The Arid Diagonal (Spanish: diagonal árida/arreica) is a contiguous zone of arid and semi-arid climate that traverses South America from coastal Peru in the Northwest to Argentine Patagonia in Southeast including large swathes of Bolivia and Chile. The Arid Diagonal encompasses a number of deserts, for example: Sechura, Atacama, Monte and the Patagonian Desert.
The Arid Diagonal acts to isolate the temperate and subtropical forests of Chile and southern Argentina from other forests of South America. Together with the Quaternary glaciations in the Southern Andes, the diagonal has controlled the distribution of vegetation throughout Chile and Argentina.
The northern portion of the Arid Diagonal is a result of the blocking of the trade winds by the barrier formed by the Central Andes and the South Pacific High and to the south in the westerlies the rain shadow the Southern Andes casts over eastern Patagonia. South of Mendoza (32°53' S), the driest parts of the diagonal move away from the Andes as the mountains lose height, causing some humidity to penetrate; thus, at more southern latitudes the driest parts of the diagonal lie on the Atlantic coast of Patagonia.
The Arid Diagonal has existed since the Neogene. The origin of the aridity of northern part of the diagonal is linked to two geologic events: a) the rise of Andes—an event that led to the permanent block of both the westward flow of moisture along the tropics, and the eastward flow of moisture in Patagonia and b) the permanent intrusion of cold Antarctic waters (the Humboldt Current) along South America's west coast. Together with the Quaternary glaciations in the Southern Andes the diagonal controls the distribution of the vegetation types over Chile and Argentina.