Further Studies of Ocean-Atmosphere Coupled Dynamics in the Tropical Pacific

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This project is an investigation of some of the fundamental processes behind interannual and decadal variability in the coupled ocean-atmosphere system of the tropical Pacific. One focus of the proposed research is whether there are energetic coupled modes in different frequency bands: "fast" - on the order of 1 year; "intermediate" - the ENSO mode; "slow" - a decadal mode, and whether, together these modes and their nonlinear interactions can produce some of the complex time dependence seen in the tropical Pacific. Several approaches will be used to study the problem. First, the linear modes of a hierarchy of different linearized coupled models will be determined. The most complex of these models is a coupled MOM3 multi-level ocean and a multi-level spectral atmospheric model, linearized around a steady state. The response of the linearized models to high-frequency stochastic forcing will also be examined. The upper-ocean energy budgets of these models will be diagnosed in order to understand the dynamics responsible for each mode. A modified version of the Cane-Zebiak intermediate model will be used to examine nonlinear interactions between the different modes. It is anticipated that the fast coupled mode and the ENSO mode will interact primarily through the effects of zonal advection on temperature while the ENSO and decadal mode will interact through ocean thermocline displacement. The effects of using more general stochastic wind stress and heat flux forcing (using observed spatial EOFs with amplitudes that are white noise processes) will also be examined. The process modeling approach will be augmented with analysis of decadal variability in climate model output through a collaboration with Dr. Kimoto of Tokyo University (CCSR). This research is important because it has the potential to improve the physical understanding of the observed behavior of coupled ocean-atmosphere climate phenomena in the tropical Pacific and, hence, projections of climate phenomena such as El Nino.