1. Interannual Variability of North American Summer Precipitation in NASA/NSIPP and NCAR/CAM2.0 AMIP Simulations By Alfredo Ruiz-Barradas and Sumant Nigam Department of Meteorology University of Maryland September 3, 2003

2. Goal To assess interannual variability of precipitation over North America in AMIP- like runs of CAM2.0 and NSIPP models during summer months (June, July, August).

3. Data Precipitation: –Retrospective US and Mexico analysis. –Hulme (University of East Anglia) data set. –Xie/Arkin precipitation data set. SST from Hadley Center. NCAR/NCEP Reanalysis. AMIP simulation (ensemble no. 5) from the NSIPP model. AMIP simulation (case newsstamip06) from the CAM model.

4. Method Reanalysis and simulations extrapolated to a 5°  2.5  grid on 17 pressure levels. Monthly climatology for the 1950-1998 period. Monthly anomalies wrt 1950-1998 climatology. JJA is the mean of June, July, August. Assessment through: –Standard Deviation –Precipitation Index –Multivariate analysis

6. ~12 years peak ~5 years peak

16. Remarks Large precipitation variability in observations and simulations over central US. Although it is shifted in simulations. Great Plains precipitation indices from simulations do not correlate with the observed index. SST regressions on the Great Plains index suggest linkage with Pacific midlatitude variability. CAM however emphasizes the tropical influence.

17. Remarks Multivariate analysis indicates: –Great Plains precipitation variability is the main mode of summer variability in observations; –This is however not the case in both model simulations; –Wet/dry events are cold/warm events in both observed and simulated summers.

18. Remarks PC regressions on moisture fluxes and geopotential heights indicate: –Observed precipitation variability is linked to a coherent, barotropic circulation that enhances/diminishes southerly stationary moisture flux from the Gulf of Mexico; –Model simulated variability does not have such circulation linkages.