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).
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 3-4°C
below-normal, falling below the lowest 10th percentile in the historical record
dating back to 1950. Over the eastern U.S. temperatures averaged 1-2°C below-average
during the month, which is below the 30th percentile. Farther north,
temperatures over large portions of both western and eastern Canada averaged 1-3°C
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
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 regions 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 regions rainy season. The above-average rains in southeastern Brazil,
southeastern Africa, and northeastern Australia are all consistent with the weak Pacific
cold episode (La Niña).
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
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.