Scales of Climate in Designing with the Desert - Design with the Desert: Conservation and Sustainable Development - page 61

the earth-sun relationships, the resultant earth-atmosphere's general circulation pattern
(trade winds and Hadley cells, jet streams, the polar front activity, air masses that develop
and collide), and the two-thirds-oceans, one-third-land global configuration, with extreme
differences in the northern and southern hemispheres.
The key to desert design, and coping with desert climates on earth, relates to regions of
the subtropics as part of the general atmospheric circulation due to the Hadley Cell motion
system between the equator and the Tropics of Cancer and Capricorn (23.5° north and
south of the equator). In these subtropical latitudes, the air is hot and dry due to subsidence
(downward moving air to the surface) and compressional warming. In the context of the
deserts in the American Southwest, surface high pressure and clear skies exist most of the
time, interrupted only by dynamics of a summer monsoon from the south, fall tropical
storms and hurricanes, and upper-level low-pressure systems occasionally, or higher-
latitude intrusions of frontal storms in winter. Thus, we are used to expecting persistent
diurnal, seasonal, and annual climate rhythms in desert regions, as well as in other
climates. When there is a change in the timing, intensity, and persistence of these rhythms
due to global reverberations and changes as well as local changes, regional changes may
be induced that must be understood in order to anticipate and cope with any anomalies of
normally expected climate variability.
One of the best examples is our increased appreciation in only the last decade or so of the
impacts on the desert southwest of the Southern Oscillation and Pacific Decadal variations
in the Pacific Ocean basin. 10 The Southern Oscillation is a surface atmospheric pressure
difference across the southern Pacific that reverses atmospheric circulation and, as a result,
affects surface ocean currents. * The El Niño (EN) and La Niña (LN) phenomena represent
extensive areas of warm and cold water, respectively, off the west coast of South America,
notably Peru, and induce regional changes in the supply of moisture to our region on a
quasi-periodic and variable time scale, often less than a decade in length. The U.S. govern-
ment now monitors and forecasts these conditions on a seasonal basis since their impacts
are sizeable. In fact, several scientists consider these oscillations good analogs for study
on possible future societal impacts due to expected future regional climate change. The
Pacific Decadal Oscillation is longer than the El Niño-La Niña cycle and, in concert with
the Southern Oscillation, may cause longer-term changes in drought or flooding on a time
scale over decades.
In Arizona, when EN events are observed, winds reverse and come toward South
America. There is a low pressure, and we tend to receive above-normal precipitation in
the winter months. When LN occurs, drought may be more extensive (see Figure 3.2 as
an example). The teleconnections between EN/LN and Southwest precipitation are not
strong correlations but are statistically significant (e.g., the r 2 in Figure 3.2 is only 0.272,
thus a large unexplained variance still exists). Coping with moisture extremes of the sum-
mer monsoon and variable winter supplies of moisture is a challenge to designing livable
spots in the desert and to avoid dangerous zones that may flood periodically. Community
developments must not only protect against these conditions, but should develop further
schemes to store water during the wet times to combat the dry times. This surely would be
important in the rapid growth scenario of our future, with possible limited reservoir and
ground water supplies.
* When El Niño occurs, the Southern Oscillation Index (SOI) is negative—Tahiti minus Darwin pressure
normalized—meaning that in Darwin, Australia air pressure is higher than at Tahiti in mid-Pacific. When
La Niña occurs, the SOI index positive.