Graduate Thesis Or Dissertation
 

A study of El Niño/southern oscillation : numerical experiments and data analysis

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  • For the purpose of investigating the fundamental nature of the interannual oscillation observed in the tropical Pacific in conjunction with El Nino/Southern Oscillation (ENSO), the Comprehensive Ocean Atmosphere Data Set was analyzed. Based upon this analysis, an "intermediate" tropical atmosphere-ocean coupled model was developed. For reason of their particular importance to atmosphere-ocean interactions, moisture processes such as condensation-convergence, evaporation-sea surface temperature and evaporation-wind feedbacks were given special emphasis in both the data analysis and modeling processes. The atmospheric component of the model is based on the first baroclinic mode, which is driven by the atmospheric internal heating. The oceanic model consists of two layers with an imbedded oceanic mixed-layer, by which SST is predicted. The present study has demonstrated that despite the structural simplicity of the oceanic model, it is capable of simulating mean oceanic circulation. In preparation for coupling, individual models were first tested with the use of appropriate time-dependent boundary conditions specified from the composite ENSO data. Both model simulations reproduced the major features associated with the ENSO events. Coupling was performed following the imposition of wind stress anomalies over the western Pacific for a given time period and the removal of all external forcing for a period of eight years thereafter. The coupled responses simulated during the the first two-year period provided reasonable simulations of the following ENSO-like features: the appearance of warm sea-surface temperature anomalies in the central Pacific, deepening/shallowing of mixed-layers in the eastern/western Pacific, weakening of the trades in the central Pacific, strengthening/weakening of the North Equatorial Counter Current/South Equatorial Currents, and enhanced convective activities around the dateline. The long-term coupled integration showed a pattern of interannual oscillation over a period of approximately three years. The results obtained from this coupling study have illustrated (1) that the necessary condition for the interannual oscillation is the interaction between the atmosphere and the ocean and (2) that it is likely that oceanic wave dynamics plays a crucial role in the determination of the growth and decay of ENSO events.
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