Alfredo Ruiz-Barradas Sumant Nigam

1. Simulation of 20th Century North American Hydroclimate Variability by the Drought Working Group Models:A Quick First Look Alfredo Ruiz-Barradas Sumant Nigam University of Maryland March 8, 2008

2. Objective Assess capabilities of the DWG atmospheric models in simulating seasonal and low-frequency summer hydroclimate variability over North America Models examined: • NSIPP (NASA/GSFC; 2.5lon x 2.0lat, 5th AMIP ens mem, 1930-2004) • CCM3 (LDEO; T-42 goga_new runs atm, 1st ens mem, 1856-2007) • CAM3.5 (NCAR; T-85, one run completed recently, 1871-2006) • GFS (NOAA/NCEP; T62L64 version of current CFS, 1950-2001) • AM2.1 (NOAA/GFDL; AMIP run data requested/awaited xxxx-yyyy)

3. JJA CLIM Monthly STD Summer Precipitation(1950-2000)CI: 1.0 mm/day (CLIM); >2.0 shadedCI: 0.3 mm/day (STD); >1.5 shaded US-MEX CAM3.5 • Climatology • GFS has a dry bias while CCM3 is too wet over central United States • CAM3.5 is fairly realistic except for large values on the eastern seaboard • All models are dry over the Gulf Coast states, particularly, eastern TX & Louisiana • GFS, CCM3, and NSIPP have a wet bias over the Northern Great Plains (western & central Canadian provinces) • Standard Deviation • CAM3.5, NSIPP, and CCM3 have something resembling the observed STD maximum over central US GFS CCM3 NSIPP

4. Great Plains Precipitation AnomalySeasonal precipitation anomalies smoothed by 12 applications of 1-2-1 averagingThe smoothed PRECIP index mimics PDSI (correlation of detrended series ~0.85) Full Century (1901-2002) Half Century (1950-2000) Full Century (1901-2002) Correlations [CRU_P, CCM3_P] =0.36 (0.37 detrend) [CRU_P, CAM3.5_P]=0.32 (0.33 detrend) [CRU_P, PDSI] =0.82 (0.84 detrend) Half Century (1950-2000) Correlations [CRU_P, CCM3_P] =0.58 (0.50 detrend) [CRU_P, CAM3.5_P]=0.38 (0.19 detrend) [CRU_P, GFS_P] =0.28 (0.19 detrend) [CRU_P, NSIPP_P] =0.27 (0.04 detrend)

5. SST Correlations of the Great Plains SummerPRECIP Indices (1950-2000)All-season precipitation indices are first detrended and then smoothed(as before) to generate PDSI proxies. The summertime proxy indices are then correlated with detrended SSTs. CRUTS2.1 CAM3.5 • GFS has fairly realistic correlations over the Pacific but not the Atlantic • CCM3’s Pacific correlations are too strong, NSIPP’s too weak, and CAM3.5’s somewhere in between • Atlantic links are comparable to the Pacific ones in observations but weaker in model simulations (with less accord among them as well) • CCM3 and CAM3.5 exhibit Indian Ocean connectivity, with little support from observations CCM3 GFS NSIPP

6. Concluding Remarks This first look, based on evaluation of a SINGLE ensemble member from each center’s AMIP simulations, indicates • Significant differences in the summer precipitation climatology and monthly SD distribution • Modest skill in simulating low-frequency variability of precipitation (or proxy PDSI): Correlations for the Great Plains region are in the 0.0-0.5 range in the 1950-2000 record • SST links of the Great Plains proxy drought index are more focused in the Pacific in model simulations, but not in 1950-2000 observations which show significant connectivity to the Atlantic basin as well • The spread in models’ performance makes the CLIVAR Drought Modeling exercise worthwhile