The Pacific Ocean's influence on drought and wetness in the continental United States and the nation's breadbasket

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Tuesday, 19 January 2010: 2:45 PM
B216 (GWCC)
Brad Rippey, USDA, Washington, DC

Presentation PDF (136.6 kB)

The surface of the Pacific Ocean covers some 60 million square miles (nearly 160 square kilometers), nearly one-third of the surface of the Earth. The ocean's area is greater than that of all of Earth's land masses combined. Since the northern Pacific Ocean lies west, or atmospherically upstream, of the continental United States, influence on U.S. climate is profound. Well-known Pacific phenomena—such as the Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO)—influence U.S. temperature and precipitation patterns to varying degrees. Sometimes, ENSO and PDO act together to amplify conditions (e.g. drying, moistening) in certain parts of the U.S. For example, the negative phase of the PDO (in place during the 1950's) may have acted in tandem with the cold phase of ENSO (1954-56) to worsen U.S. drought.

This paper will explore the relationships between oceanic signals such as the SOI and the PDO and the major U.S. droughts of 1930-40, 1952-57, and 1999-2004. (Major U.S. wet spells of 1905-10, 1912-17, 1941-46, 1972-75, 1982-87, and 1992-99 will also be discussed.) Interestingly, the 1930's Dust Bowl drought occurred during a period of positive-phase PDO and an insignificant (weak, or nearly neutral) ENSO signal. Conversely, the 1950's drought—as mentioned in the previous paragraph—occurred during a period dominated by negative-phase PDO and cold-phase ENSO (La Niña). More recently, the U.S. drought that began in the late 1990's had its roots in the La Niña of 1998-2001 and occurred during a PDO transition period.

Another focus of this paper will be to look at periods of transition from U.S. drought to wetness, or wetness to drought. As the nearly neutral ENSO signal of the 1930's gave way to the warm-phase (El Niño) episode of 1939-41, there was a fairly rapid transition from the Dust Bowl era to a wet period in the early to middle 1940's. In contrast, warm-phase periods of 1991-95 and 1997-98 were followed by the protracted cold-phase (La Niña) episode of 1998-2000; during the same period, the general U.S. wetness of 1992-99 was followed by pervasive drought from 1999-2004.

Finally, shorter scale U.S. weather changes will be examined in the context of ENSO to determine if there are implications for corn yields in the nation's breadbasket. Historically, there have been several instances of hot, dry growing seasons (and Midwestern yield reductions for grain corn) embedded within an overall pattern of U.S. wetness. Recent examples include 1983 and 1995, when (following wet springs) Midwestern crops withered under unrelenting mid- to late-summer heat and short-term dryness. The agricultural droughts of 1983 and 1995, along with other Midwestern droughts such as those that occurred in 1954 and 1988, were noted in the months immediately following the end of a warm-phase (El Niño) episode. But, some major Midwestern droughts (e.g. 1980) were not tied to receding warm-phase episodes, nor were all warm-phase cessations (e.g. 1998) linked to Midwestern drought.