Intra- to Multidecadal Climate Variability over the Continental United States: 1932–99

Trend analysis is used frequently in climate studies, but 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 multidecadal (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 1970s. When applied to individual climate divisions this test 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 major IMD climate events over the United States. From this survey, four such periods emerge between 1932 and 1999: the droughts of the 1930s and 1950s, a cool 1964–79 period, and wet–warm time windows at the end of the century. More circumstantial consideration was also given here to the state of ENSO, the Pacific decadal oscillation (PDO), the winter state of the North Atlantic Oscillation, and mean annual Northern Hemisphere surface temperature during those periods. 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 1930s and 1950s. Conversely, the La Niña–like climate impacts found here during the late 1940s to mid-1950s, 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. Significant late-century warmth was found mainly in the western United States after the mid-1980s, but no evidence of a cooling trend 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 1972–79, and more concentrated in the southwest and central United States during 1982–99.