Arid Diagonal

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.[1] 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.[2] Together with the Quaternary glaciations in the Southern Andes, the diagonal has controlled the distribution of vegetation throughout Chile and Argentina.[3]

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[4] and to the south in the westerlies the rain shadow the Southern Andes casts over eastern Patagonia.[1] 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.[1]

The Arid Diagonal has existed since the Neogene.[3] 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[5] and b) the permanent intrusion of cold Antarctic waters (the Humboldt Current) along South America's west coast.[4] Together with the Quaternary glaciations in the Southern Andes the diagonal controls the distribution of the vegetation types over Chile and Argentina.[3]