THE SINGULAR VECTORS OF A COUPLED OCEAN-ATMOSPHERE MODEL OF ENSO .2. SENSITIVITY STUDIES AND DYNAMICAL INTERPRETATION

We have studied the singular vectors of an intermediate coupled ocean- atmosphere model of El Nino Southern Oscillation in an attempt to unde rstand the factors which influence the growth of perturbations in the coupled system in the tropics. The singular vectors are important beca use they are the fastest growing perturbations that can exist in the s ystem before nonlinearity becomes important. In this study, we have ex amined how the model singular-vectors are affected by variations in ce rtain model parameters, some of which mathematically define the singul ar vectors. The important findings of this study can be summarized as follows: The behaviour of the model singular-vectors can be understood in terms of a superposition of the linear modes of the coupled system which are non-orthogonal. In some cases, the gravest modes are oscill atory in time and this leads to resonances in the singular-vector grow th-factors. The singular-vector spectrum is generally dominated by its fastest-growing member whose wind structure is relatively insensitive to the parameters that define the singular vectors mathematically Thi s result is in accord with the findings of Part I, and suggests that, in the tropics, the coupled system has a preferred response to perturb ations, and that this response will readily manifest itself if conditi ons are favourable. Favourably configured perturbations can undergo ra pid transient growth over the entire regime in which the coupled model is asymptotically stable (i.e. when there are no unstable modes in th e system). In this regime, rapid transient growth can occur over a wid e range of timescales from 1 week to 24 months. The results and ideas presented here have important ramifications for the way in which the r eal coupled ocean-atmosphere system will respond in the tropics to per turbations arising from stochastic forcing inherent in the system. The dynamical operators that govern the early stages of linear perturbati on-development are non-normal which means that the ubiquitous stochast ic noise present in the tropics can be amplified. The dynamical mechan isms by which this can occur are the same as those responsible for the growth of the system singular-vectors. The dominant singular-vectors of the coupled model bear a remarkable resemblance to the tropical-cyc lone pairs that are often observed spanning the equator in the western Pacific Ocean accompanied by strong westerly wind bursts, and thought to be precursors of El Nino events. We propose that both westerly and easterly wind bursts occuring in nature are a preferred response of t he coupled system in the tropics to its inherent stochastic noise comp onent.