The 500-hPa circulation during
January was dominated by above-normal heights across central Canada, the central latitudes
of the eastern Pacific, and from Scandinavia southward to the eastern Mediterranean Sea (Fig. E9). The circulation also featured a continuation of
negative height anomalies over the high latitudes of the North Pacific, over the eastern
North Atlantic and western Europe, and over central Russia and central Siberia.Over
North America, this anomalous circulation resulted in significantly above-average
temperatures across Canada and the northern tier of the United States (Fig. E1). In Europe, it contributed to a continuation of 1)
above-average temperatures throughout the continent, and 2) above-average precipitation
over southern Europe (Figs. E3, E4).
In contrast, exceptionally cold temperatures were again observed during the month over
portions of north-central Russia and central Siberia.
a. North America
A strong split-flow pattern characterized the circulation over western North America,
with the northern branch of the flow bringing milder, marine air into western and central
Canada. Farther east, a weakening of the climatological mean "Hudson Bay Low"
was accompanied by a reduced northwesterly flow from Canada into the northern United
States (Figs. E9, E10). This
circulation pattern resulted in well above-average temperatures across Canada and the
northern half of the United States (Fig. E1), with the
largest anomalies averaging 7°-9°C over western Canada. This anomalous warmth is in
dramatic contrast to the extremely cold temperatures observed during December 2000, in
association with a pronounced flow of arctic air from northern Siberia into Canada and the
United States.
b. Europe
Above-average heights covered Scandinavia and the high latitudes of the North Atlantic
during January, while below-average heights covered the east-central North Atlantic (Fig. E9). This overall anomaly pattern is consistent with a
positive phase of the East Atlantic (EA) teleconnection pattern (Table
E1, Figs. E6, E7). The positive
phase of this EA pattern has persisted for four consecutive months, and has been
particularly pronounced for the past two months. This pattern has contributed to an
amplified flow of mild, marine air into Europe, which has resulted in significantly
above-average temperatures across the continent. It has also contributed to a southern
shift of the main storm track toward southern Europe, which has resulted in enhanced
rainfall across southwestern and southern Europe during the period.
c. Asia
Exceptionally cold temperatures have covered central Siberia and north-central Russia
since November 2000. During January, surface temperatures averaged 2°-5°C below normal
throughout the region. This excessive cold resulted from a persistent pattern of
below-average heights at 500-hPa throughout the region, which has produced a sustained
flow of polar air into the region at lower levels (Fig. E8).
2. Southern Hemisphere
The circulation during January featured above-average heights in the lower-
midlatitudes, and an expanded areal extent of the polar vortex (Fig.
E15) in association with below-average heights over large portions of the higher
midlatitudes. Over Antarctica, near-average heights were observed during January, which
contrasts with the extremely large positive height anomalies that dominated the region
last month.
The primary regions of above-average temperatures during January were observed along
the 30°S latitude band (Fig. E1, bottom). Particular areas
of significantly above-average temperatures included southeastern Brazil and the southern
half of Australia. This overall anomalous warmth was linked to a poleward extension of the
subtropical ridges around much of the hemisphere (Fig. T22),
and with a concurrent reduction in the strength of the upper-level westerly winds in the
vicinity of 30°S (Fig. T21).
