Evaluation of the simulations of the North American monsoon in the NCAR CCM3

The six-year average of a ten-year integration of the National Center for A tmospheric Research (NCAR) Community Climate Model version 3 (CCM3), forced with prescribed, climatological sea surface temperatures, was compared wit h the Legates-Willmott precipitation climatology and the National Centers f or Environmental Prediction (NCEP)-NCAR reanalysis product. Summertime prec ipitation associated with the North American Monsoon (NAM) circulation is l argely underrepresented in simulations using the CCM3. The CCM3 simulates e xcessive amounts of tropical eastern Pacific and Caribbean precipitation, d epressed precipitation over Mexico and the southwestern United States, and largely misrepresents the summertime circulation pattern over North America as compared to the reanalysis climatology fields. Basic diagnostic analyse s suggest that excessive convection over tropical waters in the eastern Pac ific and the Caribbean alters the summertime circulation pattern which prod uces excess subsidence over much of Northern Mexico and the southwestern U. S. and prohibits the northward transport of atmospheric moisture into the N AM region. The vertically integrated moisture flux and precipitable water e stimated by the CCM3 are significantly different in amount and direction (i n the case of fluxes) than those observed in the reanalysis data. Introduci ng anomalously wet land-surface conditions over the NAM region at model ini tialization yields minimal improvement. Suspected causes for the erroneous simulation of the summertime circulation in the CCM3 are discussed.