The circulation during February
featured a persistent pattern of above-normal heights across the central North Pacific,
the Gulf of Alaska, the southeastern United States, and the high latitudes of the North
Atlantic, and below-normal heights over eastern Siberia and the western United States (Figs.
E10, E12). One aspect of this
large-scale pattern was a strong negative phase (-1.1) of the North Atlantic Oscillation (Table E1, Figs. E7, E8).
A second aspect of the pattern was a well-defined jet exit region over the central North
Pacific of the East Asian jet, and a confinement of the jet core to well west of the date
line (Fig. T22). These features of the East Asian jet and
jet exit region are typical of mature cold episode conditions during the NH winter and
early spring season, and are linked to the La Niņa-related enhancement of the subtropical
ridge over the western North Pacific and the subtropical trough over the central North
Pacific (Fig. T22).On a regional basis this overall
anomaly pattern contributed to exceptionally warm temperatures over Alaska and the
southeastern United States during February, and to below-average temperatures over the
northern plains and south-central Canada (Fig. E1).
Above-average temperatures were also observed across central and much of southern Europe
during the month.
The prominent precipitation anomalies during February reflected a
continuation of exceptionally dry conditions across both the southeastern United States
and the Pacific Northwest (Fig. E3). Below-average
precipitation was also observed over portions of southern Europe (Fig.
E4), while extreme northern Europe, southeastern Scandinavia, and western Russia
recorded above-average precipitation.
a. North America
The circulation over North America featured a persistent pattern of above average
heights over the southeastern U.S. and below-average heights over the western U.S. (Fig. E10). These conditions were associated with a pronounced
split flow pattern that covered western North America. The northern branch of this split
flow extended northward to Alaska and then southeastward to the Great Lakes region, and
the southern branch extended from Southern California to the mid-Atlantic States. Farther
downstream the confluence of these two flow streams over the central and eastern United
States contributed to an axis of heavy precipitation extending from northeastern Texas to
the eastern Great Lakes.
Above-average precipitation during February also extended from the Great Lakes to
northeastern Canada. This area was situated in a region of increased cyclonic curvature
within the base of the Hudson Bay low.
Precipitation was again significantly below average across Florida and the southeastern
U.S. during February (Fig. E3). A persistent pattern of
above-average heights and anomalously low rainfall has prevailed across these areas for
much of the past three years in association with ongoing La Niņa conditions. During the
last year (March 2000 February 2001), Florida recorded its second lowest rainfall
total in the historical record dating back to 1895. The southeastern U.S. also recorded
anomalously warm conditions (1°-2° C above average) during February (Fig.
Above-average heights covered the high latitudes of the North Atlantic during the
second half of February, which contributed to an overall negative phase of the North
Atlantic Oscillation (NAO) (Table E1, Figs. E7, E8). This circulation was accompanied
by anomalously warm and dry conditions across southern Europe (Figs. E1 and E3), and above-average
temperatures over central Europe.
Strong winter stratospheric warming over the Northern Hemisphere polar region reached
peak levels at the end of January and early February (Fig. S4).
The unusually long duration of strong vertical transport of wave energy during January and
February (Fig. S7) contributed to the strength of the
stratospheric warming. By mid-February, strong positive height anomalies (Fig. S1) prevailed over the north polar region in the upper
troposphere and stratosphere, as the polar anticyclonic circulation strengthened
throughout the stratosphere. Associated with these temperature (Fig.
S3) and circulation anomalies, there were also strong positive total ozone
anomalies which prevailed over the north polar region (Fig. S6).
Similar features accompanied a strong north polar winter warming in February 1999, which
was the first major warming to occur since 1991.
2. Southern Hemisphere
The circulation during February was dominated by a persistent pattern of above-average
heights extending from southeastern Australia to central South America, and by
below-average heights across the high latitudes of the South Pacific and southern South
America (Figs. E16, E18). This
anomalous circulation was associated with above-average temperatures over central
Argentina and below-average temperatures over southern South America (Fig.
E1). It was also associated with significantly warmer-than-average conditions
across southeastern Australia, with temperatures in many areas exceeding the 90th
percentile (Fig. E3).
The rainy season in southern Africa typically extends from October to April, with the
peak in rainfall typically occurring during January-February. During February 2001
significantly above-average rainfall was recorded in southeastern South Africa and
Mozambique (Fig. E3). These rains were preceded by
below-average rainfall during January 2001 (Fig. E4).