1. Northern Hemisphere
Very persistent and large-amplitude circulation anomalies covered the middle latitudes
of the Northern Hemisphere during November (Figs. E9, E11), which contributed to anomalous temperature (Fig. E1) and rainfall patterns (Fig. E3) over large
portions of the hemisphere. The most prominent circulation anomalies included large
regions of above-normal heights over the central Pacific, North America and the eastern
Atlantic, and below-normal heights over the eastern Pacific and the Caspian Sea sector.
This predominance of above-normal heights is consistent with an ongoing pattern of
anomalous anticyclonic circulations which have dominated large portions of the middle
latitudes for the past several months.
a. North America
The circulation over North America was dominated by an amplified ridge that covered
most of the continent, with the largest positive anomalies observed over central Canada
and the central United States (Fig. E9). This pattern
contributed to above-normal temperatures across the continent (Fig.
E1), with departures averaging more than 4°C above normal over most of central
North America. The amplified ridge also contributed to significantly below-normal rainfall
over large portions of the United States (Fig. E3), with
the largest percentile deficits observed in the following regions (Fig.
E5): the Southwest, the Inter-mountain region, the Great Plains, the Midwest, the
Gulf Coast, the mid-Atlantic, and the Northeast. For the midwestern region as a whole,
rainfall totals have been significantly below normal throughout the July-November 1999
period (Fig. E5).
b. Europe/ Asia
Portions of western Europe, northern Africa and the Middle East experienced
significantly above-normal temperatures during November (Fig. E1),
with temperatures above the 90th percentile covering large parts of these
regions. In northern Africa and the Middle East, this warmth represents a continuation of
extremely warm conditions that have prevailed for the last several months. This
persistence of well above-normal temperatures has been linked to an extremely persistent
and anomalous anticyclonic circulation at upper levels (Fig. T22,
bottom) which, in many months, has been associated with the previously-noted
large-scale pattern of above-normal heights over the middle latitudes.
Farther north, the warmth that was observed over northwestern Europe during November
also prevailed during September and October. This warmth has been linked to above-normal
heights across the eastern North Atlantic, Scandinavia and northwestern Russia, which have
persisted for much of the period. In the region downstream of this amplified ridge,
anomalously cold temperatures prevailed during November across eastern Europe and much of
western Russia (Fig. E1).
2. Southern Hemisphere
The Southern Hemisphere circulation during November featured a continuation of
above-normal heights over large portions of the middle latitudes (Fig.
E15), with the largest positive anomalies observed in the region poleward of
Australia, and in the regions to the east of New Zealand and southern South America.
Another prominent feature of the Southern Hemisphere circulation during November was a
large region of negative height anomalies over the high latitudes of the eastern South
Pacific. This overall pattern of height anomalies extending from Australia to the eastern
South Pacific has persisted for the past four months. During the August-October period
this pattern contributed to extremely warm conditions across large portions of Australia,
New Zealand and southern South America, which continued during November across New Zealand
and southern South America but replaced by anomalously cold conditions over nearly all of
The transition to anomalously cold conditions over Australia during November was
associated with a poleward shift of positive height anomalies to the region south of the
continent, and to the development of negative height anomalies across the central
latitudes of the country. This anomaly pattern was linked to an amplified jet stream (Fig. T21) with an anomalous jet entrance region across central
and eastern Australia (Fig. T21, bottom). An anomalous
thermally-direct transverse ageostrophic circulation, which is a dynamically required
component of this anomalous jet entrance region, likely contributed to a strong dipole
pattern consisting of anomalously dry conditions over southeastern Australia and extremely
wet conditions over northeastern Australia during the month (Fig.
b) South America
In South America, most of cental Argentina and southeastern Brazil experienced a
continuation of anomalously dry conditions during November (Fig. E3).
Rainfall deficits have been observed in these regions in each of the past four months.
This dryness has been linked to an overall poleward shift of the main frontal boundary
away from central Argentina and southeastern Brazil, in association with an overall
poleward displacement of the mean storm track across the eastern Pacific and southern
South America. This anomalous circulation has also contributed to anomalously warm
conditions across southern South America during the period (Fig. E1),
and is consistent with ongoing La Niņa conditions.