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

1. Northern Hemisphere

During November the 500-hPa circulation pattern featured positive height anomalies over eastern Siberia , the high latitudes of the North Pacific, the southwestern and central U.S. , eastern Canada , and south-central Europe , and negative height anomalies over western Canada , Iceland , and south-central Russia (Figs. E9, E11). Over the North Pacific and North America , the anomalies reflected the negative phases of the Pacific/ North American (PNA) and West Pacific (WP) teleconnection patterns (Table E1, Fig. E7). Over the North Atlantic , the circulation reflected the ongoing positive phase of the East Atlantic (EA) teleconnection pattern. This pattern has been a prominent feature of the circulation since April. In the subtropics, the 200-hPa circulation was consistent with El Niņo conditions, with anticyclonic streamfunction anomalies in both hemispheres flanking the regions of enhanced convection over central and east-central Pacific, and cyclonic streamfunction anomalies over southeastern Asia in association with suppressed convection across Indonesia (Fig. T22).

The main surface temperature departures during November reflected warmer-than-average conditions across the southwestern U.S. , Europe , China , and southeastern Asia , and cooler-than-average conditions over western Canada and eastern Alaska (Fig. E1). The main precipitation anomalies included above-average totals in the northwestern, mid-Atlantic, and northeastern states, and below-average totals in the southwestern, Plains, and Intermountain states, south-central Europe , and southeastern Asia (Figs. E3, E5, E6)


a. Pacific/ North America

In the subtropics, the mean 200-hPa circulation during November was consistent with El Niņo, with anticyclonic streamfunction anomalies flanking the regions of enhanced convection over the central and east-central equatorial Pacific (Fig. T22). In the extratropics the 500-hPa circulation featured a high-latitude blocking pattern (Fig. E9), which projected onto the negative phases of both the PNA and WP teleconnection patterns (Table E1, Fig. E7). The surface temperature anomaly pattern generally followed the 500-hPa height anomalies (Fig. E1). Below-average temperatures were observed downstream of the blocking ridge across eastern Alaska and western Canada , while above-average temperatures from the southwestern U.S. to eastern Canada coincided with a broad area of positive height anomalies.  

A strong under-cutting of the blocking ridge and a persistent downstream trough over western Canada were present during November. This pattern led to an amplified and persistent storm track across southwestern Canada and the Pacific Northwest U.S., resulting in well above-average precipitation in these regions (Fig. E3). Monthly totals in the Pacific Northwest were above the 90th percentile of occurrences, following four consecutive months of well below-average precipitation (Fig. E5). Because the storm track during November was focused in the north, the Southwestern, Intermountain, and Plains States recorded generally less than 25% of normal precipitation (Fig. E6). This is the first time since July that monthly precipitation totals were below average in the Southwest and Intermountain regions.


b. North Atlantic / Europe

The 500-hPa circulation pattern during November featured below-average heights near Iceland and above-average heights over south-central Europe . Enhanced westerlies south of the mean upper-level trough contributed to well above-average surface temperatures during November. Monthly temperatures across the continent generally exceeded the 90th percentile of occurrences, with the largest departures of 2°-3°C observed in central Europe .

Well above average sea-surface temperatures were again observed across the high latitudes and tropical latitudes of the North Atlantic during November. This pattern is associated with the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995, and with the warming trend in global temperatures during the past 100 years.


c. China

An anomalous upper-level trough was again centered over China during November. This feature reflected a weakening of the mean subtropical ridge and is consistent with the El Niņo-related pattern of suppressed convection across Indonesia and Southeast Asia (Fig. T25). Surface temperatures across China and Southeast Asia were well above average, with departures in central China exceeding the 90th percentile of occurrences.


2. Southern Hemisphere

The mean 500-hPa circulation pattern during November featured above-average heights across southern Australia and south of Australia , and below-average heights south of New Zealand . (Fig. E15). In the subtropics, anticyclonic streamfunction anomalies extended across the South Pacific consistent with ongoing El Niņo conditions.

In Australia , an enhanced mid-and upper-level ridge contributed to a continuation of anomalously warm and dry conditions. The temperature departures were especially significant in the west, where monthly values were in the upper 90th percentile of occurrences. The most significant precipitation anomalies occurred in the east and northeast, where totals were in the lowest 10th percentile of occurrences. While these below average totals are consistent with the El Niņo, they were accentuated by large-scale subsidence downstream of the upper-level ridge axis.

In South Africa , surface temperatures were also above average during November, and precipitation was near average. To date, the 2006-07 South African rainy season, which extends from October to April, has featured below-average precipitation in October and near-average totals in November.



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