AGCM hindcasts with SST and other forcings: Responses front global to agricultural scales

Multiple realizations of the 1969-1998 time period have been simulated by t he GISS AGCM to explore its responsiveness to accumulated forcings, particu larly over sensitive agricultural regions. A microwave radiative transfer p ostprocessor has produced the AGCM lower tropospheric, tropospheric, and lo wer stratospheric brightness temperature (T-b) time series for correlations with microwave sounding unit (MSU) time series. AGCM regional surface air temperature and precipitation were also correlated with GISTEMP temperature data and with rain gage data. Seven realizations by the AGCM were forced s olely by observed sea surface temperatures. Subsequent runs hindcast Januar y 1969 through April 1998 with an accumulation of forcings: observed sea su rface temperatures (SSTs), greenhouse gases, stratospheric volcanic aerosol s, stratospheric and tropospheric ozone, and tropospheric sulfate and black carbon aerosols. Lower stratospheric T-b correlations between the AGCM? an d the MSU for 1979-1998 reached as high as 0.93 globally given SST, greenho use gases, volcanic aerosol, and stratospheric ozone forcings, Midtroposphe ric T-b correlations reached as high as 0.66 globally and 0.84 across the e quatorial, 20 degrees S-20 degrees N band. Oceanic lower tropospheric T-b c orrelations were less high at 0.59 globally and 0.79 across the equatorial band. Of the sensitive agricultural areas considered, Nordeste in northeast ern Brazil was simulated best with midtropospheric T-b correlations up to 0 .80. The two other agricultural regions, in Africa and in the northern midl atitudes, suffered from higher levels of non-SST-induced variability. Zimba bwe had a maximum midtropospheric correlation of 0.54, while the U.S. Corn Belt reached only 0.25. Hindcast surface temperatures and precipitation wer e also correlated with observations, up to 0.46 and 0.63, respectively, for Nordeste. Correlations between AGCM and observed time series improved with addition of certain atmospheric forcings in zonal bands but not in agricul tural regions encompassing only six AGCM grid cells.