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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - November 2004

1. Northern Hemisphere

The 500-hPa circulation pattern during November featured above-average heights across central North America, the high latitudes of the North Atlantic, and the western North Pacific, and below-average heights over the southwestern United States , the central North Atlantic , and portions of the polar region (Figs. E9, E11). The main temperature departures during November included warmer-than-average conditions over the eastern half of the United States, most of Canada, northwestern and southeastern Europe, and most of Asia (Fig. E1). The main precipitation departures included above-average totals over the southwestern and south-central U.S., below-average totals over western Europe (Fig. E3).

a. North America

The mean 500-hPa circulation pattern during November featured persistent positive height anomalies across central North America and an anomalous trough over the southwestern United States (Figs. E9, E11). This circulation pattern was associated with a pronounced split-flow configuration over the western portion of the continent. In the northern branch of the flow, an enhanced influx of marine air (Fig. E10) contributed to anomalously warm temperatures over much of Canada (Fig. E1).

In the southern branch of the flow, broad southwesterly winds downstream of the mean upper-level trough axis covered the southern United States . Several storm systems within this enhanced low-latitude flow contributed to above-average precipitation across the south-central, southwestern, and Intermountain states (Figs. E3, E5). Rainfall in the south-central U.S. exceeded the 70th percentile of occurrences, with totals over eastern Texas and western Louisiana exceeding the 90th percentile of occurrences. The Southwest and Inter-Mountain regions have recorded above-average precipitation for three consecutive months (Fig. E5), which has helped to moderate the long-term drought conditions plaguing these regions.

b. North Atlantic/Europe

Above-average 500-hPa heights covered the high latitudes of the North Atlantic during November and below-average heights spanned the central North Atlantic . This anomaly pattern reflected a complete disappearance of the mean Icelandic Low (Fig. T21). These conditions were associated with an enhanced flow of marine air into northern Europe and Scandinavia . They were also associated with large-scale sinking motion and significantly below-average precipitation over western Europe in the area immediately downstream of the mean upper-level ridge axis. 

Also downstream of the ridge axis, a split-flow configuration was present over Europe and western Russia . The northern branch of the flow constituted the main belt of westerlies that extended eastward across northern Asia . This zonal flow likely transported the anomalous marine air entering northern Europe to the Far East, thereby contributing to the well above-average temperatures recorded across central and eastern Asia . Mean temperatures in these areas generally exceeded the 70th percentile of occurrences, with the largest departures (3-5C) observed over central and eastern Siberia. Within the southern branch of the split flow, anomalous southwesterly winds across southeastern Europe and northeastern Africa brought well above-average temperatures to these regions.

2. Southern Hemisphere

The 500-hPa circulation pattern during November featured above-average heights over the high latitudes of the South Pacific and the western South Atlantic , and below-average heights over most of the middle latitudes (Fig. E15). At 200-hPa, cyclonic streamfunction anomalies and an enhanced subtropical jet stream covered the South Pacific from central Australia to South America (Figs. T21, T22). These conditions contributed to above-average precipitation over large portions of central South America .

In southern Africa the rainy season normally lasts from October to April. During November area-averaged precipitation totals were in the lowest 10th percentile of occurrences (Fig. E4), and regional totals in eastern South Africa , Mozambique , and southern Tanzania were generally in the lowest 30th percentile of occurrences. This below-average rainfall was associated with an area of anomalous upper-level convergence (Fig. T23) downstream of an amplified upper level ridge (Fig. T22). Surface temperatures were also well above-average across over most of South Africa , with monthly departures generally exceeding the 90th percentile of occurrences. This excessive warmth is consistent with the combination of the anomalous upper-level ridge and below-average rainfall.

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