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A conceptual model is proposed to explain the observed aperiodicity in the short term climate fluctuations of the tropical coupled ocean-atmosphere system. This is based on the evidence presented here that the tropical coupled ocean-atmosphere system sustains a low frequency inter-annual mode and a host of higher frequency intra-seasonal unstable modes. At long wavelengths, the low frequency mode is dominant while at short wavelengths, the high frequency modes are dominant resulting in the co-existence of a long wave low frequency mode with some short wave intra-seasonal modes in the tropical coupled system. It is argued that due to its long wavelength, the low frequency mode would behave like a linear oscillator while the higher frequency short wave modes would be nonlinear. The conceptual model envisages that an interaction between the low frequency linear oscillator and the high frequency nonlinear oscillations results in the observed aperiodicity of the tropical coupled system. This is illustrated by representing the higher frequency intra-seasonal oscillations by a nonlinear low order model which is then coupled to a linear oscillator with a periodicity of four years. The physical mechanism resulting in the aperiodicity in the low frequency oscillations and implications of these results on the predictability of the coupled system are discussed.

The stability of a simple coupled ocean-atmosphere system similar to the one studied by Hirst with general ocean thermodynamics is investigated in which the atmospheric heating is determined by sea surface temperature anomalies as well as the convergence feedback (low level moisture convergence by the waves themselves). It is shown that the unstable coupled mode found by Hirst (UH mode) is profoundly modified by the convergence feedback. The feedback increases the unstable range of the UH mode and can increase its growth rate several folds. The maximally growing UH mode can become westward propagating for certain strength of convergence feedback. If the convergence feedback strength exceeds a critical value, several new unstable intraseasonal mode are also introduced. These modes are basically `advective' modes. For relatively weak strengths of the convergence feedback the growth rates of these modes are smaller than that of the UH mode. As the atmosphere approaches `moist neutral' state, the growth rates of these modes could become comparable or even larger than that of the UH mode. It is argued that these results explain why the El Nino and Southern Oscillation signal is clear in the eastern Pacific but not so in the western Pacific.

The role of convergence feedback on the stability of a coupled ocean-atmosphere system is studied using model III of Hirst (1986). It is shown that the unstable coupled mode found by Hirst is greatly modified by the convergence feedback. If the convergence feedback strength exceeds a critical value, several new unstable intraseasonal modes are also introduced. These modes have very weak dependence on the wave number. These results may explain the behaviour of some coupled models and to some extent provide a mechanism for the observed aperiodicity of the El-Nino and Southern Oscillation (ENSO) events.

The nature of the coupled ocean-atmospheric instability is investigated within the framework of a simple coupled model. The coupled instability depends on the strength of the thermodynamic processes, namely upwelling (proportional to $K_T$), zonal advection of the mean temperature (proportional to $T_x$) and the meridional advection of the mean temperature (proportional to $T_y$). The relative importance of these processes varies from the western side of the Pacific basin to the eastern side, as also with different seasons. For other parameters representative of the central Pacific, an increase in the strength of the upwelling process (i.e., $K_T$) is seen to increase the growth of the instability. However, the unstable region in the wavenumber domain initially decreases with decrease of $K_T$ but again increases above some critical value of $K_T$. Moreover, for very small values of $K_T$ the unstable mode is westward propagating for all wavenumbers. A larger negative gradient of the mean zonal temperature is more favourable for instability growth. The meridional advection of the mean temperature has a stabilising effect. The three regions of the Pacific Ocean are considered separately, using representative values of the thermodynamic parameters corresponding to the west, central or east Pacific. The conditions in the west Pacific do not favour instability growth. On the other hand, the conditions in central Pacific are always favourable for the growth of instability. The conditions of east Pacific are favourable for instability growth only if the thermocline is as deep as in the central Pacific. The seasonal dependence of the unstable modes in the three regions of the Pacific basin is also studied.