Graduate Project
 

A point-data analysis of the Southern Oscillation Index (SOI) and its relationship to seasonal precipitation in the northwest United States

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  • El Niflo and La Nifia are oceanographic and atmospheric phenomena that have been catalogued for well over a centuly. In El Nifio years, Peru's otherwise thy west coast is subjected to torrential rainfall. Higher ocean temperatures off the coast are deemed the culprit. Attempts to quantify this began in the early part of the 20th Century in the western Pacific/eastern Indian Ocean region. By approximately the 1980's, what is known as the Southern Oscillation Index (501) became established as a benchmark for understanding circulation in the Pacific Ocean. This present study compared summer SOl averages with average fall and winter precipitation at 260 locations in Montana, Washington, Oregon, and Idaho. The time periods analyzed included 1948-97, 1951-80, 1961-90, 1951-65, 1966-80, 1961-75, 1976-90, and 1983-97. Linear regression and means-difference analysis (t-tests) were the statistical methods employed. The former test was used to test hypothesized positive correlations (r>0) for each station. A p-value less than or equal to 0.01 was established as verifying the correlations. This same level of significance was used to verify a second hypothesis using t-tests. This hypothesis states that mean seasonal precipitation values following La Nifla episodes exceed that which follows El Niflo episodes. It was theorized that there would be geographical correspondence between results of the two models. In addition, these relationships were theorized to remain regardless of the period that was analyzed. Results were also hypothesized to yield notable spatial variation. The hypothesis pertaining to the two statistical models were largely rejected. For approximately half of the time periods analyzed, however, high levels of statistical significance were noted in western portions of Montana, Washington, and Oregon, as well as northern parts of Idaho. Most sites appear to be near the western slopes of major mountain ranges, or are located near mountainous provinces. The hypothesis regarding uniformity of results for all time periods was also rejected. However, three out of eight time periods analyzed utilizing the means-difference model showed roughly the same geographic distribution of high significance as noted with the linear regression model. The lack of correspondence between the two models is thought to be the result of marked diversity in precipitation averages that correspond to El Niflo and La Nifia events. The high West Coast precipitation totals that followed the El Niflo of 1982-83 demonstrated how an event can violate the assumption that accompanies use of the statistical models. This fundamental assumption is that low precipitation totals follow El Niflo events, while higher totals characteristically follow La Nifia events. Marked spatial variation of the statistical results confirmed the final hypothesis.
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