Mechanisms of Climate Variability - Natural Hazards - page 37

In this respect, the NPO is probably the temperate
north Pacific extension of the Southern Oscillation,
with sea surface temperature changes in the north
Pacific lagging those in the tropics by three months.
However, the NPO does have its own independent
features. Of all atmospheric indices, the North Pacific
Oscillation shows the greatest well-defined trend over
time (Figure 2.8). The index has been steadily declin-
ing, although there is significant year-to-year variation.
Concomitantly with this trend, the Aleutian Low has
intensified and shifted eastward in winter. As a result,
storm tracks have shifted southward. This has resulted
in warmer and moister air being transported north-
ward along the west coast of North America into
Alaska. Sea surface temperatures in the north Pacific
have cooled, but sea-ice has decreased in the Bering
Sea.

Other phenomena characterize our sun. Solar flares
- representing ejection of predominantly ionized
hydrogen in the sun's atmosphere at speeds in excess
of 1500 km s -1 - develop in sunspot regions. Flares
enhance the solar wind that ordinarily consists of
electrically neutral, ionized hydrogen. Accompanying
any solar flare is a pulse of electro-magnetic radiation
that takes eight minutes to reach the Earth. This
radiation, in the form of soft X-rays (0.2-1.0 nm
wavelengths), interacts with the Earth's magnetic
field, increasing ionization in the lowest layer of the
ionosphere at altitudes of 65 km. The enhanced solar
wind arrives one to two days after this magnetic pulse
and distorts the magnetosphere, resulting in large,
irregular, rapid worldwide disturbances in the Earth's
geomagnetic field. Enhanced magnetic and ionic
currents during these periods heat and expand the
upper atmosphere. The solar wind also modulates
cosmic rays affecting the Earth. When the solar
wind is strong, cosmic rays are weak - resulting
in decreased cloud formation and warmer surface
air temperatures. Cloud cover decreases around
3-4 per cent between troughs and peaks in geo-
magnetic activity . Solar flare activity and the strength
of the solar wind are weakly correlated to the number
of sunspots (Figure 2.15b). Thus, geomagnetic activity
is a better indicator of solar influence on climate than
the number of sunspots. While solar activity on a
timescale of weeks can affect the temperature
structure of the stratosphere, there is no proven
mechanism linking it to climatic change over longer
periods near the surface of the Earth. This fact has
tended to weaken the scientific credibility of research
into the climatic effects of solar cycles.
Despite this, the literature is replete with examples
showing a correlation between climate phenomena and
solar activity in the form of sunspots. Much of the
emphasis has been upon a solar sunspot-global
temperature association. High sunspot activity leads
to warmer temperatures and more precipitation,
although this relationship is consistent neither over
time nor the surface of the globe. This discussion is
beyond the scope of this text and the curious reader
should see Hoyt and Schatten (1997) for further infor-
mation. More substantial is the fact that thunderstorm,
lightning and tropical cyclone activity worldwide
increases during periods of sunspot activity. In
addition, 15 per cent of the variation in the position of
storm tracks in the north Atlantic and Baltic Sea can be

ASTRONOMICAL CYCLES

Sola r cycles

(Shove, 1987; Hoyt & Schatten, 1997; Boberg & Lundstedt,
2002)

Sunspots are surface regions of intense disturbance of
the sun's magnetic field: 100-1000 times the sun's
average. The spots themselves appear dark, because
the magnetic field is so strong that convection is inhib-
ited and the spot cools by radiation emission. However,
the total amount of radiation given out by the sun
increases towards peaks in sunspot activity. The
sun has a 22-year magnetic cycle, consisting of two
11-year sunspot cycles. In Figure 2.15, these are plotted
back to 1500 AD. The 22-year periodicity known as the
Hale Cycle is evident as an enhanced peak in sunspot
numbers in this figure. There are also other periodici-
ties, at 80-90 years and 180 years, in the number of
sunspots. The interaction of these cycles has produced
periods where there was little sunspot activity. Two
of these, the Maunder and Dalton Minimums, in the
late seventeenth and early nineteenth centuries,
respectively, are evident in Figure 2.15. Both are
correlated to periods of cooler climate, at least in the
northern hemisphere. The Maunder Minimum is also
known as the Little Ice Age . Periods of high activity
correlate with warmer climate. This occurred in the
thirteenth century during the Medieval Maximum and
over the last 150 years concurrent with modern global
warming.