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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.