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 90^th 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 10^th 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 80^th 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 90^th percentile of occurrences. It also contributed to exceptionally warm surface temperatures in southern Scandinavia , with departures exceeding the 90^th 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 70^th percentile of occurrences. Rainfall totals in portions of the southeast were below the 10^th percentile of occurrences for the second consecutive month.