The stratus cloud deck is often observed off the west coast of South America, and it plays an important role in the regional and global climate by substantially reducing the amount of solar radiation absorbed by the ocean. Coupled ocean-atmosphere models have also shown the potential impact of this cloud deck on the SST and the positioning of the ITCZ over the eastern Pacific. A regional climate model developed at International Pacific Research Center (IPRC), University of Hawaii, is used to study the effects of the narrow and steep Andes on the stratus cloud deck over the southeastern Pacific in the dry season (August-October, 1999) and on precipitation in the eastern Pacific and South America in the wet season (March-May, 1999), respectively. The regional climate model is a primitive equation model with sigma as the vertical coordinate. The model resolution is 0.5X0.5°in longitude/ latitude and 28 vertical levels with 10 levels below 850hPa. The model is initialized with NCEP/NCAR reanalysis data and daily NCEP/NCAR reanalysis data is also used as the lateral boundary conditions.

In the dry season, the model successfully simulates the stratus cloud deck over the southeastern Pacific off South America, the surface wind field as well as precipitation when compared with observations (TRMM cloud liquid water, QuikSCAT surface wind and CMAP precipitation). A sensitivity experiment was carried out by removing the Andean mountain range. Comparison of the control with the No-Andes experiment indicates that the presence of the mountains substantially enhance cloud deck over the southeastern Pacific. It is found that the presence of the Andes blocks low-level warm easterly winds from South America, leading to the development of divergence and an enhanced temperature inversion off South America that favor the formation of stratus clouds. In the wet season of southeastern Pacific, on the other hand, the presence of the Andes not only changes the precipitation over the South America but also affects the precipitation in the eastern Pacific ITCZ. The Andes act to enhance the northern ITCZ and weaken the southern one. This suggests that the poor representation of the steep and narrow Andes in current global GCMs is a likely cause of persistent double ITCZ in their simulation.