Dynamics and Thermodynamics of Interannual Variability in the Tropical Atmosphere/ocean System.

A series of coupled atmosphere/ocean models are developed to investigate the El Nino/Southern Oscillation (ENSO) phenomenon. A coupled numerical ocean-atmosphere model is used to examine the dynamic and thermodynamic processes associated with ENSO. The model warm events are initiated in the spring prior to the event peak, and are well described as an instability of the coupled system. Oceanic wave dynamics determines the fate of the growing instability. The warming of the SST produces westerly wind anomalies in the equatorial central Pacific, forcing equatorially trapped Rossby waves that propagate freely to the western boundary. These waves reflect at the western boundary, sending upwelling equatorial Kelvin waves back to the central basin which act to terminate instability growth and rapidly plunge the coupled system into a cold regime. The system returns from a cold regime via reduced heat flux to the atmosphere and by wave induced processes like those that lead to the warm event termination. Analog models are derived (from the numerical model) which highlight the properties that produce the coupled atmosphere/ocean instability localized in the eastern ocean basin, and the equatorial wave dynamics in the western ocean basin that are responsible for a delayed, negative feedback into this instability growth. Together, these processes determine the growth rate of the coupled system and, when the solutions are oscillatory, the period of the oscillation. The simple analog models are used to interpret a set of experiments using the nonlinear, numerical model of the Pacific coupled atmosphere/ocean system, in which we examine the effects of the assumed basic state and ocean geometry on the model ENSO events. In all of the experiments, the simple models give a correct estimation of the changes in interannual variability displayed in the full numerical model. The essential processes that describe the local instability growth rate and period of the interannual oscillations are found to be linear. Nonlinearities primarily act as a bound on the amplitude of the final state oscillations. The numerical and analog models help to explain why interannual variability analogous to ENSO is not prominent in the Atlantic and Indian basins.