MECHANISMS OF VARIABILITY AND PREDICTABILITY OF THE TROPICAL COUPLED OCEAN-ATMOSPHERE SYSTEM

A conceptual model is proposed to explain the observed aperiodicity in the short term climate fluctuations of the tropical coupled ocean-atm osphere system. This is based on the evidence presented here that the tropical coupled ocean-atmosphere system sustains a low frequency inte r-annual mode and a host of higher frequency intra-seasonal unstable m odes. 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 wa ve intra-seasonal modes in the tropical coupled system. It is argued t hat due to its long wavelength, the low frequency mode would behave li ke a linear oscillator while the higher frequency short wave modes wou ld be nonlinear. The conceptual model envisages that an interaction be tween the low frequency linear oscillator and the high frequency nonli near oscillations results in the observed aperiodicity of the tropical coupled system. This is illustrated by representing the higher freque ncy intra-seasonal oscillations by a nonlinear low order model which i s then coupled to a linear oscillator with a periodicity of four years . The physical mechanism resulting in the aperiodicity in the low freq uency oscillations and implications of these results on the predictabi lity of the coupled system are discussed.