Geoscience Reference
In-Depth Information
They propose that such synoptic support is important because the pure katabatic
wind layer is only a few hundred meters deep. A positive feedback on the katabatic
system can occur when a mesocyclone develops off the coast.
Numerical simulation of Polar Lows is still in its infancy. Renfrew (
2003
) also
points out that polar mesocyclones, including Polar Lows, may play an important
role in the high-latitude climate system through strong coupling of the atmosphere
and ocean via air-sea heat exchange.
4.7
Modes of Atmospheric Variability Affecting the Arctic
4.7.1
The North Atlantic Oscillation (NAO)
The NAO has long been recognized as a major mode of atmospheric variability
affecting the Arctic. The NAO describes the tendency for the Icelandic Low and
Azores High, the two “centers of action” in the atmospheric circulation of the North
Atlantic, to covary in strength. When the Icelandic Low is strong, the Azores High
also tends to be strong; in this case, the NAO is in its positive phase. When the
Icelandic Low and Azores High are both weak, the NAO is in its negative phase. In
the negative phase, the Icelandic Low also tends to be shifted to the southwest. This
covariability is associated with significant high-latitude signals in SAT, cyclone
activity, moisture transport, precipitation, SSTs, and ocean heat transport; some of
these impacts will be examined here while others will be addressed in subsequent
chapters as appropriate. When its index is based on SLP anomalies between two
fixed points (as has traditionally been the case [more on this shortly]), the NAO
is best expressed during winter. Research has naturally focused on this season. As
reviewed by H. van Loon and J. Rogers (
1978
), manifestations of the NAO have
been recognized for centuries. For example, the missionary Hans Egede Saabye
recorded in a diary kept in Greenland (1770-1778) that the Danes were well aware
that when winters in Greenland were especially severe, the winters in Denmark
tended to be mild, and vise-versa. Description of this temperature seesaw associated
with the NAO can be found in writings throughout the nineteenth century.
The first formal description of the NAO is credited to Sir Gilbert Walker. In
a paper with E.W. Bliss (Walker and Bliss,
1932
), a selected set of highly cor-
related SAT, SLP, and precipitation time series at widely separated stations over
eastern North America and Europe were combined to develop an NAO index. The
same approach was used to identify the North Pacific Oscillation and the Southern
Oscillation. Walker and Bliss described the NAO in terms of a tendency for sea
level pressure to be low near Iceland in winter when it is high near the Azores and
southwest Europe, attended by unusually high temperature in northwest Europe and
low temperatures off the Labrador coast” (1932). When Iceland pressures are low
(high), the NAO index is positive (negative). The NAO index is hence a measure
of the meridional gradient in SLP over the north Atlantic, which can also be inter-
preted in terms of the strength of the mid-latitude westerly (west to east) winds.
