Geoscience Reference
In-Depth Information
700mb pattern shows very weak subtropical highs,
deep meridional troughs and a blocking anti-
cyclone off Alaska (see
Figure 7.24D
). The cause
of these variations is still uncertain, although it
would appear that fast zonal flow is unstable and
tends to break down. This tendency is certainly
increased in the Northern Hemisphere by the
arrangement of the continents and oceans.
Detailed studies are now beginning to show
that the irregular index fluctuations, together
with secondary circulation features, such as cells
of low and high pressure at the surface or long
waves aloft, play a major role in redistributing
momentum and energy. Laboratory experiments
with rotating 'dishpans' of water to simulate
the atmosphere, and computer studies using
numerical models of the atmosphere's behavior,
demonstrate that a Hadley circulation cannot
provide an adequate mechanism for transporting
heat poleward. In consequence, the meridional
temperature gradient increases and eventually the
flow becomes unstable in the Hadley mode,
breaking down into a number of cyclonic and
anticyclonic eddies. This phenomenon is referred
to as
baroclinic instability
. In energy terms, the
potential energy in the zonal flow is converted
into potential and kinetic energy of eddies. It
is also now known that the kinetic energy of
the zonal flow is derived
from
the eddies, the
reverse of the classical picture, which viewed
the disturbances within the global wind belts as
superimposed detail. The significance of atmo-
spheric disturbances and the variations of the
circulation are becoming increasingly evident. The
mechanisms of the circulation are, however,
greatly complicated by numerous interactions and
feedback
processes, particularly those involving
the oceanic circulation discussed below.
temperatures between Western Europe and
western Greenland associated with the north-
south change in pressure gradient over the North
Atlantic. The phenomenon at work here is the
North Atlantic Oscillation (NAO). While Walker
originally defined an index of the NAO from a set
of highly correlated surface air temperature, sea-
level pressure and precipitation time series at
widely separated stations over eastern North
America, Walker and Bliss later suggested that a
simpler index could be based on the pressure
difference between Iceland and the Azores.
The NAO index based on this idea describes the
mutual strengthening and weakening of the
Icelandic low (65
N).
When both are strong (weak) the NAO is taken to
be in its positive (negative) mode, or phase. While
the NAO can be identified in all seasons, most
research has focused on the winter season when it
tends to have its strongest expressions. The
relationship between the positive and negative
modes of the NAO noted by Walker, and the
associated temperature and other anomaly
patterns, are shown in
Plate 7.3
for two contrast-
ing Januarys. When the two pressure cells are well
developed as in January 1984, the zonal westerlies
are strong. Western Europe has a mild winter,
while the intense Icelandic Low gives strong
northerly flow in Baffin Bay, low temperatures in
western Greenland and extensive sea ice in the
Labrador Sea. In the negative phase the cells are
weak, as in January 1970, and opposite anomalies
are formed. In extreme cases, pressure can be
higher near Iceland than to the south, giving
easterlies across Western Europe and the eastern
North Atlantic.
Through the late 1990s and early 2000s,
following the work of D. Thompson, considerable
debate arose as to whether the NAO should be
considered as part of a more general pressure
(mass) oscillation between the north polar region
and mid-latitudes, known variously as the Arctic
Oscillation (AO) or Northern Annular Mode
(NAM). Part of the argument for considering the
NAM as the more fundamental mode is its
°
N) and the Azores high (40
°
North Atlantic Oscillation
The relative strength of the Icelandic Low and
Azores High was first observed to fluctuate on
annual to decadal scales by Sir Gilbert Walker in
the 1920s. Fifty years later, van Loon and Rogers
discussed the related west-east 'see-saw' in winter
