Response of the upper tropospheric humidity and moisture transport to changes of tropical convection. A comparison between observations and a GCM over an ENSO cycle

Comparison of the upper tropospheric relative humidity (UTH) of CCM2 forced by observed sea surface temperatures (SSTs) with data from TOVS, MSU, and the GEOS-1 assimilation shows that the model reproduces the observed large-scale pattern of interannual changes of UTH in the tropics, but gives too much drying during the 1987 El Niilo. How UTH responds to the changes of precipitation is well-simulated in the areas of anomalous subsidence and drying in the subtropical Pacific. However, the exaggerated increase of precipitation i the model in the equatorial central Pacific leads to excessive amplitudes of vertical motion, which dries the subtropical upper troposphere. Since the uplift does not result in excessive moistening in the equatorial Pacific, the effect of the too vigorous hydrological cycle is an unreali•c•y large reduction of UTH in the tropics during an E1 Nilto in CCM2.