Effects of the Andes on Eastern Pacific Climate: A Regional Atmospheric Model Study*

A regional atmospheric model is used to study the effects of the narrow and steep Andes on the eastern Pacific climate. In the Southern Hemisphere cold season (i.e., August–October 1999), the model reproduces key climatic features, including the intertropical convergence zone (ITCZ) north of the equator and an extensive low-level cloud deck capped by a temperature inversion to the south. Blocking the warm easterly winds from South America, the Andes help maintain the divergence and temperature inversion and, hence, the stratocumulus cloud deck over the southeast Pacific off South America. In an experiment where the Andean mountains are removed, the warm advection from the South American continent lowers the inversion height and reduces the low-level divergence offshore, leading to a significant reduction in cloud amount and an increase in solar radiation that reaches the sea surface. In March and early April 1999, the model simulates a double ITCZ in response to the seasonal warming on and south of the equator, in agreement with satellite observations. Under the same sea surface temperature forcing, the removal of the Andes prolongs the existence of the southern ITCZ for 3 weeks. Without the mountains, the intrusion of the easterlies from South America enhances the convergence in the lower atmosphere, and the transient disturbances travel freely westward from the continent. Both effects of the Andes removal favor deep convection south of the equator. The same sensitivity experiments are repeated with orography used in T42 global models, and the results confirm that an underrepresentation of the Andes reduces the stratus clouds in the cold season and prolongs the southern ITCZ in the warm season, with both acting to weaken the latitudinal asymmetry of eastern Pacific climate. The implications of these results for coupled modeling of climatic asymmetry are discussed.