1. Northern Hemisphere
The 500-hPa circulation during January 2004 featured above-average
heights at high latitudes and below-average heights in the middle latitudes
(Fig. E10). This anomaly pattern is
referred to as the negative phase of the Arctic Oscillation (AO) (Fig.
A2.1), and reflects a meridional mass exchange between middle and
high latitudes. A pronounced stratospheric warming is partly responsible
for the exceptionally high stratospheric penetration of the AO signal
during January, which extended up past the 30-hPa level (Fig.
S1). Over the North Atlantic this stratospheric warming spread
southward to 60EN, where mean lower
stratospheric temperatures ranged from 8E-11EC
above average (Fig. S2, bottom).
Regional aspects of this hemispheric anomaly pattern included blocking
flow configurations over Alaska and Greenland, amplified troughs across the
central North Pacific and North Atlantic, and a large-amplitude trough over
northern and eastern Europe. These regional anomalies reflected the
negative phases of several prominent teleconnection patterns, including the
West Pacific (WP) Oscillation, the East Pacific pattern, the North Atlantic
Oscillation (NAO), the East Atlantic/ West Russia pattern, and the
Scandinavia pattern (Table E1, Figs. E7,
Prominent temperature departures during January included significantly
colder than average conditions across Alaska, Canada, the northeastern
United States, and eastern Europe, and a continuation of warmer than
average SSTs over large portions of the North Atlantic (Fig.
E1). Overall, Northern Hemisphere land-only temperatures averaged
slightly below normal (Fig. E2), which is
only the fourth such occurrence since early 1997. Prominent precipitation
anomalies during the month included above-average totals in the
northwestern U.S., and both northwestern and southeastern Europe, and
below-average totals in the southeastern and eastern U.S. (Figs. E3,
a. Pacific/North America
The mean upper-level circulation during January featured an amplified
ridge over Alaska and an amplified trough over eastern Canada (Fig.
E10). This circulation contributed to anomalously cold air across
Canada and the northeastern U.S., with surface temperatures throughout the
region averaging 2o to 4oC below average (Fig.
E1). In southeastern Canada and the New England states monthly mean
temperatures were below the 10th percentile of occurrences.
b. North Atlantic/Eurasia
Over the North Atlantic positive 500-hPa height anomalies at high
latitudes reflected a continued absence of the mean Icelandic Low. These
anomalies, in combination with below-average heights in the middle
latitudes, reflected the negative phase of the NAO (Table
E1). This circulation, combined with the exceptionally warm SSTs at
high latitudes, contributed to above-average surface temperatures over
Great Britain and across large portions of southern Europe (Fig.
E1). It also contributed to above-average precipitation in
northwestern Europe where the mean jet core entered the continent, and over
southeastern Europe in the vicinity of the mean upper-level trough (Fig.
2. Southern Hemisphere
In the Southern Hemisphere the 500-hPa circulation during January
featured below-average heights over western Antarctica, and a zonal wave-3
pattern in the middle latitudes with above-average heights over the three
ocean basins. (Fig. E16). Anomalously warm
and dry conditions were observed over southern South America during the
month, with many locations reporting mean temperatures in the warmest 30th
percentile of occurrences (Fig. E1) and
precipitation totals in the driest 30th percentile of
occurrences (Fig. E3).
In southern Africa the rainy season normally lasts from October to
March. Rainfall was above-average during January (Fig.
E3) with area-average totals exceeding the 80th
percentile (Fig. E4). Overall, the 2003-2004
rainy season has been below-average as area-averaged totals during November
and December were in the lowest 10th percentile of occurrences.