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 happened to produce low pressure over the middle Atlantic, which pushed the jet stream south into the southeastern United States.
Together these shifts pinned the jet stream for much of the summer in a single giant wave, rising over western Canada and falling over the eastern United States. Since the jet stream is what brings moisture in from the Pacific, its northward shift produced a brutal drought in the Southwest--while dumping the rain instead on the Northwest. The jet’s southward shift on the other side of the continent, meanwhile, contributed to heavy rains in the Northeast. We’ve seen this kind of coincidence of La Niña and the negative phase of the North Atlantic oscillation maybe six or eight times, says Rich Tinker of the National Weather Service, but it’s never been this intense before.