Through much of the late 20th century, those interested in crop prospects have followed sea surface temperatures because of the strong correlation between shifts in this measure and agricultural production in many areas. Shifts between El Niño, the term for higher sea temperatures, and La Niña, the corresponding term for lower temperatures, continue of great importance in determining whether growing seasons will tend toward dryness or wetness.
In his weather column in the current issue of Milling & Baking News, Drew Lerner of World Weather Inc. discusses a very different phenomenon that appears to be exerting a marked impact on global weather – known as Arctic Oscillation. “Arctic Oscillation is determined by the strength of a prevailing high pressure system anchored over the arctic,” he writes. He explains that when the A.O. is negative, which is the case this late winter, cold temperatures tend to extend from the Canadian Prairies deep into U.S. growing areas. A positive A.O. a year earlier was associated with no snow on the ground and record warm temperatures, he says.
Recognition of the importance of A.O. in driving weather patterns is relatively new, identified and named only in the late 1990s. Unlike La Niña and El Niño, which emerge gradually, A.O. has a more random, abrupt character, fluctuating on a daily, weekly or monthly basis and associated with extreme weather.