The Prominence of Decadal and Multidecadal Variability in North American Precipitation

Recent studies have suggested that the risk of drought over North America changes at timescales of one to several decades, and that these changes are coherent over large areas. We present a complementary perspective that measures the importance of these signals at the local scale. We use singular spectrum analysis to identify regions in North American where decadal to multidecadal (D2M) signals make up a significant fraction of the total variance in annual or seasonal precipitation. In most regions, D2M variability is not significant, as precipitation is dominated by interannual variability and secular trends. Decadal variability is significant at the regional scale in some seasons, most prominently in Minnesota and northern California during winter, and the central Rocky Mountains in autumn. Eastern Quebec is the only region where precipitation exhibits significant variance in the multidecadal band. D2M variability in these four regions is generally not coherent, and does not resemble major modes of climate variability. Decadal signals in the discharge of the Sacramento River and, to a lesser degree, the Colorado River are coherent and in phase with similar signals in regional precipitation. It is possible that D2M signals in other aspects of the climate system or amplification of weak D2M signals in precipitation by landscape response could create large changes in drought severity on decadal or multidecadal timescales. However, our results indicate that changes in precipitation cannot by themselves be the cause of significant D2M variability in drought over most parts of North America.