|1. Northern Hemisphere
The extratropical circulation during January featured
above-normal heights over much of the North Pacific, the southwestern United States, and
the high latitudes of the North Atlantic (Figs. E9, E11), and below-normal heights over western Canada, eastern
Europe and northern Africa. The first half of the month also featured significant blocking
activity over the high latitudes of the North Pacific (Fig. A2.3b)
in association with a pronounced westward retraction of the East Asian jet, while the
second half featured strong blocking activity over the high latitudes of the North
Atlantic (Fig. A2.3c).
In the subtropics, there was again considerable symmetry of the upper-level circulation
anomalies in both hemispheres during January (Fig. T22, bottom).
Notable aspects of this anomaly pattern included amplified troughs over the mid-Pacific of
both hemispheres, and anticyclonic circulation anomalies extending eastward in the
Northern Hemisphere from the Atlantic Ocean to Asia and in the Southern Hemisphere from
the Atlantic Ocean eastward to Australia. This overall pattern was also prominent
throughout 1999 in association with ongoing La Niņa conditions.
In the Northern Hemisphere, the anomalous low-latitude height field over the North
Pacific contributed to a confinement of the East Asian jet stream to the area well west of
the date line (Fig. T21). It also contributed to an
extremely strong diffluent flow in the exit region of the East Asian jet, along with an
amplification of the mean thermally-indirect transverse ageostrophic circulation
throughout the jet exit region. These dynamical changes in the wintertime East Asian jet
represent a fundamental aspect of the extratropical atmospheric response to mature cold
a. North America
The atmospheric circulation, temperature and precipitation patterns over the central
U.S. and Canada exhibited considerable variability during the month, in association with
strong blocking activity over the high latitudes of both the North Pacific and North
Atlantic (Figs. A2.3b, c). Farther south, above-normal
heights dominated the southwestern U.S. and Mexico during the month (Fig.
E9), which contributed to anomalously warm (Fig. E1)
and dry (Fig. E3, bottom) conditions in these regions.
Rainfall has been substantially below-average in the southwestern U.S. and northwestern
Mexico since August (see box labeled Southwest in Fig. E5).
This dryness reflects an early end to the regions monsoon rains in August, and a
nearly complete absence of measurable rainfall since October. This region typically
receives below-average rainfall during the cool season in association with La Niņa
conditions.The Gulf Coast region also recorded significantly below-normal rainfall during
the November 1999 - January 2000 period (Fig. E5), which is
also typical of mature La Niņa conditions. However, it is likely that the extreme
rainfall deficits observed during November and December in this region were also partly
linked to the strong positive phase of the NAO (Table E1).
b. North Atlantic/ Europe
The atmospheric circulation featured strong blocking activity over the high latitudes
of the North Atlantic during the second half of the month (Fig.
A2.3c). This blocking activity contrasts with the strong positive phase of the
North Atlantic Oscillation (NAO) observed during November and December (Figs. E6, E7, Table E1).
For the month as a whole, temperatures averaged near-normal over most of Europe, with
below-normal temperatures confined to the extreme south. Much of Europe also recorded
below-average precipitation for the month, which contrasts with the exceptionally stormy
conditions observed in northern Europe in December.
Below-normal heights dominated northern Africa and portions of the Middle East during
January, which contributed to below-average temperatures across portions of northern
Africa and the eastern Mediterranean Sea, and to near-average conditions over large
portions of the Middle East. They also contributed to near-to-above average rainfall over
large portions of the Middle East and eastern Mediterranean Sea. These conditions contrast
with the extremely warm temperatures and suppressed rainfall that had dominated these
regions for the past several months, in association with an anomalous anticyclonic
circulation at upper levels throughout the region.
2. Southern Hemisphere
In the Southern Hemisphere extratropics, the circulation during January featured a
continuation of above-normal heights over large portions of the middle latitudes (Fig. E15), with the largest and most persistent positive
anomalies observed in the region poleward of Australia and over the east-central South
Pacific (Fig. E17, left) Over the central and eastern
South Pacific, the above-normal heights were located immediately poleward of the La
Niņa-related subtropical cyclonic circulation anomalies, which is similar to the anomaly
pattern observed in the Northern Hemisphere (Fig. T22, bottom).
This strong inter-hemispheric symmetry of circulation features over the central and
eastern Pacific is typical of mature La Niņa episodes. At high latitudes, another
prominent feature of the Southern Hemisphere circulation was a continuation of negative
height anomalies throughout Antarctica (Fig. E17, right),
in association with an amplified polar vortex.
Anomalously cool (Fig. E1) and wet (Fig.
E3) conditions covered most of western Australia during January, while anomalously
cool and dry conditions were observed in the east. Each of these regions has experienced
cooler-than-normal conditions since November.
b. Southern Africa
The rainy season in southern Africa typically extends from October to April, with many
regions recording more than 75% of their mean annual rainfall during this period. The
heaviest rains are generally observed over southeastern Africa, while considerably less
rainfall is observed farther west in areas such as the Kalahari Desert of south-central
Africa and the Namib Desert of southeastern Africa. Rainfall throughout southern Africa
exhibits a strong relationship to the ENSO cycle, with above-normal rains generally
observed during La Niņa episodes and below-normal rainfall observed during El Niņo
Rainfall during January was above-average over central and southeastern Africa, with
totals exceeding the 70th percentile throughout the region (Fig. E3, bottom). Overall, area-averaged rainfall was above the
90th percentile during the month, when compared to the 1979-1995 base period
means. Above-normal rains have also been observed across the region since November (Fig. E4), following a late start to the rainy season during