Collaborative Research: A Precipitation Dipole in Eastern North America: Issues of Space-Time Variability and Physical Mechanisms

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Recent studies suggest that precipitation over Eastern North America (ENA) exhibits a dipole pattern of wet and dry conditions between the central United States and eastern Canada, with particular prominence at decadal timescales and considerable contribution to local trends ? but which is largely erased in the usual calculations of area-averaged trends. The associated trend toward increased Central US precipitation is not well reproduced in current models and is contrary to the usual expectations of drying in the continental interior with a warmer climate. Decreasing precipitation over eastern Canada is also contrary to model projections of generally wetter conditions nearer the coasts. Several fundamental questions have yet to be addressed: What are the underlying dynamics and key physical mechanisms? How closely is it related to large-scale climate variability? What processes are setting the decadal timescale?What are the intellectual merits of the proposed activity?The goals of this project are to determine the underlying dynamics of the dipole pattern and its connections to large-scale climate variability and trends. The three primary objectives of the proposed work are to: 1) determine the timescales, seasonality, and structure of the dipole pattern, 2) examine the dynamics of the precipitation changes in terms of the hydrologic budget, moisture transport, and storm track variability, and 3) use a hierarchy of models to investigate the influence of large-scale variability on the dipole mode. To investigate the underlying mechanisms, we will test a set of hypotheses on the local forcing of the precipitation via moisture flux and thermodynamically-forced vertical velocity, and on the large-scale controls on the regional circulation via both baroclinic and barotropic response to tropical convection.We will expand our observational data analyses to establish and clarify the link among precipitation variations, circulation anomalies, and boundary forcing that may create a precipitation dipole over ENA. We will then investigate the links to large-scale climate variability and tropical forcing. This is perhaps one of the first attempts to understand and characterize the existence of a precipitation dipole over ENA. Our methodology -- which builds on our ongoing and previous work -- was briefly tested and some preliminary results are presented in the proposal. We will conduct observational analysis of a range of hydrologic and atmospheric variables to determine the structure of the seasonal and spatial pattern. We will analyze the observed hydrologic budget, moisture transport, shifts in the storm tracks, and transient-mean flow interaction to investigate the internal dynamics of the dipole, using both simple compositing and pattern-based analysis techniques such as Principal Component Analysis. We will use multiple estimates of precipitation and atmospheric circulation to alleviate known data quality issues. For dynamical investigations, we will use a range of models of increasing complexity: a global barotropic (one-layer) model linearized about a zonally-varying mean flow, a tropical Gill-Matsuno model with generalized heating, and the NCAR Community Atmospheric Model, which we have modified to allow imposition of convective anomalies.What are the broader impacts of the proposed activity?This collaborative project between the Tufts University and University of Massachusetts builds on mutually synergistic expertise in water cycle research, atmospheric dynamics, and hydrology. This partnership will be further expanded through co-mentoring of a post doctoral fellow, PhD students and involvement of undergraduate and high school students through summer internships. The proposed research addresses several important scientific questions, including the dynamics of large-scale atmospheric influences on hydrology, mechanisms of hydrologic variability at decadal and longer timescales, and trend attribution over different regions of ENA. There is also considerable societal relevance: understanding these regional changes in precipitation and their long-term variations has important implications on how, and to what extent, long-term variations in precipitation over ENA can be predicted and managed. The results are also relevant to agriculture (e.g., winter wheat) and, potentially, to water trade with Canada and increased terrestrial carbon fluxes to stream.