Posted on 05/03/2004 8:42:22 PM PDT by ckilmer
The Denver Post perspective
Drought's a tricky phenomenon By Klaus Wolter
Sunday, May 02, 2004 -
In much of Colorado, the 1980s and '90s were years of benign climate conditions. Dry years were rare, and some of the wettest years on record were observed along the Front Range. If you moved into our state during this period, you might have gotten a misleading impression about typical conditions here.
Beginning in September 1999, Colorado experienced severe drought conditions, part of a global drought pattern that covered a large fraction of the United States, the Mediterranean, and southwest Asia. (It led to two of the worst wildfire seasons ever recorded in the western United States, 2000 and 2002.)
The following year was somewhat more benign, but precipitation continued to be below normal through the summer of 2001. With reservoirs (containing water from previous wet years) being drawn down, it was easy to ignore the initial drought.
However, during the third and worst drought year (2001-02), it was Colorado's turn to hit rock bottom, with record-low streamflows and record-high wildfire activity. It was the driest year ever observed for much of Colorado, including the foothills southwest of Denver that suffered through the Hayman fire in June 2002. (The longer drought periods of the 1950s and 1930s were more extreme for much of the Eastern Plains.)
Moisture returned during the subsequent year over northern Colorado, but much of southern Colorado suffered through a fourth drought year. Since September 2003, a reversal in drought patterns gave the southern mountains a decent rebound, while the South Platte and Upper Colorado basins recorded a prolonged dry spell that peaked statewide in March. However, this year's soggy April has gone a long way toward erasing the moisture deficits east of the divide, but the Upper Colorado basin was not nearly so lucky.
What do we know about droughts? Do they come in cycles? Despite a tendency for Western U.S. droughts to occur about once every two decades - most notably during the 1930s and '50s - careful analysis reveals much more complex behavior, especially over northern Colorado. The duration of droughts can vary from a brief, so-called "flash" drought like last July's heatwave to a seven-year drought like the Dust Bowl period of the 1930s, and to even longer droughts found in tree-ring records. Therefore, the length of a recent dry spell should not be used to predict the duration of the drought.
How do droughts get broken? In much of Colorado, especially along the Front Range and toward the San Juans, long-lasting La Niña events have been associated with drought, while a switch to El Niño is one of the more reliable indicators for a possible recovery. (The term "El Niño" refers to warmer-than-normal sea surface temperatures in the eastern tropical Pacific basin, while "La Niña" refers to the opposite conditions.) For example, the 1930s drought was replaced by wet conditions during the long-lasting El Niño of the early 1940s, and the 1950s drought came to an abrupt end with the emergence of the 1957-58 El Niño event. Last year's March "Storm of the Century" along the Front Range was consistent with impacts expected from the El Niño of 2002-03.
Aside from La Niña, there are many other influences that contribute to widespread and prolonged drought conditions. Several important ones involve land-surface feedback mechanisms: The longer a region stays dry, the more the soil dries out, and the sun's energy goes into heating up the atmosphere instead of evaporating moisture. During the warmer half of the year, this results in heat waves that tend to dry out the soil even further. Thus, moisture has to be imported rather than recycled to make it rain, and this phenomenon can perpetuate a drought over the interior of a continent. The 1930s Dust Bowl years were a prime example of this.
During extreme wildfire seasons, drought gets reinforced by large-scale forest fires. They release huge amounts of soot into the atmosphere that may "overseed" clouds, thus resulting in drizzle rather than regular rain. Suspended haze high above the ground tends to anchor temperature inversions and reduce the sunshine reaching the ground, thus hindering thunderstorm development. Much of the western United States was affected by this during the 2000 and 2002 fire seasons.
As a third feedback mechanism, late-season snowpack may have an influence on the monsoon. If snow lingers into the summer, as in 1995, it appears to inhibit the development of a surface heat low, one of the key ingredients of the monsoon system. On the other hand, if an anemic snowpack melts out early, it opens the door for an early onset of the monsoon season - to the extent that it can overcome the soil moisture and forest fire impacts described above. This mechanism provides an escape hatch out of prolonged drought conditions.
Other factors play a role and are the subject of ongoing research. Oceans have been called the flywheels of the climate system by maintaining long-lasting sea surface temperature anomalies that influence the atmosphere above them. Aside from the Pacific, with its El Niño and La Niña events, warm Indian Ocean and Atlantic Ocean temperatures may be detrimental by diverting the storm track away from us. Solar influences like the 11- and 22-year sunspot cycles have been correlated with precipitation anomalies around the world, including Colorado, but as yet there is no ready physical explanation for this apparent relationship. Much research remains to be done.
What will the future bring? While there is broad scientific consensus that global warming is likely to occur over the next century, it is much less clear how this will affect the climate of any given region, including Colorado. Our climate is shaped by occasional storms that cross our region, followed by dry spells that can last for weeks. Are the dry spells going to be more intense due to higher temperatures, while the stormy periods drop more moisture on us, as has been suggested by scientific reports?
While confidence with regard to precipitation prospects is low, most models suggest that Colorado's climate will become warmer. My concern is that this could lead to an earlier snowmelt in the mountains, and longer growing (and irrigation) seasons in the Plains, translating into higher water demand. Even if we receive as much moisture as we have in the past, this may pose a challenge for Colorado's future.
Klaus Wolter is a research scientist at the NOAA-CIRES Climate Diagnostics Center at the University of Colorado.
I am so sick of seeing this statement going unchallenged.
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