The influence of tropical east Pacific Ocean temperatures on the atmosphere

Abstract A version of the nine‐level hemispheric model of the atmosphere developed at the Geophysical Fluid Dynamics Laboratory has been used to test Bjerknes' hypothesis that fluctuations of ocean temperatures in the tropical east Pacific are responsible for major variations in the position and intensity of the Aleutian surface low. The results of four 30‐day integrations, two with warm and two with cool tropical east Pacific (maximum differences 3.5°C) are analysed, and verified against observational data. The ocean variations had important effects on the model atmosphere, both tropical and extra‐tropical. The warmed ocean heated the air above and induced a surface low near the sea temperature maximum; associated low‐level convergence and ascent gave increased tropical rainfall over the central and east Pacific with decreases over the west Pacific and parts of South America. The related upper flow changes generated a subtropical jet maximum with persistent troughing to the north over the mid‐latitude east Pacific. The Aleutian low was shifted east and deepened, confirming Bjerknes' hypothesis. There was strong similarity between the mid‐latitude flow over the Pacific in the warm cases despite major differences between the two cases with cool tropical east Pacific. The importance both of tropical surface conditions and of extra‐tropical influences on the model Tropics is indicated and an interaction mechanism proposed. The early development of differences between the experiments suggests that it is important to use correct tropical sea temperatures for short‐range numerical prediction in the Tropics. However, mid‐latitude effects were not significant till a week after the onset of deep tropical convection.