P2C2: Continental Scale Droughts in North America: Their Frequency, Character and Causes Over the Past

Pan-continental North American droughts, defined as droughts that impact multiple water resource and agricultural areas, can exert severe stress on society in excess of more localized, but equally severe, droughts. For example, the drought of 2012 struck both the Plains and the Midwest and caused tens of billions of dollars in crop losses. However, events like the 2012 pan-continental drought take both the public and climate researchers by surprise. They do not appear to be easily explained in terms of ocean forcing, and cannot be predicted by current operational seasonal forecasting systems. However, pan-continental droughts have occurred repeatedly over the instrumental record and also appear in tree-ring reconstructions of climate and precipitation in North America over the last thousand years.

This project will examine the causes and dynamical mechanisms of pan-continental droughts in North America on seasonal to multidecadal timescales. The work will use instrumental records, proxy climate records of the past millennium, atmosphere models forced by historical sea surface temperatures, coupled general circulation models and idealized model experiments. The time period of interest will be the last millennium and the next few decades. The hypothesis is that preferred states of the coupled Pacific-North America-Atlantic atmosphere-ocean system cause pan-continental droughts, including potential contributions from internal atmospheric modes that are unpredictable on seasonal and longer timescales. The project will also determine the role of soil moisture and vegetation conditions on the occurrence and evolution of pan-continental droughts. The record of regional and pan-continental drought in the newly updated millennium-long tree-ring based North American Drought Atlas will be examined. The research team will assess whether the current climate system can also explain the record of the past millennium or whether fundamentally different modes of operation (e.g. climate shifts in the Pacific and Atlantic Oceans) are required. The ability of current generation climate models to simulate pan-continental droughts, and with what mechanisms, will be examined. Finally, analyses of the past and current model projections will be used to determine changes in the likelihood of pan-continental drought in the coming decades.

This project is supported by funding from: