Influence functions analysis of climate model errors during El Nino episodes

Atmospheric monthly circulation anomalies during El Nino episodes in the period 1963-1992, produced by a climate model driven by observed SST are compared with the circulation anomalies obtained from the NCEP/NCAR reanalysis data. The ensemble mean of eight ECHAM3 model runs with resolution T42 is used in this analysis. Composites of monthly streamfunction anomalies at 850 hPa and 200 hPa, as well as precipitation and divergence anomalies for El Nino events, are generated with data from these sources, along with an assessment of their consistency. They show some significant differences. The focus of this paper is on the differences between anomalies near Southern South America during November of El Nino episodes, when the impact on precipitation in this region is strongest. The possible sources for these differences are examined using influence functions of a vorticity equation model. They indicate that the differences may be due partly to errors in the heat sources (and associated upper level divergence) and partly to the differences between the atmospheric basic states of the model and from the reanalysis data.