Climatic factors influencing Last Glacial Maximum and modern glacial conditions in the tropical and subtropical Andes

Geomorphic evidence indicates that the presently unglaciated subtropical Andes (18.5° - 27°S) have previously sustained glaciers. However, the timing of glaciation and the mechanisms driving it are still poorly known. This study uses a full surface energy and mass balance model, driven using general circulation model output, to better understand the potential for past glaciation in the region and to identify the climatic forcings that may drive glaciation in the tropical and subtropical Andes. Model results show average Last Glacial Maximum (LGM) equilibrium line altitude (ELA) depressions to be approximately 600 m in the tropics and 800 m in the subtropics, consistent with studies that suggest the subtropical Andes were glaciated at LGM. While previous studies have found that the subtropical Andes are presently unglaciated due to low precipitation, idealized experiments show that lower LGM temperatures play the largest role in overall lowering of ELAs at that time. Furthermore, results show that shortwave radiation plays a significant role in driving tropical and subtropical ELAs. Results indicate that LGM decreases in shortwave radiation contributed to over 30% of total ELA depressions in the tropical Andes. In the subtropical Andes, decreases in shortwave radiation are shown to be as important as increases in precipitation during LGM. While previous modeling shows that a 4-5 fold increase in precipitation would have been necessary to glaciate the subtropical Andes, our results suggest that only a 50% increase in precipitation, in conjunction with a decrease in shortwave radiation and decrease in temperatures, would have been sufficient