Impact of atmospheric horizontal resolution on El Niño Southern Oscillation forecasts

In this paper, we assess the skill of a seasonal forecast system to predict El Niño Southern Oscillation (ENSO) anomalies and the impact that the horizontal resolution of the atmospheric component has on the forecasts of the growing phase of strong ENSO events. The analysis has been carried out on retrospective six-month forecasts for the period 1973–2001 performed with the Istituto Nazionale di Geofisica e Vulcanologia (INGV) coupled ocean–atmosphere general circulation model (SINTEX) in the framework of the DEMETER project. Each forecast is formed by an ensemble of nine integrations that start from perturbed initial conditions. In order to consider the possible impact of the seasonal cycle on the forecasts, four different dates of the year have been chosen as initial conditions. The results indicate that, in general, the model is reasonably good in predicting the distribution of the seasonal anomalies in the tropical Pacific. However, it is also shown that the model predictions underestimate substantially the amplitude of the anomalies during the developing phase of intense El Niño episodes. The forecasts of strong ENSO events in the period here considered have been repeated, increasing the horizontal resolution of the atmospheric component. In these experiments, both the ocean model component and the oceanic initial conditions are unchanged. During the decaying phase of the oscillation, the high-resolution and low-resolution version