2010 Articles

On the causes and dynamics of the early twentieth century North American pluvial

Cook, Benjamin I.; Seager, Richard; Miller, Ronald L.

The early twentieth century North American pluvial (1905-1917) was one of the most extreme wet periods of the last five hundred years and directly led to overly generous water allotments in the water-limited American West. Here we examine the causes and dynamics of the pluvial event using a combination of observation-based data sets and general circulation model (GCM) experiments. The character of the moisture surpluses during the pluvial differed by region, alternately driven by increased precipitation (the southwest), low evaporation from cool temperatures (the central plains), or a combination of the two (the Pacific northwest). Cool temperature anomalies covered much of the west and persisted through most months, part of a globally extensive period of cooler land and sea surface temperatures (SST). Circulation during boreal winter favored increased moisture import and precipitation in the southwest, while other regions and seasons were characterized by near normal or reduced precipitation. Anomalies in the mean circulation, precipitation, and SST fields are only partially consistent with the relatively weak El Nin ̃o forcing during the pluvial, suggest- ing a significant role for internal variability or other forcing agents. Differences between the reanalysis dataset, an independent statistical drought model, and GCM simulations high- light some of the remaining uncertainties in understanding the full extent of SST forcing of North American hydroclimatic variability.