The 500-hPa circulation during January featured a persistent pattern of
positive height anomalies over western North America, Greenland, the low
latitudes of the eastern North Atlantic, and central Asia, and negative
height anomalies over the central North Pacific, eastern North America,
southern Europe and Scandinavia (Figs. E10, E12). These anomalies
project onto the positive phases of the Pacific/ North American (PNA) and
the East Atlantic (EA) teleconnection patterns, each of which has persisted
since October (Table E1, Figs. E7,
E8). The persistence of these
teleconnection patterns is consistent with the ongoing El Ni�o
and with an overall negative phase of the Arctic Oscillation (Fig. A2.1).
In the subtropics a marked inter-hemispheric symmetry of the 200-hPa
circulation anomalies was also evident during January in association with
the ongoing El Ni�o (Fig.
T22). This symmetry included anticyclonic streamfunction anomalies in
both hemispheres flanking the region of enhanced convection over the
central equatorial Pacific, and cyclonic streamfunction anomalies in both
hemispheres over the subtropical eastern Pacific and over most of the
subtropical Atlantic.
The prominent temperature anomalies during January included much warmer
than normal conditions across the western United States, Canada, and
central Asia, and cooler than normal conditions across the eastern United
States and Scandinavia (Fig. E1). Prominent precipitation departures
during the month included above-average precipitation across portions of
southern Europe, and below-average precipitation over the southwestern and
eastern United States, and eastern Canada (Figs. E3,
E4).
a. Pacific/North America
The strong positive PNA (+1.5) pattern during January (Table E1)
reflected amplified upper-level ridges over the subtropical North Pacific
and western Canada, and upper-level troughs across the central North
Pacific and the southeastern United States (Figs. E10,
T22). These
anomalies were associated with an extension of the East Asian jet stream
across the eastern North Pacific, and with a shift of that jet exit region
and accompanying split-flow configuration to well east of the date line (Figs.
E11, T21).
The amplified upper-level ridge over western North America during
January contributed to above-average surface temperatures across the
western United States, Alaska, and Canada. In Alaska and Canada
temperatures ranged from 2�C above
average while over the western U.S. they ranged from 2-4�C
above average (Fig. E1). This anomalous warmth is a typical winter
feature during El Ni�o
episodes.
b. North Atlantic and Europe
The circulation over the North Atlantic again featured
above-average heights at high latitudes and in the subtropics, and
below-average heights in the middle latitudes (Fig. E10). This
circulation was associated with high-latitude blocking and a pronounced
split-flow configuration over the eastern North Atlantic, and with a
southward shift of the mean wintertime jet axis to southern Europe (Figs.
E11, T21). This overall anomaly pattern has persisted since October,
and reflects the ongoing positive phase of the East Atlantic teleconnection
pattern (Table E1, Figs. E7,
E8). During January these conditions
were associated with a continuation of warmer and wetter than normal
conditions across southern Europe, and cooler than normal conditions in
Scandinavia (Figs. E1, E3).
2. Southern Hemisphere
Above-average 500-hPa heights were evident over southeastern Australia
and the high latitudes of the South Atlantic during January, and
below-average heights covered the mid-latitudes of the South Atlantic and
the area south of Africa (Fig. E16). In the subtropics a strong
inter-hemispheric symmetry of the 200-hPa circulation anomalies was again
evident during the month in association with the ongoing El Ni�o
(Fig. T22). This symmetry included anticyclonic streamfunction
anomalies in both hemispheres flanking the region of enhanced convection
over the central equatorial Pacific, and cyclonic streamfunction anomalies
in both hemispheres over the eastern Pacific and most of the Atlantic.
The dominant temperature and precipitation anomalies during January
reflected a continuation of warmer and drier than average conditions over
eastern Australia and across northeastern South America. These conditions
are consistent with El Ni�o (Fig. E4) but were also linked to the local 200-hPa circulation features,
with the anomalous warmth coinciding with the upper level ridge axes in
both regions, and the anomalous upper-level convergence (Fig.
T23) and
implied sinking motion situated between the upper-level ridge and
downstream trough axes.