1. Seasonal Moisture Flux Variability over North America in NASA/NSIPP’s AMIP Simulation and Atmospheric Reanalysis By Alfredo Ruiz-Barradas and Sumant Nigam University of Maryland --------o-------- Mississippi River Climate and Hydrology Conference New Orleans, LA May 13-17, 2002 1. Introduction. Seasonal variability of stationary moisture fluxes over the United States in the NASA Seasonal-to-Interannual Prediction Project (NSIPP) atmospheric simulation are compared with NCEP and ECMWF reanalysis. The annual cycle of moisture flux into the US is dominated by contributions from the Gulf of Mexico due to the existence of a low-level jet during spring and summer months. The horizontal and vertical structure of the jet and its seasonal evolution is the focus of this study. 2. Data. Monthly data sets are on a 5°  2.5° horizontal grid with 8 vertical levels at 1000, 925, 850, 700, 600, 500, 400, and 300 mb spanning the period 1979-1993. Climatology is based on this period. An NSIPP Atmospheric simulation is compared with the reanalysis data sets and Xie-Arkin precipitation. ITCZ in the eastern Pacific does not move north in summer in the simulation. ITCZ over Central America/Gulf of Mexico, on the other hand, moves too far north in the simulation. 3. Results. Vertically Integrated Moisture Fluxes Mean JJA Mean DJF Reasonable simulation of the large scale onshore (offshore) fluxes over the US west (east) coast during both winter and summer. Deficient simulation of moisture fluxes over the tropics: +Excessive flux convergence over Central America, Gulf of Mexico, and Caribbean regions in JJA. +Diminished flux convergence near the eastern tropical Pacific in JJA. Meridional Moisture Flux Profiles at 30°N in boreal summer The simulated jet is too diffuse, particularly zonally. The jet is strongest in July in both simulation and observation, but it is to anemic in the May and June simulations. Annual cycle at 120°W and 90°W Temperature profiles at 90°W and 20°N in boreal summer Anomalous upper level warming over the Gulf of Mexico in May and June Anomalous warming reaches surface in July and August Correspondence with the anomalous evolution of the ITCZ over the region 850mb Geopotential Height in JJA Correspondence between anomalous surface warming and anomalous low at 850mb over the Gulf of Mexico Anomalous weakening of the western extension of the Bermuda High. Weakening of the anticyclone induces weakening of the southerly moisture flux from the Gulf of Mexico into the Great Plains region. 4. Conclusions. The NSIPP atmospheric model reasonably simulates the large scale fluxes over the US. Errors in the simulation of the semipermanent midlatitudes anticyclones introduce errors in the moisture fluxes over the US. In particular, weak moisture fluxes from the Gulf of Mexico seem to be related to a deficient simulation of the ITCZ in the eastern tropical Pacific. It is suggested that propagation of anomalous warming from upper to surface levels, due to anomalous release of latent heat, induces a weakening of the westward extension of the Bermuda anticyclone which impacts on the meridional moisture fluxes toward the US.