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 2011

1. Northern Hemisphere

The 500-hPa circulation during August featured above average heights over the polar region and below average heights in the northern mid-latitudes (Fig. E9). Regional aspects of this pattern included below average heights over Alaska, Canada, northeastern Europe, and central Siberia. Over the Atlantic Ocean, the circulation reflected a strong negative phase (-1.85) of the North Atlantic Oscillation (NAO) (Fig. E7, Table E1).

The main surface temperature signals during August included warmer-than-average conditions over most of the U.S., eastern Canada, Europe, and large portions of eastern Asia (Fig. E1). The main precipitation signals included above-average totals in the northeastern U.S. and northwestern Europe, and below-average totals in the southern and mid-western U.S., and in southern Europe (Fig. E3).

a. North Pacific and North America

The mean 500-hPa circulation during August featured a persistent ridge over the central U.S., an anomalous trough extending from Alaska to eastern Canada, and mean troughs over both the west and east coasts of North America (Fig. E9). At 200-hPa, the most prominent circulation feature was an enhanced summer-time ridge east of the Rocky Mountains (Figs. T21, T22), which controlled the surface temperature (Fig. E1) and precipitation (Fig. E3) anomalies across much of the U.S.

Exceptionally warm surface temperatures covered much of the U.S. during August, with the southern half of the country recording departures in the upper 90^th percentile of occurrences (Fig. E1). The largest actual temperature anomalies were observed in the south-central states, where monthly departures exceeded 5^oC.

Significant precipitation deficits also accompanied the amplified summer-time ridge, with below average totals observed in the southern Plains states, the Gulf Coast states, and large portions of the Midwest (Fig. E3). For the state of Texas monthly totals were generally less than 25% of normal (Fig. E6), while temperatures were generally 3^oC-5^oC above average. According to NOAA’s U.S. Drought Monitor, exceptional drought continued during August across Texas, Oklahoma, southern Kansas, eastern New Mexico, and portions of western Louisiana. Extreme drought was evident in southeastern Colorado and central Georgia.

The mid-Atlantic and Northeast regions of the U.S. recorded above-average precipitation during August (Fig. E5), in association with the mean upper-level trough over the eastern U.S. Another contributing factor to this enhanced rainfall was Hurricane Irene, which made landfall late in the month in both North Carolina and central New Jersey. Irene produced heavy rains exceeding 175 mm (7 inches) and wide-spread flooding from eastern North Carolina northward to southern New York. The largest totals exceeded 250 mm (10 inches) across eastern North Carolina, portions of the Delmarva Peninsula, and southern New York. In Vermont, storm rainfall totals of 125mm – 175 mm (5-7 inches) also caused wide-spread flooding.

b. North Atlantic and Europe

The 500-hPa circulation during August featured above average heights over Greenland and below average heights over northwestern Europe and eastern North America (Fig. E9). This persistent pattern (Fig. E11) projected strongly onto the negative phase (-1.85) of the North Atlantic Oscillation (NAO) (Fig. E7, Table E1). Consistent with this pattern, northwestern Europe recorded increased precipitation in the area immediately downstream of the mean upper-level trough, and southern Europe recorded decreased precipitation (Fig. E3).

2. Southern Hemisphere  

The 500-hPa circulation during August featured above average heights in the area south of Australia and Africa, and also across Antarctica, and below average heights over the central South Pacific Ocean and South Atlantic Ocean (Fig. E15).  The most significant surface temperature signals were observed in southern and central Australia, where departures ranged from +2.0^oC to +3.0^oC, and were in the upper 90^th percentile of occurrences (Fig. E1). 

The Antarctic ozone hole typically develops during August and reaches peak aerial extent in September and October. By the end of August, the ozone hole (Fig. S6) spanned approximately 20 million square kilometers, which is roughly equal to the 2001-1010 mean (Fig. S8).