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 Atmospheric Research (NCAR) Community Climate Model version 3 (CCM3), forced with prescribed, climatological sea surface temperatures, was compared with the Legates-Willmott precipitation climatology and the National Centers for Environmental Prediction (NCEP)-NGAR reanalysis product. Summertime precipitation associated with the North American Monsoon (NAM) circulation is largely underrepresented in simulations using the CCM3. The CCM3 simulates excessive amounts of tropical eastern Pacific and Caribbean precipitation, depressed 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 analyses suggest that excessive convection over tropical waters in the eastern Pacific and the Caribbean alters the summertime circulation pattern which produces excess subsidence over much of Northern Mexico and the southwestern U.S. and prohibits the northward transport of atmospheric moisture into the NAM region. The vertically integrated moisture flux and precipitable water estimated by the CCM3 are significantly different in amount and direction (in the case of fluxes) than those observed in the reanalysis data. Introducing anomalously wet land-surface conditions over the NAM region at model initialization yields minimal improvement. Suspected causes for the erroneous simulation of the summertime circulation in the CCM3 are discussed.