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Extratropical Highlights - November 2000

1. Northern Hemisphere
The Northern Hemisphere circulation during November featured a persistent pattern of above-normal heights at high latitudes and below-normal heights in the middle latitudes (Figs. E9, E11). One component of this large-scale pattern was a relatively strong negative phase (-0.7) of the North Atlantic Oscillation (NAO) (Table E1, Figs. E6, E7).

On a regional basis this overall anomaly pattern contributed to exceptionally warm temperatures over both western and eastern Canada and throughout Europe, and to significantly below-average temperatures across the United States and large portions of central Russia (Fig. E1). These large areas of below-average temperatures contributed to near-average temperatures for the Northern Hemisphere as a whole (Fig. E2), marking the first cool-season month since November 1996 of near-average hemispheric-mean temperatures.

a. North America

The circulation over North America featured above-average heights across Canada and below-average heights over much of the United States (Fig. E9). In western Canada this anomaly pattern reflected an amplification and westward shift of the mean ridge axis. In the United States it reflected a pronounced southwestward extension of the mean Hudson Bay trough to the southwestern United States and northern Mexico, such that much of the United States was under the influence of the large-scale trough. The anomalous southward extension of the Hudson Bay trough is an important structural and dynamical feature of the negative phase of the NAO (Table E1, Figs. E6, E7). In particular, it produces an increased geopotential height gradient over the southeastern U.S. and western Atlantic, which supports the observed southward shift of the North Atlantic jet stream from the eastern U.S. across the central North Atlantic (Fig. E10). The anomalous trough also contributes to an amplification and southward shift of the region of strongest confluence over the southeastern United States, which is also dynamically required to support the observed changes in the North Atlantic jet stream.

During November the anomalous circulation over North America was associated with significantly below-average temperatures across the United States (Fig. E1). Temperatures over the western half of the country averaged 3-4C below-normal, falling below the lowest 10th percentile in the historical record dating back to 1950. Over the eastern U.S. temperatures averaged 1-2C below-average during the month, which is below the 30th percentile. Farther north, temperatures over large portions of both western and eastern Canada averaged 1-3C above-normal, which ranks above the 70th percentile during the month.

b. Europe and Asia

Above-average heights covered Greenland and the high latitudes of the North Atlantic, while below-average heights covered both the west-central and east-central Atlantic (Fig. E9). This overall anomaly pattern was accompanied by a pronounced southward shift of the mean North Atlantic jet stream over much of the Atlantic basin (Fig. E10), and is consistent with the negative phase of the NAO (Table E1, Figs. E6, E7). During November these conditions contributed to significantly warmer (Fig. E1, bottom) and wetter than average conditions across large portions of Europe and Scandinavia (Fig. E3, bottom).

2. Southern Hemisphere

a. Troposphere

The circulation during November featured a pronounced wave-3 pattern throughout the middle and high latitudes of the Southern Hemisphere (Fig. E15), with positive height anomalies covering the polar region and the areas poleward of the three continents, and negative height anomalies covering the central ocean basins. Negative height anomalies also covered southern South America and southern Africa during the month. In South America this anomalous circulation contributed to cooler (Fig. E1) and wetter (Fig. E3) than average conditions in Argentina, and to above-average rainfall in southeastern Brazil. In Africa the negative height anomalies reflected a large-scale trough situated over central South Africa. This feature contributed to above-average rainfall over southeastern South Africa, Mozambique and Tanzania, which represents another strong start to the region’s wet season (Fig. E4).

In Australia the eastern half of the continent recorded significantly above-average rainfall during November, with totals exceeding the 90th percentile throughout the area (Fig. E3). In the Northeast this second consecutive month of near-record (compared to the 1979-1995 base period means) rainfall totals (Fig. E4) indicates another very strong start to that region’s rainy season. The above-average rains in southeastern Brazil, southeastern Africa, and northeastern Australia are all consistent with the weak Pacific cold episode (La Nia).

b. Stratosphere

The Antarctic ozone hole, defined by total ozone values less than 220 Dobson Units (DU), reached a record areal extent in early September 2000 (Fig. S7, top). The size of the ozone hole then declined rapidly from October through mid- November, after which it completely disappeared. This evolution contrasts to almost every year since 1990, in which the very low ozone values persisted into late November or early December.

The demise of the ozone hole during October– mid-November 2000 was linked to a substantial warm-up near 50-hPa throughout the polar region (Fig. S3, bottom right). This warm-up was associated with a significant lessening in the magnitude and areal extent of the circumpolar polar vortex (Fig. S7, middle), as indicated by positive 50-hPa height anomalies throughout Antarctica in both months (Fig. S1).

The extent to which the atmospheric circulation also influenced the regional distribution of ozone anomalies at high latitudes during November is indicated by the close correspondence between these anomalies (Fig. S6, left) and the 50-hPa height anomalies (Fig. S1). For example total column ozone in the negative height anomaly regions of the southern South Atlantic Ocean and southern Indian Ocean were reduced by as much as 26% compared to the pre-ozone hole (1979-1986) average (Fig. S6, left). Also, total ozone in the positive height anomaly region over western Antarctica was up to 12% higher than the long-term average.


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