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Drought Onset and termination Across North America: Mechanisms and predictability

Introduction to the problem of drought onset and termination (DO&T). Much advance has been made identifying the mechanisms that cause drought in North America. The tropical oceans are key drivers but internal atmospheric variability often plays a de- termining role while land surface feedbacks can be important. Less clear are the processes that determine DO&T and whether these have similar controls and predictability. However
society always faces questions of when a drought will end or begin?

Rationale for proposed work. To develop understanding of the mechanisms in the at- mosphere, land and ocean that cause DO&T to enable more accurate prediction of DO&T.

Brief summary of proposed work. DO&T will be defined in terms of change in soil moisture over timescales ranging from a season to a year. The observational basis will be soil moisture data from the atmosphere-forced NLDAS-2 land surface models, multiple atmospheric reanalyses, and sea surface temperature (SST) datasets, for the overlapping 1979 to present period. Multiple statistical methods will be deployed to identify continental scale associations within observations between DO&T and changes in atmospheric circula- tion, storm tracks, moisture transports/convergence, evapotranspiration, runoff and SSTs. DO&T mechanisms in key regions of interest (southwest, Texas/Southern Plains, Cornbelt)
will be analyzed. We will identify the observed mechanisms of DO&T for case studies (the 1999-2004 southwest, the 2010/11Texas/Southern Plains, the 2012 Cornbelt droughts). Identification of physical mechanisms will come from interpretation of ensembles of simulations with multiple SST-forced models and the North American Multimodel Ensemble coupled model hindcasts and forecasts. The relative roles of internal atmosphere variability and SST forcing in causing DO&T will be determined as well as whether models are capable of capturing observed mechanisms of DO&T and how predictable DO&T is. For the three case studies we will generate large ensembles of the SST-forced NCAR CAM5.3 model (i) with and (ii) without soil moisture initialized to the observed state from NLDAS-2 prior to DO&T and (ii) a run of the land surface model component with initialization and forced by observed meteorology and simulate the period of DO&T. Comparison of simulations and observations will determine the mechanisms and predictability of DO&T.

Relevance to Competition and NOAA’s long term climate goals. The work addresses MAPP goals “Developing a better understanding of sources of predictability towards improving predictions of drought onset, evolution, and termination on subseasonal to in- terannual timescales” and “the role of atmospheric, oceanic and land processes and coupled interactions in providing predictability for droughts in North America” and “how predictability sources and processes linked to drought are represented in state-of-the-art modeling and prediction systems”. The work contributes to NOAA long term goals of “scientific understanding, monitoring, and prediction of climate and its impacts, to enable effective decisions” within the identified themes of “Research to advance scientific understanding” and “Modeling and prediction” focused on “weather and climate extremes” and “climate impacts on water resources”.

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