Unlike other mid-latitude mountain ranges of high elevation, the Andes do not seem to significantly modify the large-scale atmospheric circulation around them. Instead, the prominent Bolivian high-pressure system that dominates South American meteorology is ruled largely by heating over the Amazon basin.

Yet, along the eastern side of the Andes mountains sweeps a northerly wind that would not be there without the mountain chain, according to Nadja Insel and colleagues (Clim. Dyn. doi:10.1007/s00382-009-0637-1; 2009). These northerly winds bring moisture and energy from the tropical Amazon basin to the more arid subtropical landscapes farther south.

Credit: © 2009 SPRINGER

However, simulations with a high-resolution regional climate model, in which the height of the Andes is manipulated, suggest that these northerly winds have not always existed. In the model, the Andes had to reach up to at least half their present elevation for the wind system to develop. Therefore, before sufficient Andean uplift, the eastern flank would have been much drier.

In the model without the Andes, only 2 to 4 mm of rain falls per day over the eastern rises of the range. This is meagre, compared with 15 mm per day that falls in a simulation with a modern-sized mountain chain. Insel and colleagues attribute this potentially vital difference to the southerly transport of moisture and energy in the mountain-induced wind system.

The rise in precipitation over the central Andes as the northerly winds strengthened should have left its imprint in the geological record. If so, dating the change could help constrain the point in time when the Andes reached half their present height.