STUDY OF THE PREDICTABILITY OF TROPICAL PACIFIC SST IN A COUPLED ATMOSPHERE-OCEAN MODEL USING SINGULAR VECTOR ANALYSIS - THE ROLE OF THE ANNUAL CYCLE AND THE ENSO CYCLE

The authors examine the sensitivity of the Battisti coupled atmosphere -ocean model-considered as a forecast model for the El Nino-Southern O scillation (ENSO)-to perturbations in the sea surface temperature (SST ) held applied at the beginning of a model integration. The spatial st ructures of the fastest growing SST perturbations are determined by si ngular vector analysis of an approximation to the propagator for the l inearized system. Perturbation growth about the following four referen ce trajectories is considered: (i) the annual cycle, (ii) a freely evo lving model ENSO cycle with an annual cycle in the basic state, (iii) the annual mean basic state, and (iv) a freely evolving model ENSO cyc le with an annual mean basic state. Singular vectors with optimal grow th over periods of 3, 6, and 9 months are computed. The magnitude of m aximum perturbation growth is highly dependent on both the phase of th e seasonal cycle and the phase of the ENSO cycle at which the perturba tion is applied and on the duration over which perturbations are allow ed to evolve. However, the spatial structure of the optimal perturbati on is remarkably insensitive to these factors. The structure of the op timal perturbation consists of an east-west dipole spanning the entire tropical Pacific basin superimposed on a north-south dipole in the ea stern tropical Pacific. A simple physical interpretation for the optim al pattern is provided. In most cases investigated, there is only one structure that exhibits growth. Maximum perturbation growth takes plac e for integrations that include the period June-August, and the minimu m growth for integrations that include the period January-April. Maxim a in potential growth also occur for forecasts of ENSO onset and decay , while minima occur for forecasts initialized during the beginning of a warm event, after the transition from a warm to a cold event, and c ontinuing through the cold event. The physical processes responsible f or the large variation in the amplitude of the optimal perturbation gr owth are identified. The implications of these results for the predict ability of short-term climate in the tropical Pacific are discussed.