Atmospheric circulation changes in the tropical Pacific inferred from the voyages of the Manila galleons in the sixteenth-eighteenth centuries

Historical accounts of the voyages of the Manila galleons derived from the Archivo General de Indias (General Archive of the Indies, Seville, Spain) a re used to infer past changes in the atmospheric circulation of the tropica l Pacific Ocean. It is shown that the length of the voyage between Acapulco , Mexico, and the Philippine Islands during the period 1590-1750 exhibits l arge secular trends, such that voyages in the middle of the seventeenth cen tury are some 40% longer than those at the beginning or at the end of the c entury, and that these trends are unlikely to have been caused by societal or technological factors. Analysis of a series of "virtual voyages," constr ucted from modem wind data,,indicates that sailing time to the Philippines depended critically on the strength of the trade winds and the position of the western Pacific monsoon trough. These results suggest that the atmosphe ric circulation of the western Pacific underwent large, multidecadal fluctu ations during the seventeenth century.