A Framework for Evaluating Model Credibility for Warm-Season Precipitation in Northeastern North America: A Case Study of CMIP5 Simulations and Projections

Abstract

uture projections of northeastern North American warm-season precipitation [June?August (JJA)] indicate substantial uncertainty. Atmospheric processes important to the northeast-region JJA precipitation are identified and a first evaluation of the ability of five phase 5 of the Coupled Model Intercomparison Project (CMIP5) models to simulate these processes is performed. In this case study, the authors develop a set of process-based analyses forming a framework for evaluating model credibility in the northeast region. This framework includes evaluation of models? ability to simulate observed spatial patterns and amounts of mean precipitation; dynamical atmospheric circulation features, moisture transport, and moisture divergence important to interannual precipitation variability; long-term trends; and SST patterns important to northeast-region summer precipitation.Wet summers in the northeast region are associated with 1) negative 500-hPa geopotential height anomalies centered near the Great Lakes; 2) positive 500-hPa geopotential height anomalies over the western Atlantic east of the Mid-Atlantic states; 3) northeastward moisture flow and increased moisture convergence along the Eastern Seaboard; 4) increased moisture divergence off the U.S. Southeast coast; and 5) positive sea level pressure (SLP) anomalies in the western Atlantic, possibly related to cold tropical Atlantic SSTs and southwest ridging of the North Atlantic anticyclone. Models are generally able to simulate these features but vary compared to observations. Models capture regional moisture transport and convergence anomalies associated with wet summers reasonably well, despite errors in simulating the climatology. Identifying sources of intermodel differences in future projections is important, determining processes relevant for model credibility. In particular, changes in moisture divergence control the sign of northeast-region summer precipitation changes, making it a critical component of process-level analyses for the region.