Relationship between Amazon and High Andes rainfall

A regional climate model is utilized to better understand the relationship between Amazonian and high Andes rainfall. Simulations from the present day and the Last Glacial Maximum (LGM; ~21,000 years ago) provide two cases. Paleo-proxy evidence generally suggests that the high Andes were wet during the LGM, while conditions over the Amazon were drier than today, although there is more uncertainty about the latter.

Model results for the LGM indicate that a delay in the onset of convection in the Amazon during austral spring is associated with an enhancement of rainfall over the high Andes. In the absence/delay in the onset of Amazon convection, the zonal height gradients between the Andes and the Atlantic Ocean are more intense than those in the present day simulation, resulting in anomalous easterly flow between the Amazon and the high Andes. In the present day climate, the low-level flow runs parallel to the Andes (e.g., northwesterly), but in the LGM simulation it is directed up the eastern slopes of the Andes (e.g., northeasterly) and transports lower-tropospheric moisture from the Amazon up into the mountains.

This circulation mechanism helps explain how atmospheric moisture, and rainfall, over the high Andes of Peru and Bolivia increases despite the large-scale drying that was prevalent during the LGM over South America. Our study as well as previous work (e.g., Fuenzalida and Rutlland 1987, Vuille et al. 1998, Chaffaut et al. 1998, Garreaud 1999) indicates that the moisture source for the Andes region is the Amazon basin, but that rainfall variability over the central Andes is not directly controlled by moisture changes in the Amazon basin (Garreaud et al. 2003). This most likely is related to the elevation differences between the Amazon and central Andes. Regardless of the lower tropospheric variability of moisture over the Amazon, any transport of moist air from low-levels (i.e., below 800 hPa) onto the Altiplano (i.e., 600 hPa or less) will generally result in an increase in moisture. Our study of the LGM climate provides an example of this disconnection between moisture variability in the Andes and the Amazon. In this case, the atmospheric moisture content over the Amazon basin is lower during the LGM, yet the moisture content increases during the austral spring months over the central Andes.

Once the LGM summer rains become established over the Amazon, simulated rainfall rates over the Andes quickly become similar to, or even slightly lower, than present day values for the rest of the season. This suggests that the dynamical circulation changes associated with the Amazonian convection during the austral spring onset are important for rainfall variability over the high Andes. However, the opposite does not appear to hold true, as there is no clear evidence from these simulations suggesting that the variations in high Andes rainfall influences the Amazonian response.

This mechanism identified in the regional climate model for the LGM can also be observed to be operating in the present day variability. Comparison of the austral springs of 2002 with 2003 reveals a rainfall and circulation response comparable to the LGM model response.