In the eastern tropical Pacific, where the phenomenon of El Niño is best developed, the atmospheric circulation undergoes a marked annual cycle as well. Analysis of data from a recently compiled atlas elucidates the role of the annual cycle in modifying the great 1972–73 Niño, and confirms that off South America upwelling and sea surface temperature are not directly linked. Meridional profiles derived from the atlas reveal that oceanic heat loss to the air is a function of wind speed rather than of sea surface temperature and that rainfall can be accounted for in terms of the horizontal convergence of moisture. Distribution of tropical cyclones is not significantly related to El Niño, while predicting El Niño has proved much more difficult than had been expected. Abstract In the eastern tropical Pacific, where the phenomenon of El Niño is best developed, the atmospheric circulation undergoes a marked annual cycle as well. Analysis of data from a recently compiled atlas elucidates the role of the annual cycle in modifying the great 1972–73 Niño, and confirms that off South America upwelling and sea surface temperature are not directly linked. Meridional profiles derived from the atlas reveal that oceanic heat loss to the air is a function of wind speed rather than of sea surface temperature and that rainfall can be accounted for in terms of the horizontal convergence of moisture. Distribution of tropical cyclones is not significantly related to El Niño, while predicting El Niño has proved much more difficult than had been expected.