An Expert’s Look at Drought
Science News recently interviewed Kelly Redmond, Deputy Director and Regional Climatologist of the Western Regional Climate Center, Desert Research Institute in Reno, for his take on California’s current drought.
SN: What is a drought?
KR: Drought is a nexus of two things: the physical environment (what it’s doing or not doing), and how society and the environment are affected. That part is determined by impacts that aren’t meteorological or climatological.
The broad definition of drought is “insufficient water to meet needs.” Sometimes there is a deficiency in water supplies, and sometimes there is too much water demand (by both people and vegetation). It’s that relationship between supply and demand where drought is found. If water supplies were the same as in a previous dry period, but the demand situation is different, there could be better or worse consequences. There are also considerations arising from the sequencing of climatic events, how they were ordered in time.
SN: How does California’s current drought compare to past droughts?
KR: Although California’s current drought is comparable to some of the state’s more severe droughts, it’s not the worst, although it’s probably in the top five. There have been droughts in California’s history lasting just a year or two where maybe one winter did not provide sufficient precipitation. The winter periods of 1897-99, 1923-24, and 1975-77 all exceed or rival the current drought in terms of lack of precipitation over one to two years. The ‘granddaddy’ of California droughts is the 1975-76 and 1976-77 combination. But as the state has developed its water resources more thoroughly, it’s built in buffers against that kind of behavior, so it takes two to three years for dry conditions to really hurt - like the drought from winter 1986-87 through 1993-94, with an interruption in 1992-93. There are still large numbers of people in Reno and on both east and west sides of the Sierra who remember that extended drought.
SN: What causes California droughts?
KR: California droughts are winter droughts, because there’s only one main precipitation season. During drought winters, storms that approach the West Coast often head north to Washington, Oregon, and Canada, or else they split and a piece goes north, another piece goes south. They don’t slam directly into the Sierra Nevada. In drought winters a lot of storms that come in do not reach the Sierra, but rather dive down to the south along the coast, to about the latitude of Los Angeles or so, and then head east-taking an unusual trajectory.
The Sierra Nevada Mountain Range helps cause its own precipitation since it’s elevated and air has to go up over it-rising up those long slopes. Like a ramp, it starts at sea level at Sacramento, rising up the entire way, losing its moisture. This provides a more effective process for removing moisture than if the elevation were the same all the way. The presence of the mountains greatly accentuates precipitation, especially over the Sierra and to a lesser extent, the Coast Range, when wind is blowing straight up the mountains following the path of Interstate 80. When the winds blow somewhat parallel to the mountains, roughly along California Highway 49, someone at the northern end of the valley will usually be favored with precipitation.
In a typical winter, the Sierra experiences maybe 20-25 storms of some consequence, with three to five of them usually heavy-duty storms. If you have a winter with less frequent storms or not as many of those big guys, that will lead to a loss of precipitation. A storm that drops a foot of snow helps a little, yet we still need some of those bigger storms like we had in late February and early March where they drop three to six, or even 10 feet of snow. A few such storms help make a difference, and the importance of their contribution to total winter precipitation is greater farther south in the Sierra.
SN: Are there things going on in the Pacific Ocean now that are contributing to California’s continued drought?
KR: Whenever California has a drought, there’s something in the Pacific that’s helping to cause that. Several parts of the Pacific Ocean have a bearing on what goes on in California: the equatorial region to the south where El Niño and La Niña occur, factors in the Gulf of Alaska, and thirdly the far western Pacific near Indonesia and even the Indian Ocean all act to govern or influence storms that eventually reach the West Coast. (El Niño is a warming of surface water in the Pacific Ocean right on the equator between South America and the Date Line; La Niña is a cooling of the ocean surface in the same place).
We’re having a La Niña winter now. La Niña winters have a “funny personality” to them. In general, they lead to drier than normal winters in the southern third of the state, but we also see when we look at California floods—particularly those big ones that can only happen from the Sierra-they seem more prevalent in La Niña winters than El Niño winters. The types of storms that produce precipitation in El Niño and La Niña years are also somewhat different. El Niño and La Niña tend to fluctuate back and forth over an irregular interval of four to seven years.
“Weather is like a football game in the NFL, and the climate is like the season--the sum of all the games.”
We weren’t “rescued” from really severe drought by the storm sequence that came in during late February, early March, but it helped quite a bit. It still looks like this year will come in deficient unless we get more storms. By coming into winter 2008-09 after two years of loss, we have a bigger hydrological hole to get out of. The 2009 storms helped prevent this hole from getting dug any deeper. Part of what got California into the situation it’s in is the recent dry springs. For example, last winter, once we got into March, it shut off entirely and we had the driest spring-into-summer period in recorded history.
This year, February turned out better than usual, March looks like it will come in about average, and “we’ll see” what April brings. Once you get past April, climatologically, you’re not going to expect much help.
SN: What’s the difference between climate and weather?
KR: With weather, the atmosphere soon forgets its starting conditions. It doesn’t have a memory of its past that lasts very long-maybe a range of a week or two. The conditions that lead to what follows become forgotten after a while. Climate, on the other hand, starts around two to three weeks from today and extends on out to months, and years, and decades and centuries. The two are inextricably linked. Climate can be thought of as the distillation of weather-the summary of weather. Or conversely, climate can be thought of as enabling the detailed weather we experience. The climate system gets into certain modes and those modes favor or disfavor the occurrence of certain types of weather.
Weather is like a football game in the NFL, and the climate is like the season-the sum of all the games.