Environmental Engineering Reference
In-Depth Information
Figure 3.13
Drought and sunspot cycles in western North America
Source:
After Schneider and Mtesirow (1976), Lockwood (1979)
drought prediction much time and effort has gone
into the study of the physical causes of drought.
The immediate causes commonly involve changes
in atmospheric circulation patterns. Drought in
the western prairies of Canada and the United
States, for example, is promoted by a strong zonal
airflow, which brings mild Pacific air across the
western mountains. As it flows down the eastern
slopes, it warms up, and its relative humidity
decreases, causing the mild, dry conditions in
winter and the hot, dry conditions in summer,
which produce drought (Sweeney 1985).
Seasonal drought in the Sahel was long linked to
the failure of the ITCZ to move as far north as
normal during the northern summer (see Figure
3.14). This is no longer generally accepted as
sufficient to explain the lengthier dry spells,
however, and the Sahelian drought is now being
examined as part of the broader pattern of
continent-wide rainfall variability, associated
with large-scale variations in the atmospheric
circulation (Nicholson 1989).
This type of knowledge is, in itself, of limited
help in predicting or coping with drought, since,
by the time the patterns are recognized, the
drought has already arrived. It is necessary to
move back a stage to try to find out what caused
the circulation change in the first place. Over
North America, for example, circulation patterns
seem to be related to changing sea surface
temperatures in the North Pacific, which cause
the course of the upper westerlies to be altered,
creating a zonal flow across the continent. The
failure of the ITCZ to move as far north as
normal into the Sahel has also been linked to a
strengthening of the westerlies in the northern
hemisphere. This prevents the poleward
migration of the sub-tropical anticyclone lying
over the Sahara, and it remains in place to block
the rain-bearing winds which would normally
flow over the area from the Atlantic (Bryson
1973). The drought in Africa has also been
Figure 3.14
Variations in the northward penetration of
the monsoon rains in the Sahel 1950-72
Source:
After Bryson and Murray (1977)
