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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - September 2006

1. Northern Hemisphere

During September the 500-hPa circulation pattern featured positive height anomalies over western and central Canada , Scandinavia , and the high latitudes of the central and western North Pacific, and negative height anomalies from the north-central U.S. to Great Britain and over central Siberia (Figs. E9, E11). For the second month in a row, the anomaly pattern from eastern North America to Europe has reflected a strong negative phase of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7).

The main surface temperature departures during the month reflected warmer-than-average conditions across Canada , Scandinavia and Europe , and cooler-than-average conditions across the central United States (Fig. E1). Above-average precipitation was recorded in the Inter-Mountain, Ohio Valley , and Mid-Atlantic regions of the U.S. , and across portions of southern Europe (Figs. E3, E6). Below-average precipitation occurred in portions of the southern Plains and Midwestern regions of the U.S. , central Canada , southern Scandinavia , northeastern Europe , and southwestern China (Figs. E3, E4).

a. North America

The mean 500-hPa circulation pattern during September featured an amplified ridge over western and central Canada and a broad trough across the central and eastern U.S. (Fig. E9). This pattern contributed to above-average temperatures over much of Canada , and to below-average temperatures across much of the United States . The most significant positive temperature departures were observed in portions of north-central and northeastern Canada , where values exceeded the 90th percentile of occurrences (Fig. E1). In the United States , the most significant negative temperature departures occurred in the southern Plains region, where monthly temperatures were in the lowest 10th percentile of occurrences.

The pattern of anomalous precipitation was also consistent with the upper-air circulation, with above-average precipitation in the U.S. coinciding with regions of increased storminess in the vicinity of the mean trough axis. The most significant precipitation surpluses occurred in the Inter-Mountain, Ohio Valley , and Mid-Atlantic regions, where area-average totals generally exceeded the 80th percentile of occurrences (Fig. E6). In northern Canada , large areas of below-average precipitation were observed within and downstream of the amplified upper-level ridge (Fig. E3).

b. North Atlantic / Eurasia

The 500-hPa circulation pattern during September featured an anomalous ridge over Scandinavia and an anomalous trough over the east-central North Atlantic (Fig. E9). This circulation contributed to a continuation of significantly above-average precipitation throughout Europe and western Russia , with totals in many areas exceeding the 90th percentile of occurrences. It also contributed to exceptionally warm surface temperatures in southern Scandinavia , with departures exceeding the 90th percentile of occurrences in many areas.

An anomalous north-south dipole pattern of precipitation was also evident over Europe , with below-average precipitation coinciding with the high-latitude ridge, and above-average precipitation across portions of southern Europe coinciding with broad upper-level westerly flow undercutting the ridge.


2. Southern Hemisphere

      The mean 500-hPa circulation pattern during September featured above-average heights across southern Australia and New Zealand , over portion of Antarctica , and over the central and eastern South Atlantic , and below-average heights over the high western longitudes of the three ocean basins (Fig. E15). In southern Australia , this pattern contributed to anomalously warm and dry conditions. Temperatures in that region generally exceeded the 70th percentile of occurrences. Rainfall totals in portions of the southeast were below the 10th percentile of occurrences for the second consecutive month.



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