El Nino and La Nina

The conditions of high sea-surface temperature, weak trade winds and small differences in surface atmospheric pressure across the tropical Pacific of El Nino precede and follow the generally benign and complementary conditions of La Nina, the two phenomena constituting the irregular Southern Oscillation. The conditions of interannual sea-surface temperatures which cause El Nino or La Nina are explained in terms of surface wind fluctuations. Schematic views of atmospheric conditions in the tropical Pacific and the way in which thermal circulation causes shifts in the Intertropical Convergence Zone (ITCZ) of the Pacific and Atlantic demonstrate this development. Knowledge from general circulation models, from studies of the ocean's adjustment to changes in the wind, and from ocean-atmosphere models, are discussed with reference to modelling the development of the Southern Oscillation, and the existence of a similar phenomenon in the Atlantic is mentioned. -J.W.Cooper