A defining feature of U.S. drought variability is its regime-like behavior. The 1930s, 1950s, and the 2000s stand out as national-scale dry epochs, while the 1980’s and 1990’s were comparatively wet periods. The dry epochs are not merely the result of a single, especially severe drought event, but are typically plagued by a series of recurring droughts. These sometimes strike the same portion of the nation year-over-year (e.g., southern Plains in the 1950s), while at other times, drought tends to progress and appears to spread across the contiguous U.S. over an extended period (e.g. in the 2000s). Concerns that a new regime of sustained and severe drought may now be taking hold over the U.S. grain belt, harkening perhaps to the 1930s, has been triggered by the recent drought events in the Great Plains that began in 2011 over southern portions and expanded to central portions in 2012.
This proposed project seeks to understand why the contiguous U.S. experiences distinct decadal-scale modulations in drought. While individual seasonal drought events have been extensively studied, and for which considerable progress is being made to improve their predictability, less is known about the regime-like behavior. This project seeks to understand if drought regimes are a consequence of slowly evolving forcings of climate, and whether those forcings and their accompanying impacts possess predictability. In particular, are drought regimes due principally to low frequency oceanic forcings (e.g. Atlantic and Pacific decadal-like ocean variations), or are they mainly a statistical residual (or rectified response) of higher frequency ocean fluctuations linked to El Niño – Southern Oscillation (ENSO)? Can prolonged dry periods arise from atmosphere-land coupled variability alone without ocean forcing, and if so what are the implications for predictability? In this sense, the project seeks to understand whether the various US drought epochs of the last century have had common causes. Further, given the recent proliferation of severe U.S. drought during a period in which national temperatures were their warmest on record, the question arises if the role of climate change is now exerting an appreciable effect on sustained drought risks.
The project proposes mechanistic studies using model simulations and initialized decadal predictions to understand the causes for, and predictability of, US drought regimes. The focus is on long-term factors, such as decadal ocean variability and anthropogenically-driven trends, though cognizant of possible rectification resulting from seasonal SST variations. In parallel with climate simulations that will explore the physical processes causing decadal dry and wet epochs, a set of decadal predictions will assess existing capabilities to anticipate such epochs. Climate simulations spanning the last century will be used to understand how climate change forcing has acted to influence the risks of sustained droughts.
The goals of the proposal are closely aligned with the CPO/MAPP FY14 announcement of opportunity research foci of “Research to Advance Understanding, Monitoring, and Prediction of Drought – Understanding Predictability of Past Droughts over North America.” More specifically to “…consider mechanistic studies involving model simulations and predictions to examine processes such as the role of land surface conditions, oceanic conditions, and atmospheric feedbacks; long-term factors, such as decadal variability or anthropogenically3 driven trends versus immediate meteorological causes.” Proposal objectives also align closely with the NOAA’s Next Generation Strategic Plan (NGSP) objectives to (a) “Improved scientific understanding of the changing climate system and its impacts”, and (b) “Assessments of current and future states of the climate system that identify potential impacts and inform science, service, and stewardship decisions.”