This week, the largest wildfire in its history is scorching New Mexico. Nine Western states, suffering from last winter’s drought, are at high risk for fire.
The U.S. Forest Service created Smokey Bear during the last long dry cycle in the West. SOURCE: USDA
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Giant forest fires are nothing new to the West. Scientists are learning that these fires come in cycles that are connected to patterns in the Pacific. We are in a cycle that is reducing snowfall in the Western mountains. During the previous dry cycle, which peaked in the 1940s and 50s, the US Forest Service invented Smokey Bear and began to teach people, “Only you can prevent forest fires”.
Unfortunately, the Pacific also plays a huge role in creating or preventing forest fires. If the water off the West Coast is cool, it cools the air above it and holds less moisture. The prevailing winds blow this dry air inland and there is little snow or rain.
Two ocean patterns shift cooler waters off the North American coast.
La Niña—The La Niña is when the tropical Pacific warms up. It shapes global wind and water patterns, which includes shifting cold water off the coasts of Mexico and California. This cooling normally lasts between 6 and 18 months.
Left: The Warm PDO (1925–1946 and 1977–2006). Right: The Cool PDO (1890-1924, 1947-1976 and now). Cool PDOs create drier conditions in the West. SOURCE: USGCRP
The cool phase of the PDO—The Pacific Decadal Oscillation is a longer cycle that shapes water temperatures in the North Pacific. It usually lasts about 50 years with cooler water shifting from the Western Pacific for twenty to thirty years, over to the Eastern Pacific, and back again. A cool phase of the PDO creates decades of reduced Western snowfall. Scientists disagree whether the cool phase began in 1999 or 2006, but it is cool and the West has had years of drier weather.
During the wet years of the PDO, (1977–2006), more trees grew which created dense forests. Now, the West is drier and the forests have more trees than the water supply can support.
Following last winter’s La Niña, the Western wilderness has seen multiple wildfires. SOURCE: U.S. Forest Service
The good news is that fires are part of the natural cycle. In the 1940s and early 1950s, the era of Smokey Bear, fires naturally thinned the forests and the following two decades, were less dangerous. The fires of today are shaping a quieter, less fiery tomorrow.
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It is very dry and hot out west! I heard that Denver has been well over 100 for a string of days. Hot and dry just do not make a good combination. Our meteorologists have told us this heat in the southeast will not be going ANYWHERE anytime soon. I always thought summertime was something to enjoy, but who wants this heat? Its only June so the horrifying thought for us is thinking about July and August. Is this going to be another hot summer around the southeast and southern plains? Im begging for rain because it is deeply needed. If we go to weak el nino this fall and winter, will that give us a pattern change to a cooler than normal and wetter than normal conditions? I agree, they could really use it because theres hardly any moisture in the air
Is it true that the Nina/Nino cycles exacerbate/modify the PDO to some extent? I would think so, and, if so, how does that work?
The two conditions that you identify are separate and, while the may influence each other, are also affected by other weather systems as well. For specific definitions of the El Nino/La Nina, which occur in tropical waters and the Pacific Decadal Oscillation, which refers to the currents in that entire body of water, please see the following pages:
• http://www.almanac.com/content/teleconnections-and-oscillations
• http://www.almanac.com/content/el-niño-and-la-niña
To find even more material on these topics, we invite you to search Almanac.com with the word “nino.”
We hope this helps!
Thank you for your interest in the Old Farmer's Almanac and our Web site.
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