1. Northern Hemisphere
The 500-hPa circulation
pattern during January featured persistent positive height anomalies across
the central North Pacific, and from eastern
North America
to central
Siberia
, and persistent negative anomalies over
Alaska
, the
Mediterranean Sea
, and south-central
Russia
(Figs. E9, E11).
The main surface temperature departures reflected well above-average
temperatures across
North America
and below-average temperatures across much of
Eurasia
(Fig. E1). The main precipitation departures
included above-average totals in the northwestern
U.S.
and
Mediterranean Sea
, and below-average totals in the southwestern
U.S.
and northern
Europe
(Figs. E3, E5, E6).
a. Pacific/
North America
The 500-hPa circulation
featured a combination of above-average heights over the east-central North
Pacific and eastern
Canada
, and below-average heights over
Alaska
and western
Canada
. This anomaly pattern reflected a nearly zonal flow across North America in
association with a disappearance of the mean upper-level ridge normally
located over western North America and a marked weakening of the
climatological mean Hudson Bay trough (Fig. E9).
This zonal flow produced near-record warmth throughout the continent, with
much of the
U.S.
and
Canada
recording departures of +5°C to +8°C. For nearly the entire continent east
of the
Rocky Mountains
, monthly temperatures were above the 90th percentile of
occurrences.
The circulation also featured
a pronounced eastward extension of the East Asian jet stream and an eastward
shift of the jet exit region to just upstream of the northwestern
United States
(Fig. T22). This jet stream pattern
resulted in a very focused storm track and
exceptionally heavy precipitation in the northwestern U.S. Monthly
precipitation totals in the Pacific Northwest have exceeded the 90th
percentile in three of the last four months. In contrast, a lack of
storminess farther south resulted in well below-average precipitation in the
southwestern
United States
. Area-average precipitation totals in this region have been below the 10th
percentile for the last five months. These ongoing deficits have resulted in
severe drought conditions across most of southern
Arizona
and western
New Mexico
.
b.
Europe
The
500-hPa circulation pattern during January featured above-average heights
across
Scandinavia
and the adjacent polar region, and below-average heights across the
Mediterranean Sea
and south-central
Russia
(Fig. E11). A pronounced split flow was
also evident over the eastern
North Atlantic
. The northern branch of the jet stream headed north of
Scandinavia
and then strongly southward into western
Russia
(Fig. E10). The southern branch of the jet
stream was situated over northern
Africa
. This circulation represented a nearly complete disappearance of the normal
flow of marine air into
Europe
, and a significant flow of polar air into
Russia
. This combination resulted in below-average temperatures across much of
Eurasia
, with the largest negative departures (-5° to -8°C) centered in
south-central
Russia
and
Kazakhstan
where temperatures were below the 10th percentile of occurrences.
2. Southern Hemisphere
The mean 500-hPa circulation pattern
during January featured above-average heights from the central
Indian Ocean
to the central South Pacific, and below-average heights across the high
latitudes of the South Pacific (Fig. E15).
In
Australia
, the mean upper-level ridge axis was located over the middle of the
continent. Below-average rainfall and temperatures occurred upstream of the
ridge axis and above-average rainfall and temperatures occurred downstream
over eastern
Australia
(Fig. E3). Temperatures in eastern
Australia
were again significantly above-average (3+ °C above average) during the
month, and generally exceeded the 90th percentile of occurrences
(Fig. E1). This warmth is consistent with a
disappearance of low-level flow from the
Great Australian Bight
, and with below-average precipitation throughout the region (Fig. T20).
In southern
Africa
the rainy season normally lasts from October to April. Area-average rainfall
was above normal during January, marking four consecutive months of
above-average rains (Fig. E4). An enhanced
South African rainy season is consistent with ongoing La Niña conditions.
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