, T24), and ongoing exceptionally warm SSTs across the tropical and high latitudes of the North Atlantic (Fig. T18). This combination of factors contributed to exceptionally strong hurricane activity in the Atlantic Basin, and to well below average in eastern North Pacific hurricane region.

The main surface temperature signals during September included warmer than average conditions across the southern half of the United States, Greenland, and western Russia (Fig. E1). Monthly precipitation totals (Fig. E3) were above-average from Mexico northward to the U.S. Great Lakes, across southern Canada, in eastern Europe, and in eastern China (Figs. E5, E6). Monthly precipitation was below average along the U.S. Gulf Coast and East Coast.

a. North America

The mean 500-hPa circulation during September featured troughs in the western and eastern U.S., and a ridge across the southwestern U.S. (Fig. E9). This pattern was associated with above average temperatures across the southern half of the country, with September departures in many locations exceeding the 70^th percentile of occurrences (Fig. E1). Precipitation was above average in the Great Plains and Midwest regions of the country, and throughout eastern Texas (Figs. E3, E5), with departures in many regions exceeding 175% of normal (Fig. E6). Conversely, monthly precipitation totals were below average in the southwest, and along an axis extending from the central Gulf Coast northeastward to New England. The most significant deficits were observed in portions of Mississippi, Tennessee, and Indiana, where monthly totals were less than 25% of normal.

b. North Atlantic

Over the subtropical North 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 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 that extended from the eastern North Pacific across the tropical Atlantic (Fig. T20).

This combination of conditions is consistent with an enhanced west African monsoon circulation, which was again present during September (Figs. T23, T24) and has been a prominent feature of the circulation since 1995. These conditions contributed to reduced vertical wind shear and to exceptionally conducive wind patterns that extended westward from Africa, resulting in considerable Atlantic hurricane activity throughout the month.

The above conditions are typical of the high activity era for Atlantic hurricanes that began in 1995. They are also consistent with a continuation of exceptionally warm SSTs in the Atlantic basin, which have also prevailed since 1995 in association with the warm phase of the Atlantic Multi-decadal Oscillation (AMO). Since March 2010, record warm SSTs have persisted across the tropical North Atlantic.

Over the eastern tropical North Pacific, atmospheric anomalies associated with La Niña contributed to increased vertical wind shear, and to a confinement of the eastern Pacific ITCZ to the southwest coast of Mexico. These conditions contributed to well below average hurricane activity in the eastern Pacific, with only one named storm (minimal Tropical Storm Georgette) forming during the month.

 2. Southern Hemisphere  

     The 500-hPa circulation during September reflected a zonal wave-2 pattern of height anomalies. Main aspects of this pattern included above average heights southeast of Australia and over the South Atlantic Ocean, and below average heights south of New Zealand and over the eastern Indian Ocean (Fig. E15). In the subtropics, the upper-level (200-hPa) streamfunction pattern reflected an anomalous trough across the western and central South Pacific, and an anomalous ridge extending from the eastern 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).

    The Antarctic ozone hole typically develops during August, and reaches peak aerial extent during September. During 2010, the ozone hole did not form until late August, making this the latest formation date since 2000 (Fig. S8). This delay of onset reflected a significant decrease in polar stratospheric clouds that occurred during July and August in association with a sharp increase in polar stratospheric temperatures at both 10-hPa and 2-hPa (Fig. S4). During September 2010, the ozone hole covered 14 million square kilometers early in the month, and expanded to 20 million square kilometers late in the month. This latter size was approximately equal to the 2000-2009 mean (Fig. S8).