Thursday, 17 January 2002: 4:00 PM
Intra- to multidecadal (IMD) climate variability over the continental United States: 1932–1999
Although trend analysis is used frequently in climate studies, it is vulnerable to a number of conceptual shortcomings. This analysis of U.S. climate division data uses an alternate approach. The method used here subjects time series of annual average temperature and total precipitation to tests of Mann-Whitney U statistics over moving sampling windows of intra- to multi-decadal (IMD) duration. In applying this method to time series of nationally averaged annual rainfall, a highly significant incidence of wet years is found after the early 1970's. When applied to individual climate divisions this approach provides the basis for a climate survey method that is more robust than linear trend analysis, and capable of objectively isolating the timing and location of IMD climate events over the United States. From this survey, four major impact periods emerge between 1932 and 1999: the droughts of the 1930's and 1950's, a cool 1964-1979 period, and wet-warm time windows at the end of the century. More circumstantial consideration was also given here to the state of the El Niño-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) , the winter state of the North Atlantic Oscillation, and mean annual northern hemisphere surface temperature during those periods. Significant late century warmth was found mainly in the western U.S. after the mid-1980's, but no evidence of cooling was evident in the southeast, as reported elsewhere. The late century wet regime appears to have occurred in two phases, with wetness confined to the east during 1972-1979, and more concentrated in the southwest and central U.S. during 1982-1999. Anecdotal evidence presented here suggests that wet years associated with warm phase ENSO conditions and the positive phase of the PDO may have played a role in ending the drought periods of the 1930's and 1950's. Conversely, the La Niña-like climate impacts found here during the late 1940's to mid-1950's, and the increased incidence of cold phase ENSO and negative phase PDO conditions during that time, suggests connections between that ocean state and severe drought.