Mechanisms of Climate Variability - Natural Hazards - page 22

of monsoon rains and resulting drought in the Sahel in
the early 1970s. In the Sahara region, rain normally
falls as the intertropical convergence moves seasonally
northward. If westerlies expand, then such movement
of the intertropical convergence is impeded and
drought conditions prevail. Drought in Britain can also
be linked to displacement northward of the jet stream
and polar-front lows - shifting into the Arctic instead
of crossing Scotland and Norway. This displacement
leaves Britain under the influence of an extension of
the Azores high-pressure system that is stable and
rainfall-deficient. Blocking of this high leads to drought
for several months. Blocking highs off the Californian
coast were also responsible for drought in the mid-
United States in 1997, while highs over eastern
Australia have led to some of the worst droughts in that
continent's recorded history. The social response to
these types of droughts will be examined in Chapter 5.

stream and the polar front is approximate, and a clear
relationship has not been established between the jet
stream in the upper troposphere and the formation of
low pressure at the Earth's surface. Indeed, no one
theory adequately explains the initial formation of lows
at mid-latitudes. The Palmén-Newton general circula-
tion model is a good teaching model, but it is not an
ideal model for explaining the causes of climate
change.
Conceptually, the Palmén-Newton model in its
simplest form has two areas of forcing: upward air
movement at the equator from heating of the Earth's
surface, and sinking of air at the poles because of
intense cooling caused by long wave emission. At the
equator, air moves from higher latitudes to replace
the uplifted air, while at the poles subsiding air moves
towards the equator as a cold dense mass hugging
the Earth's surface. The lateral air movement at the
equator is slow and weak, while that at the poles is
strong and rapid. Polar surging can reach within 10° of
the equator, such that some of the lifting of air in this
region can be explained by the magnitude and location
of polar outbursts. Hence, the dominant influence on
global air circulation lies with outbursts of cold air
within polar high-pressure cells. Each of these out-
bursts forms an event termed a mobile polar high .
There is no separate belt of high pressure or Hadley
cell in the subtropics. These cells, as they appear on
synoptic maps, are simply statistical averages over time
of the preferred pathways of polar air movement
towards the equator.
Mobile polar highs developing in polar regions are
initially maintained in position by surface cooling, air
subsidence and advection of warm air at higher alti-
tudes. When enough cold air accumulates, it suddenly
moves away from the poles, forming a 1500 m thick
lens of cold air. In both hemispheres, polar high out-
breaks tend to move from west to east, thus conserving
vorticity. Additionally, in the southern hemisphere,
pathways and the rate of movement are aided by the
formation of katabatic winds off the Antarctic icecap
(Figure 2.5). In the northern hemisphere, outburst
pathways are controlled by topography with mobile
polar highs tending to occur over the Hudson Bay
lowland, Scandinavia and the Bering Sea (Figure 2.5).
The distribution of oceans and continents, with their
attendant mountains, explains why the mean trajectories
followed by these highs are always the same. For
instance, over Australia, a polar outbreak always

Mobile polar highs (Leroux, 1993, 1998;
Bryant , 1997)

The monsoonal circulation described above does not
fit well within the Palmén-Newton general circulation
model. In fact, the positioning of Hadley cells, and the
semi-permanency of features such as the Icelandic or
Aleutian Lows, are statistical artefacts. There is not an
Icelandic Low, locked into position over Iceland. Nor
are there consistent trades or polar easterlies. Air
circulation across the surface of the Earth is dynamic,
as is the formation and demise of pressure cells. In
reality, the Icelandic Low may exist as an intense cell of
low pressure for several days, and then move eastward
towards Europe within the westerly air stream.
Climate change in the Palmén-Newton model implies
the movement, or change in magnitude, of these
centers of activity. For example, weakening of the Ice-
landic Low conveys the view that winter circulation is
less severe, while expansion of Hadley cells towards
higher latitudes suggests that droughts should
dominate mid-latitudes.
There are other problems with terminology in the
Palmén-Newton model; these have an historical basis.
The depression, extra-tropical cyclone, or polar low
was initially explained as a thermal phenomenon, and
then linked to frontal uplift along the polar front, to
upper air disturbances, and recently to planetary waves
or Rossby waves. The jet stream is supposedly tied
to the polar front, and related to cyclogenesis at mid-
latitudes. However, the connection between the jet