A four-component ecosystem model of the oceanic upper mixed layer (UML) forced by the annual cycle of UML depth, solar irradiation and dissolved inorganic nitrogen (DIN) entrainment from the seasonal pycnocline is presented. The model solution demonstrates the following types of temporal variability: a periodical regime with the frequency of the external forcing, a regime with a period of more than 1 year, quasi-periodic, and chaotic motion. The model results suggest that the last three types describing the interannual variability can occur only at low latitudes in regions of strong upwelling where the DIN concentration in the seasonal pycnocline is high. However, the range of external forcing parameters in which such behaviour takes place is so narrow that it is unlikely to be a common phenomenon in the ocean. The quasi-periodic or chaotic variability of the model ecosystem is very sensitive to the initial conditions, and therefore any exact prediction of model behaviour is impossible. Nevertheless, a prediction of model ecosystem behaviour can be obtained in terms of a probability density. The annual cycle of the model components calculated in this way shows that the dispersion of the trajectories during the winter period is markedly smaller than during the summer. It implies that the dynamics of the model ecosystem during the summer period is less predictable.