A strong inter-hemispheric symmetry of anomalous
upper-level circulation features was again evident during October across the lower
latitudes of the Pacific and Atlantic basins (Fig. T22,
bottom). This pattern featured anomalous anticyclonic circulations across the eastern
Pacific eastward to Africa and the Middle East, and anomalous cyclonic circulation
anomalies across the low latitudes of the western Pacific. Anticyclonic circulation
anomalies also persisted across the middle latitudes of the Northern Hemisphere during the
month. Each of these features have also prevailed during the June-September period, and
are consistent with the ongoing pattern of anomalous tropical convective activity that has
persisted throughout the period. This pattern of anomalous tropical rainfall included
suppressed convection over the central equatorial Pacific (Fig.
T25), an amplified southeast Asian monsoon system, above-normal rains over central
America, and increased precipitation across interior northern Africa near 10°N.
a. North America
The circulation over North America was dominated by an amplified ridge over the western
United States and by an amplified trough over eastern Canada (Fig.
E9). This pattern contributed to above-normal temperatures across the western U.S.
(Fig. E1, top) and to below-normal temperatures across
eastern Canada. It also contributed to below-normal precipitation across the central
United States (Fig. E3), which is located between the
persistent upper-level ridge and trough axes. In portions of this anomalously dry region,
precipitation totals were below the 10th percentile during the month. For the
midwestern region as a whole, rainfall totals have been significantly below normal
throughout the June-October 1999 period, with record or near-record dryness observed in
August and September (Fig. E5).
b. Europe/ Asia
Most of Europe, northern Africa and the Middle East experienced significantly
above-normal temperatures during October (Fig. E1), with
temperatures above the 70th percentile observed throughout the region. In
southern Europe, northern Africa and the Middle East, this warmth represents a
continuation of extremely warm conditions that have prevailed during the last several
months. Virtually all of northern Europe also experienced well above-normal temperatures
This anomalous warmth has been linked to the persistence of a large-amplitude anomalous
circulation at upper levels, which featured a trough just west of Portugal and an
anomalous ridge across Scandinavia. In the south, the continuation of excessive warmth
across southern Europe, northern Africa and the Middle East was also linked to the
persistence of an anomalous upper-level anticyclonic circulation throughout the region (Fig. T22, bottom).
2. Southern Hemisphere
The Southern Hemisphere circulation during October (Fig.
E15) featured above-normal heights over large portions of the middle latitudes,
which is similar to the anomaly pattern observed in the Northern Hemisphere (Fig. T22, bottom). In both hemispheres, this anomaly pattern has
persisted since March 1999. In the Southern Hemisphere, the anomalous height pattern
extending from eastern Australia to the eastern Pacific has been particularly persistent
during the last three months. This pattern has contributed to exceptionally warm
conditions across the eastern half of Australia , with large portions of eastern Australia
and New Zealand recording values above the 70th percentile throughout the
period (Fig. E1, bottom).
In South America, most of cental Argentina and southeastern Brazil experienced a
continuation of anomalously dry conditions during October (Fig. E3).
Rainfall deficits have been observed in these regions in each of the past three months.
This dryness has been linked to an overall poleward displacement 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 is consistent with ongoing La Niņa