A scorching drought this year swept across the southwestern United States and the southern Plains. Aggravated by temperatures that ran 2 to 6 degrees above average, the drought damaged crops, drove down reservoir levels, and kindled millions of acres of forest fires. Parts of Nevada, Utah, Arizona, and California were still dry in October.
Meteorologists believe the drought was caused by an unusual combination of two of the planet’s major climatic cycles. In the equatorial Pacific, a swath of cool water known as La Niña--the flip side of the warm and more familiar El Niño--made its periodic appearance. The cool water induced a high-pressure system in the upper atmosphere that pushed the jet stream north, taking it from its normal course over the Southwest all the way up to the Pacific Northwest and Canada. Meanwhile the upper atmosphere over the Atlantic was going through its own pressure oscillation. That cycle ...
