The principal large-scale systems responsible for the variability of precipitation over South America are: the Alta de Bolivia (AB), the South Atlantic Convergence Zone (SACZ), the Intertropical Convergence Zone (ITCZ), the South Pacific Anticyclone
(SPA), and the South Atlantic Anticyclone (SAA).
While the Humboldt Current creates a stable atmosphere, the eastern Pacific anticyclone, a high-pressure cell, determines the patterns of winds that reach the coast.
The Andes desiccate the west coast from 30[degrees] S north almost to the equator, undoubtedly amplifying the effects of the Humboldt Current and the eastern Pacific anticyclone.
Because the flora is shaped largely by the aridity of the region, the study of the area's biogeography is tied to the development of the Humboldt Current, the eastern Pacific anticyclone, and the Andes.
Therefore, the eastern Pacific anticyclone that affects climatic patterns of western South America so strongly probably has been in place since at least the mid-Miocene.