South American Low-Level Jet, Vapor Transport, and Precipitation Variability
Mountain ranges are known to have a first-order control on mid-latitude climate, but previous studies have shown that the Andes have little effect on the large-scale circulation over South America. Paleoclimate proxies for the Neogene indicate that climate conditions along the eastern margin of the Andean Plateau changed from arid to humid, while the western flanks of the plateau became hyperarid. To determine whether Neogene paleoclimate was responding mainly to surface uplift or some other forcing requires an understanding of the evolution of interactions between the Andes and regional climate.
We are using limited-domain general circulation models (RegCM3, REMOiso) to evaluate the effect of the Andes on regional-scale atmospheric dynamics and precipitation. Experiments in which Andean heights are specified at 250m, and 25, 50, 75, and 100% of their modern values indicate that the Andes have a significant influence on moisture transport between the Amazon Basin and the central Andes, deep convective processes, and precipitation over much of South America through mechanical forcing of the South American Low Level Jet (LLJ) and topographic blocking of westerly flow from the Pacific Ocean (see Figure). When the Andes are absent, the LLJ is absent and moisture transport over the central Andes is mainly northeastward. As a result, deep convection is suppressed and precipitation is low along the Andes. Above 50% of the modern elevation, a southward flowing LLJ develops along the eastern Andean flanks and transports moisture from the tropics to the subtropics. Moisture drawn from the Amazon Basin provides the latent energy required to drive convection and precipitation along the Andean front.