Geoscience Reference
In-Depth Information
(NAO, see Section 2.2 ), which has a major impact on the weather in Northern
Europe. Differences in the sea surface temperature change the atmospheric
pressure distribution. There are two semi-permanent low pressure systems
that strongly influence the circulation pattern in the Arctic, namely the
Icelandic and the Aleutian Lows. When the gradient is above average (posi-
tive), strong westerly winds blowing over the relatively warm ocean (Gulf
Stream) advect warm air to Northern Europe but cold air to Alaska, Canada,
and Greenland. Such a situation occurred from 1986 to 1995. In 1995 the
NAO reversed phase, reducing the low pressure of the Icelandic Low, which
in turn reduced the advection, resulting in a far below normal winter tem-
perature (1995/6) in Northern Europe.
The Arctic Oscillation (AO, see Section 2.7 ) effect on weather patterns is
far reaching. In its negative phase, the AO's ring of air spins more slowly and
is more easily disturbed, allowing warm air from lower latitudes to penetrate
the Arctic. In the positive phase, the ring of air spins faster and impedes the
frigid Arctic air from moving south. Normally the index flops back and forth
within a winter. There has been a tendency in the last two decades for this
index to spend more time in the positive phase, reducing the outbreak of cold
air from the Arctic and, for example, the frequency of snowfall events in
Seattle, Washington.
The Pacific Decadal Oscillation (PDO, see Section 2.3 )hasasubstantial
effect on the temperature in the western Arctic. In 1975/6 the index changed
sign from negative to positive and a sudden temperature increase was observed
for Alaska. In Barrow (Figure 5.10b ) there is a 1 8C temperature difference for
the mean of the 10 years before and after the change in the index. These
differences are even more pronounced in winter, especially in January. This
sudden change is of the same order as the total change for the 81-year time
period, assuming linear regression analysis (Hartmann and Wendler 2005 ).
As mentioned previously, the Icelandic and Aleutian areas are preferred
areas for cyclonic activities. Anticyclones are located in Eastern Siberia,
Alaska/Yukon, and to a lesser extent over the central Arctic and Greenland.
These anticyclones are well pronounced in winter, while in summer the
cyclonic activity increases, as the cold core circumpolar westerly circulation
decreases in intensity due to the decreased latitudinal temperature gradient
(see section 5.11 ).
Precipitation
The precipitation is in general light with typical values of 200-300mm
annually. A great percentage of this amount falls as snow and is redistributed
by wind as drifting snow. The amount of snow is normally under-reported.
Since frequently snowfall amounts are small and accompanied by wind, it is
not easy to obtain an accurate measurement. Northern Alaska (e.g. Barrow
 
Search WWH ::




Custom Search