The NCEP regional Eta model was nested in the NCEP atmosphere general circulation model (AGCM) to investigate the ability of the one-way dynamic downscaling method (DDM) to simulate the interannual variability of seasonal climate over South America (SA). The study compared the austral summers (DJF) and winters (JJA) of an El Niņa and an El Niņo year; namely, 1988 and 1997. Each season's run consists of 5-member ensemble. The results indicated that the DDM can improve the simulation of the lower-level circulation and precipitation interannual difference for both seasons. The intensification of the SA low-level jet in the El Niņo years was simulated by the Eta, including the jet's vertical structure, which consequently resulted in better simulations of precipitation interannual variability over the subtropics. In contrast, these features are missing in the AGCM simulations.

The winter precipitation difference (JJA97-JJA88) over southeastern SA for the observation, AGCM, and Eta were 1.90, 0.23, and 1.60 mm day-1, respectively. On the other hand, the summer precipitation differences (DJF97-DJF88) over southeastern Brazil were 4.22, 0.07, and 2.11 mm day-1 for the observation, AGCM, and Eta, respectively. Precipitation intra-ensemble noise maps indicated that the Eta exhibited higher predictability (less noise) than the AGCM over subtropical SA.

The spectral decomposition of kinetic energy (KE) and moisture flux convergence (MFC) spatial patterns indicated that the DDM's improvement resulted mostly from enhanced small-scale (500 to 1200 km) MFC associated with the topography. We conjecture that the improvement of DDM in this study is mainly due to proper representation of the Andes. Sensitivity tests were carried out to test this hypothesis. The lack of interannual variability in baroclinic wave activity, as indicated by the KE spectral analysis, suggested why the DDM's subtropical precipitation differences were not as large as observed.

This study also revealed a weakness in the simulation over the tropics. Similar to our investigation on seasonal downscaling (De Sales and Xue, 2006), the Eta simulations there were worse than the AGCM's alone. Other Eta-model DDM studies have also shown a deficiency in simulating intense summertime tropical rainfall over SA. Its possible causes will be addressed in this paper.