This chapter examines the various aspects of the southern oscillation (SO) and El Nino. Over the tropical Pacific Ocean, the SO is associated with large year-to-year variations in the intensity of the trade winds and in rainfall patterns. The SO also has a signature that extends into the middle latitudes of each hemisphere during its winter season. Interannual sea-surface temperature variations in the tropical Pacific Ocean are primarily associated with a phenomenon known as El Nino. A typical El Nino-Southern Oscillation (ENSO) episode evolves in two stages. One stage involves principally the eastern tropical Pacific, and the other involves the central and western Pacific. The differences between individual ENSO events reflect the relative strengths, and on rare occasions the order of occurrence of the two stages. The eastward displacement in the tropical Pacific of the warm surface waters and the atmospheric convection affected the atmospheric circulation globally. The oceanic changes during ENSO are caused primarily by the weakening of the trade winds. The close relation between El Nino events and the seasonal cycle suggests that the phase of the initial perturbations that trigger instabilities cannot be random.
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