Summer moisture availability across North America

Maps of the monthly self-calibrating Palmer drought severity index (SCPDSI) have been calculated for the period 1901-2002 for the contiguous United States (20 degrees-50 degrees N and 130 degrees-60 degrees W) with a spatial resolution of 0.5 degrees x 0.5 degrees. The recently introduced SCPDSI improves upon the more common Palmer drought severity index by maintaining a more consistent behavior of the index and hence comparability over diverse climatological regions. Although the self-calibrating PDSI and the "traditional" PDSI averaged over the contiguous United States correlate well, large differences were found for extensive areas. Over the area as whole the 1930s and 1950s stand out as times of persistent and exceptionally dry conditions, whereas the 1970s and the 1990s were generally wet. Over 55% of the Untied States saw moderate to extreme drought in 1934 and over 40% of the area was affected during the 1954-1956 drought. The wettest year, 1983, saw over 45% of the contiguous United States affected by moderate to extreme wet conditions. Three leading modes of coherent summer moisture variability are identified, and they have their largest loading over the northwestern United States, the southern plains, and the southwestern United States, respectively. No evidence is found for a statistically significant trend in the percentage area of the contiguous United States experiencing either moderate or extreme moisture availability conditions for the 1901 2002 period.